MATERIAL COMPOSITIONS FOR SENSORS FOR DETERMINATION OF CHEMICAL SPECIES AT TRACE CONCENTRATIONS AND METHOD OF USING SENSORS . MCKGROUND OF THE INVENTION Reference to Related Application The present application claims priority under 35 U.S.C. §?19(e) to U.S. Patent Application No. 11/259,506 entitled MATERIAL COMPOSITIONS FOR SENSORS FOR DETERMINATION OF CHEMICAL SPECIES *T TRACE CONCENTRATIONS AND METHOD OF USING SENSORS end U.S. Patent Application No. 11/259,712 entitled SELF-CONTAINED PHOSPHATE SENSORS AND METHOD FOR USING SAME, both filed October 26, 2005. Field Of The Invention The present invention relates generally to sensors used in optical analysis of samples, and in particular relates to the material composition of sensors and methods for measuring trace concentrations of chemical species using the sensors'. Description of Related Art Sensor methods and sensor films for quantification of volatile and nonvolatile compounds in fluids are known in the art. Typically, quantification nf these parameters is performed using dedicated sensor systems that are specifically designed for this purpose. These sensor systems operate using a variety of principles including electrochemical, optical, acoustic, and magnetic. For example, sensor systems are used to conduct optical inspection of biological, chemical, and biochemical samples. A variety of spectroscopic sensors operating with colorimetric liquid and solid reagents have been developed. In fact, spectophotometric indicators in analytical chemistry have become the reagents of choice in many commercially available optical sensors and probes. Optical sensors possess a number of advantages over other sensor types, the most important being their wide range of transduction principles: optical sensors can respond to analytes for which other sensors are not available. Also, with optical sensors it is possible to perform not only "direct" analyte detection, in which the splctroscopic features 'oF™tfie~analyte are measured, but also "indirect" analyte determination, in which a sensing reagent is employed. Upon interaction with the analyte species, such a reagent undergoes a change in its optical property, e.g. elastic or inelastic scattering, absorption, luminescence intensity, luminescence lifetime or polarization state. Significantly, this sort of indirect detection combines chemical selectivity with that offered by the spectroscopic measurement and can often overcome otherwise troublesome interference effects. Because spectophotometric indicators were originally developed for aqueous applications, their immobilization into a solid support is a key issue for their application in optical sensing. Polymeric materials for reagent-based optical sensors are often complex multicomponent formulations. The key formulation ingredients include a chemically-sensitive reagent (indicator), a polymer matrix, auxiliary minor additives, and a common solvent or solvent mixture. However, it is difficult to predict the best formulation of the sensor material to yield a certain desired functionality. For example, phosphate is a frequently analyzed substance in the water treatment industry. Phosphate analysis is also common in environmental monitoring, in clinic diagnosis, and in other industrial places such as mining and metallurgical processes. Optical sensors are commonly used for analysis of phosphate. A commonly used optical method for phosphate determination is the molybdenum blue method. The basic mechanism of the molybdenum blue method includes the formation of a heteropoly acid (HPA) by reaction of an crthophosphate with a molybdate, A molybdic acid is formed and then reduced using a reducing agent under acidic conditions resulting in color generation Several other methods for phosphate analysis in aqueous solution based on the HPA chemistry are also known. They include vanadomolybdophosphoric acid method, molybdenum-stannous chloride method, and canonic dye-HPA complex method. The HPA method may be cclorimetric, thai '"s a color change of the sensor results after contacting with the analytc, and/or it may be photometric, that is a measurable change in the optical property of the sensor results after contacting with the analyte. The known photometric methods for phosphate analysis based on the formation of HPA require a strong acidic media, necessitating the use of concentrated sulfuric acid solutions in sensor formulations. In the case of cationic dye-HPA complex method, triphenylmethane dyes are commonly used. The absorption band of triphenylmethane solutions at a neutral pH usually overlaps with that of the dye-HPA complex. Thus, the pH of the test media for phosphate determination has to be controlled below the transition pH of the dye in order to reveal the absorbance change due to formation of the dye-HPA complex. The known photometric methods have several disadvantages, including requiring corrosive and toxic reagents and, in the case of cationic dye-HPA complex, being highly pH dependent. Silicate interference is another disadvantage of the HPA methods for phosphate analysis. A 3.0 ppm silicate in the sample water is known to interfere with cationic dye-HPA method. The commonly used molybdenum blue method is known to tolerate up to only 10 ppm silicate concentrations. Silicates are ubiquitous in natural water and hence it becomes difficult to determine low concentrations of phosphate in these cases because of the silicate interference. Moreover, the reagents employed in known photometric methods are usually incompatible, leading to a stepwise approach to phosphate determination. The sample is added to a reactor (or confined location) with pre-existing reagents and then exposed to the separately stored reducing agent. This instability and lack of chemical compatibility of the reagents hinders a one-reactor approach, thus restricting the development of self-contained sensors. For convenient and efficient application of sensors as on-site test devices, self-contained solid sensors are needed. Because optical indicators were originally developed for aqueous applications, their immobilization into a solid support is a key issue for their application in optical sensing. The incompatibility of reagents and the low pH requirement hinders this immobilization. Additionally, the sensitivity of the solid-state sensors to low concentrations is also an issue. For example, in US Patent No. 5,858,797, a phosphate test strip based on molybdenum blue chemistry was described to be sensitive toTphosphate concentration only above 6 ppm. Moreover, the molybdenum blue reagent and the reducing agent had to be deposited into separate layers to minimize reagent stability problems. Thus, there exists a strong need for simplified sensors that can easily be used to carry out optical analysis of multiple quantitative assays and/or other biological, chemical, and physical environmental parameters with high reproducibility that yield improved sensor sensitivity, decreased response to interferences, enhanced stability, and other desired parameters. SUMMARY OF THE INVENTION In one aspect, the invention is directed to a method of quantitatively measuring the concentration of a chemical species in a sample solution with a sensor film. The method includes preparing a hydrogel sensor film having a chemical composition comprising an indicator that changes its optical property in the ultra-violet, visible, near-infrared spectral range upon being exposed to the chemical species in the sample solution. The method further includes exposing the film to a fixed amount of the sample solution. The method further includes measuring the absorbance of the film at a wavelength near the maximum absorbance peak (tanax) of the indicator using optical scanning equipment. The method also includes quantifying the concentr/ition of the chemical species in the sample solution using the average absorbance measured from the sensor film. Another aspect of the invention is a method of quantitatively measuring the concentration of a chemical species in a sample solution with a plurality of sensor films. The method includes preparing a plurality of hydrogel sensor films that change their optical property in the ultra-violet, visible, or near-infrared spectral range upon being exposed to the chemical species in the sample solution, wherein the chemical composition added in the hydrogel films comprises a pH indicator, a surfactant, and an acid. The method also includes varying the acid concentration in each of the plurality of films by a predetermined pattern. The method also includes exposing the films to a fixed amount of the sample solution. The method further includes measuring the absorbance ofthe films at a wavelength near the maximum absorbance peak (A.max) of the indicator using optical scanning equipment and quantifying the concentration of the chemical species in the sample solution using the average absorbance measured from the sensor films. In another aspect, the invention is directed to sensor used in determining the concentration of chemical species in a sample at trace concentrations. The sensor includes a hydrogel sensor film comprising a quaternary ammonium salt, quaternary phosphonium salt or a quaternary imidazolium salt, and an indicator. The indicator changes its optical properly in the ultra-violet, visible, or near-infrared spectra! range upon being exposed to the chemical species in the sample solution. The indicator is immobilized in the hydrogel film by forming an ion pair with the quaternary ammonium ion, wherein the concentration of quaternary ammonium salt is substantially higher that the stoichiometric amount required to ion pair. In another aspect, the invention is directed to sensor used in determining the concentration of chemical species in a sample at trace concentrations. The sensor includes a hydrogel sensor film comprising an indicator and an additive that increases the sensor sensitivity of response to chemical species where the additive is a polymer and where the sensor film is prepared by dissolving hydrogel, indicator, and second polymer in a common solvent mixture. The indicator changes its optical property in the UV, ultra violet, visible near-infrared spectral range upon being exposed to the chemical species in the sample solution. According to another aspect, the invention is directed to a self-contained phosphate sensor is described. The self-contained phosphate sensor includes at least one analyte-specific reagent and at least one pH-modifier The self-contained phosphate sensor may be used in solution or as a solid-state device. The method of determining phosphate concentration in a test sample using the self-contained phospha?2 is also described. According to another aspect, the analyte-specific reagent includes a molybdate salt .aqd^a dye and a sulfpnicLacj(Las the pH-modifier. The self-contained phosphate sensor may further include a solvent or may be immobilized in a polymer matrix. According to another aspect, the analyte-specific reagent includes a metal complex and a dye and a sulfonic acid as the pH-rnodifier. The self-contained phosphate sensor may also include a non-aqueous solvent. According to a further aspect, the analyte-specific reagent includes a metal complex and a dye and an amine as the pH-modifier. The self-contained phosphate sensor may be immobilized in a polymer matrix. According to an embodiment of the invention, a method of determining phosphate in a test sample is described. The method includes, contacting a test sample with a self-contained phosphate-sensor described above, measuring a change in an optical property of the self-contained phosphate sensor produced by contacting the fest sample with the self-contained phosphate-sensor, and converting the change in optical property to the phosphate concentration. The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS The above mentioned and other features of this invention will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: FIG. 1 is a cross-section of a self-contained sensordisposed as a flhi on a substrate constructed in accordance with an embodiment of the invention. FIG. 2 is a cross-section of the self-contained sensor of FIG. ! in contact with a test sample. FIG. 3 is a cross-section of the self-contained sensor of FIG. 1 after contacting with thejest sample resulting in a change in the optical property of the phosphate sensor. FIG. 4 is a set of spectra at different phosphate concentrations for an embodiment of the self-contained sensor of FIG. 1 comprising h-PBMP-Zn-PCViolet Complex in Dowanol. FIG. 5 is a set of spectra at different phosphate concentrations for an embodiment of the self-contained sensor of FIG. 1 comprising h-PBMP-Zn-PCVioIe' Complex in polymer matrix. FIG. 6 is a calibration curve for an embodiment of the self-contained sensor of FIG. 1 comprising h-PBMP-Zn-PCViolet Complex in Dowanol, obtained by plotting absorbances at 650 nm as a function of phosphate concentration. FIG. 7 is a set of spectra at different phosphate concentrations for an embodiment of the self-contained sensor of FIG. 1 comprising Azure C and molybdate salt in water. FIG. 8 is a calibration curve for an embodiment of the self-contained sensor of FIG. 1 comprising Azure C and molybdate salt in water, obtained by plotting absorbarces at 650 nm as a function of phosphate concentration. FIG. 9 is a set of spectra at different phosphate concentrations for an embodiment of the self-contained sensor of FIG. 1 comprising Azure B and molybdate salt in water. FIG. 10 is a calibration curve for an embodiment of the self-contained sensor of FIG. 1 comprising Azure B and molybdate salt in water showing blue-to-violet reaction, obtained by plotting absorbances at 650 nm as a function of phosphate concentration. FIG. 11 is a calibration curve for an embodiment of the self-contained sensor of FIG. 1 comprising Brilliant Cresyl Blue and molybdate salt in water, obtdned by plotting absorbances at 622 nm as a function of phosphate concentration. FIG. 12 is a low-concentration calibration curve for an embodiment of the self-contained sensor of FIG. 1 comprising Azure B and molybdate salt in water, obtained by plotting absorbances at 650 nm as a function of phosphate concentration. . 13 is a set of spectra .aljdilferent phosphate concentrations for an embodiment of the self-contained sensor of FIG. I comprising Azure B and molybdate salt in polymer matrix. FIG. 14 is a calibration curve for an embodiment of the self-contained sensor of FIG. 1 comprising Azure B and molybdate salt in polymer matrix, obtained by plotting absorbances at 650 nm as a function of phosphate concentration. FIG. 15 is a set of spectra at different phosphate concentrations for an embodiment of the self-contained sensor of FIG. 1 comprising Malachite Green and molybdate salt in polymer matrix. FIG. 16 is a calibration curve for an embodiment of the self-contained sensor of FIG. 1 comprising Malachite Green and molybdate salt in polymer matrix, obtained by plotting absorbances at 650 nm as a function of phosphate concentration. FIG. 17 is a set of spectra at different phosphate concentrations for an embodiment of the self-contained sensor of FIG. 1 comprising Basic Blue and molybdate salt in polymer matrix. FIG. 18 is a calibration curve for an embodiment of the self-contained sensor of FIG. 1 comprising Basic Blue and molybdate salt in polymer matrix, obtained by plotting absorbances at 650 nm as a function of phosphate concentration. FIG. 19 is a set of spectra at different phosphate concentrations for an embodiment of the self-contained sensor of FIG. 1 comprising Methylene Blue and molybdate salt in polymer matrix. FIG. 20 is a calibration curve for an embodiment of the self-contained sensor of FIG. 1 comprising Methylene Blue and molybdate salt in polymer matrix, obtained by plotting absorbances at 650 nm as a function of phosphate concentration. FIG. 21 is a set of spectra at different phosphate concentrations for an embodiment of the self-contained sensor of FIG. 1 comprising Basic Blue and molybdate salt in a plasticized polymer matrix. 3. 22 is a calibration curve for an embodiment of the self-contained sensor of FIG. 1 comprising Basic Blue and molybdate salt in a plasticized polymer matrix, obtained by plotting absorbances at 650 nm as a function of phosphate concentration. FIG. 23 illustrates absorption spectra of a molybdate sensor film according to another embodiment of the invention at different molybdate concentrations; FIG. 24 illustrates a response curve for the sensor film of FIG. 23; FIG. 25 illustrates absorption spectra of a magnesium sensor according to another embodiment of the invention at different magnesium concentrations; FIG. 26 illustrates a response curve for the sensor film of FIG. 25; FIG. 27 illustrates absorption spectra of a hardness sensor according to another embodiment of the invention at different concentrations of magnesium; FIG. 28 illustrates a response curve for the sensor film of FIG. 27; FIG. 29 illustrates absorption spectra of a calcium sensor according to another embodiment of the invention at different calcium concentrations; FIG. 30 illustrates a response curves for the sensor film of FIG. 29; FIG. 31 illustrates absorption spectra of a sulfite sensor according to another embodiment of the invention at different sulfite concentrations; FIG. 32 illustrates a response curve for the sensor film of FIG. 31; FIG. 33 illustrates; a typical set of spectra of a sulfite sensor according to another embodiment of the invention at different sulfite concentrations; FIG. 34 shows a typical response curve for the sensor film of FIG. 33; FIG. 35 illustrates a calibration curve for an alkalinity sensor; FIG. 36 illustrates the improvement of sensitivity of response upon addition of increasing concentration of Nafion polymer in the pHEMA sensor film; and Flu. 37 illustrates the improvement of FIG. 36 plotted as the sensor signal upon exposure to 2 ppm of chlorine demonstrating an existence of a critical non-intuitive region of concentration of Nafion in PHEMA where a maximum sensor response is obtained. Corresponding reference characters indicate corresponding parts throughout the views of the drawings. DETAILED DESCRIPTION OF THE INVENTION The invention will now be described in the following detailed description with reference to the drawings, wherein preferred embodiments are described in detail to enable practice of the invention. Although the invention is described with reference to these specific preferred embodiments, it will be understood that the invention is not limited to these preferred embodiments. But to the contrary, the invention includes numerous alternatives, modifications and equivafents as will become apparent from consideration of the following detailed description. Disclosed are improved sensor material compositions and methods for determining the concentration of chemical species in a sample at trace concentrations. Embodiments of the self-contained sensors described herein can be used either in aqueous or non-aqueous solution or as a solid-state device. Such self-contained sensors have the advantage that no post-addition reagents are required to determine analyte concentrations and the analyte determination test requires a minimal number of procedural steps. Moreover, self-contained sensors provide enhanced sensitivity and a faster response time. Embodiments of the invention also provide a method for determining chemical species concentrations in a test sample. The concentration in a test sample can be quantified using a calibration curve generated by testing samples with known concentrations. In one aspect, the self-contained sensor is an optical sensor. Optical sensors possess a number of advantages over other sensor types, the most important being their wide range of transduction principles: optical sensors can respond to analytes for which other sensors are not available. Also, with optical sensors it is possible to perform not ertty "direct" analyfe detection; in which the spectroscopic features of the analyte are measured, but also "indirect" analyte detection, in which a sensing reagent is employed. Upon interaction with the analyte species, such a reagent undergoes a change in its optical property, e.g. elastic or inelastic scattering, absorption, luminescence intensity, luminescence lifetime or polarization state. Significantly, this sort of indirect detection combines chemical selectivity with that offered by the spectroscopic measurement and can often overcome otherwise troublesome interference effects. According to the invention, the sensor materials change their optical property in the ultraviolet (UV), visible, or near-infrared (1R) spectral range upon exposure to trace concentrations of the chemical species. The film is a polymer-based composition generally including a chemically sensitive analyte-specific reagent (for example, a fluorescent or colorimetric indicator), a polymer matrix or combination of polymer matrices, and auxiliary minor additives, wherein the film is produced from a solution of the components in a common solvent or solvent mixture. The analyte-specific reagent is immobilized within the polymer matrix to form the sensor film. Examples of additives are surfactants and internal buffers. Other additives can be also included. The polymers utilized in the sensor film are permeable to selected analytes where an analyte is a certain chemical species or class of chemical species detected by the sensor. The analyte-specific reagent undergoes changes in its optical properties (e.g., absorbance, fluorescence) as a function of analyte concentration. Desirably, the analyte-specific reagent undergoes the changes in its optical property outside- the film where the change in response is not affected by the presence of interfering species as provided by the sensor formulation. Measurements are performed using ultraviolet/visible/near-IR detection systems known to those skilled in the art. The desired response is achieved by tailoring the composition of the sensor film where the composition includes additional components in the film. For example, the desired sensor response is achieved by tailoring the oxidation potential oT the immobilized analyte-specific reagent with selection of the polymer matrix components where the polymer matrix components are additional polymers. It is desired that the sensor film be self-contained so it does not have a need for auxiliary reagents outside the film. In one embodiment, the above-mentioned self-contained sensors include an analyte-specific reagent and a pH-modifier. As used herein, "analyte-specific reagents" are compounds that exhibit change in colorimetric, photorefractive, photochromic. thermochromic, fluorescent, elastic scattering, inelastic scattering, polarization, and any other optical property useful for detecting physical, chemical and biological species. Analyte-specific reagents may include metal complexes or salts, organic and inorganic dyes or pigments, nanocrystals, nanoparticles, quantum dots, organic fluorophores, inorganic fluorophores, and their combinations thereof. pH-Modifiers in the sensors serve as buffers and maintain the pH level of the sensor formulations at a constant pH which is preferable for the sensing mechanism. The choice of pH-modifiers depends upon the nature of the analyte-specific reagent used, but pH-modifiers may include acids, bases, or salts. The self-contained sensors described herein may be used in solution or as solid-state devices. For application of the sensor as a solution, a common solven* is chosen for the different constituents of the sensor. Some examples of such a solvent include, but are not limited to, deionized water (DI water), l-methoxy-2-propanol (Dowanol), ethanol, acetone, chloroform, toluene, xylene, benzene, isopropyl alcohol, 2-ethoxyethanol, 2-butoxyethanol, methylene chloride, tetrahydrofuran, ethylene glycol diacetate, and perfluoro(2-butyl tetrahydrofuran). For application of the self-contained sensor as a solid-state device, the sensors described above are attached to or immobilized in a polymer matrix. The sensors are then disposed as a film on a substrate. It is to be appreciated that the polymeric material used to produce the sensor film may affect detection properties such as selectivity, sensitivity, and limit of detection. Thus, suitable materials for the sensor film are selected from polymeric materials capable of providing the desired response time, a desired permeability, desired solubility, degree of transparency and hardness, and other similar characteristics relevant to the material of interest to be analyzed. .Tjje, polymer matrix of the sensor film is preferably a plastic film, i.e., a resin film. The resin utilized to form the polymer support depends on the sensor applications. The resin is dissolved in the solvent so that the analyle-specific reagent becomes dispersed in the liquid medium. Alternatively, the analyte-specific reagent may be applied directly to an already formed plastic film. In one embodiment, a polymer film is made and a solvent is removed from the film by any known means such as evaporation, followed by the exposure of the dry film to a cocktail containing at least one reagent. In this way, a reagent is incorporated into the film. In one embodiment, the sensor film is prepared by coating a clear plastic surface with a thin layer of the chemical mixture and allowed to dry over a period of several hours in air the dark. The final film thickness is desirably between about 0.1 and about 200 microns, more preferably 0.5 - 100 microns and more preferably 1 - 50 microns. For evaluation of response, the film is exposed to aqueous samples of analyte. Desirably, the amount of the aqueous sample of the analyte ranges between about 30 uL and about 50 uL of sample, however other amounts are contemplated without departing from the scope of the invention. Exposure time is desirably between about 0. 5 - 1000 seconds, more preferably 1 - 500 seconds, and more preferably 5 - 300 seconds. In one embodiment, the water sample is then removed before measurement of the sensor film. Alternately, the water sample can be present during the measurement. In one yet another embodiment, the measurement is done continuously before water exposure, during water exposure, and after water exposure. In a farther embodiment, the measurement is done continuously before water exposure and during water exposure. It is understood that the polymeric material used to produce the sensor film may affect the detection properties such as selectivity, sensitivity, and limit of detection. Thus, suitable materials for the sensor film are selected from polymeric materials capable of providing the desired response time, a desired permeability, desired solubility, degree of transparency and hardness, and other similar characteristics relevant to the material of interest. Suitable polymers which may be used as polymer supports in accordance with the present disclosure are hydrogels. As defined herein, a hydrogel is a three dimensional network of hydrophilic polymers which have been tied together to form 'waTer-swellable but water insoluble structures. The term hydrogel is to be applied to hydrophilic polymers in a dry state (xerogel) as well as in a wet state as described in U.S. Patent No. 5,744,794. A number of different methods may be used to tie these hydrogels together. First, tying of hydrogels via radiation or free radical cross-linking of hydrophilic polymers may be utilized, examples being poly(hydroxyethylmethacrylates; pol "(acrylic acids), poly(methacrylic acids), poly(glyceryl methacrylatcj, poly(vinyl alcohols), poly(ethylene oxides), poly(acryl amides), po!y(N-acrylamides), po!y(N,N-dimethylaminopropyl-N'-acrylamide), poly(ethylene imines), sodium'potassium poly(acrylates), polysaccharides, e.g. xanthates, alginates, guar gum, agarose etc. poly(vinyl pyrrolidone) and cellulose based derivatives. Second, tying via chemical cross-linking of hydrophilic polymers and monomers with appropriate polyfunctional monomers may be utilized, examples including poly(hydroxyethylmethacrylate) cross-linked with suitable agents such as N,N'-nrethylenebisacrylamide. polyethylene glycol diacrylate, triethylene glycol diacrylate, tetraethylene glycoi dimethacrvlate, tripropylene glycol diacrylate, pentaerythritol tetraacrylate, di-irimethylolpropane tetraacrylate, dipentaerythritol pentaacrylate, trimethylolpropanr triacrylate, pentaerythritol triacrylate, propoxylated glyceryl tnacrylate, ethox^ iated pentaerythritol tetraacrylate, ethoxylated trimethylolpropane triacrylate, hexanediol diacrylate, hexanediol dimethacrylate and other di- and tri-acrylates and methacrylates; the copolymerisation of hydroxyethylmethacrylate monomer with dimethacrylate ester crosslinking agents; poly(ethylene oxide) based polyurethanes prepared through the reaction of hydroxyl-terminated polyethylene glycols) with polyisocyanates or by the reaction with diisocyanates in the presence of polyfunctional monomers such as triols; and cellulose derivates cross-linked with dialdehydes, diepoxides and polybasic acids. Third, tying via incorporation of hydrophilic monomers and polymers into block and graft copolymers, examples being block and graft copolymers of po!y(e!hylene oxide) with suitable polymers such as poly(ethyleneglycol) (PEG), acrylic-acid (AA), poly(vinyl pyrrolidone), poly(vinyl acetate), poly(vinyl alcohol). N,N-dimethy!aminoethyl methacrylate, poly(acrylamide-co-methyl methacrylate), poIy(N-isopropylacrylamide), ' puch reagents include light absorbing materials such as near infrared (N1R) absorbing materials. Examples of NIR absorbing materials include carbon black and Poly(styrenesulfonate)/poly(2,3-dihydrothieno(3,4-b)-l,4-dioxin). In one embodiment, the analyte-specific reagent is a light absorbing reagent absorbing light at about 620 - 670 nm. In another embodiment, the analyte-specific reagent is a light absorbing reagent absorbing light at about 750 - 820 nm. In another embodiment, the analyte-specific reagent is a light absorbing reagent absorbing light at about 380 - 420 nm. . These dyes may be used singly or in combination depending on the desired application. The choice of organic compound and amount utilized for a given application depends on the properties of the organic compound and the purpose for which it will be used. For instance, fluorescent dyes may be added to a resin binder at part-per-million concentrations as is known in the art. In one embodiment of the invention, the analyte-specific reagent is immobilized in the hydrogel matrix by forming an ion pair between the analyte-specific reagent and a lipophilic counter ion, such as a quaternary ammonium ion. It is known that quaternary ammonium ions may cause a change in the absorption spectra of analyte-specific reagents. However, it was unexpectedly discovered that the addition of quaternary ammonium ions, in concentrations substantially higher than the stoichiometric amount required to ion pair an analyte-specific reagent, produced a very significant improvement in the indicator selectivity and sensitivity. As used herein, concentrations substantially higher than the stoichiometric amount required to ion pair means the quaternary ammonium ions are added in a concentration of between about 5-1000 times greater than stoichiometric amounts relative to the indicator. By means of example and not by way of limitation, it has been determined that a preferred molar ratio of 285:1 (quaternary ammonium ion:indicator) is desirable for a particular molybdate sensor. By comparison, an optimum molar ratio of 18:1 was determined for a sulfite sensor. Without being limited to any particular explanation, it is currently believed that the physical change that occurs when amounts of quaternary ammonium ions greater than the critical micelle concentration are present in the film is the formation of micelles that bind more than a single irt&icator-analyte complex afils surface. Ligand-metal ratios greater than unity are thus formed in the presence of cationic micelles and can lead to enhancements in expected ultraviolet-visible-near-IR spectroscopy responses of the indicator-analyte. One example of an ion pair to be used below for exemplary purposes and not by way of limitation is Bromopyrogallol Red (BR) and benzyldimethyltetradecylammonium chloride (Zephirarnine) for the BP Red-Mod^ indicator system. The presence of quaternary ammonium salts has been shown to induce a significant bathochromie shift of the BP Red-Mo chelate absorption maximum, as well as intensification of the chelate absorption band. The quaternary ammonium salt used in this film was chosen with respect to structure and mass to achieve a shift in a position of peak absorption (^max) to longer wavelengths. Table 1 lists the A.max produced by selected quaternary ammonium salts on pHEMA film when wetted. Table 1. Effect of Quaternary Ammonium and Phosphonium Salt on km!a of pHEMA film when wetted with water (Table Removed) The addition of quaternary ammonium salt in concentrations significantly higher than that required to ion pair produced a very significant improvement in the indicator selectivity and sensitivity. A significant absorbance shift, desirably between about 10 nm and about 30 nm, and more desirably about 20 nm, in Xmax to higher wavelength was observed when the quaternary ammonium salts were added in greater than stoichiometric amounts relative to the dye. This shift enables significant improvement in detection sensitivity when the film is measured at a wavelength near the Xmax Desirably, the measured wavelength is within about 1 - 80 nm of >.max. Without being limited to any specific reason, the effect is believed to be the result of the formation of the BP Red-Mod^ chelate of higher order (e.g., higher ligand: metal ' falto) on the interface of cat ionic micelle. In another embodiment of the invention, multiple transparent hydrogel films are prepared that contain a chemical composition that changes color after being exposed to the sample solution to obtain a quantitative measurement of the sample solution. In one example of this embodiment, multiple films are prepared to be used to determine the alkalinity of the sample solution, and desirably contain a chemical composition that changes color after being exposed to alkaline species in the sample solution. Desirably, between about 2 and 12 transparent films, and more preferably between 2 and 8 transparent films are prepared and are exposed to the sample solution. In one embodiment, the chemical composition added in the hydrogel films comprises a pH indicator, a surfactant, and an acid. Suitable surfactan's include quaternary ammonium salt such as cetyltrimethylammonium bromide, tridodecylmethylammonium chloride, tetrabutylammonium bromide, and many others. Desirably, the surfactant reduces or substantially e'iminates the amount of indicator leaching in the film. Without the surfactant, indicator leaching will introduce undesirable errors in absorbance measurement. Suitable pH indicator dyes include bromoscresol green and bromophenol blue. It is desirable that the pH indicator dye have a pKa value near 4.3. According to the method disclosed in this embodiment, the multiple hydrogel films in this embodiment each contain a different amount of acid. Some suitable acids are carboxylic acids and aryl- and alkylsulfonic acids such as p-toluenesulfonic acid, however any acid that can be dissolved in the hydrogel media may be used. The acid concentration in each film is different and varies from film to film by a predetermined pattern. The acid concentrations desirably vary between about 0.2 and 50 wt% relative to dry pHEMA (hydrogel). Preferably, the film having the lowest concentration has a concentration between about 0.2 and 20 wt%, and more desirably between about 0.8 and 10 wt%. Preferably, the film having the highest concentration has a concentration between about 20 and 50 wt%, and more desirably between about 25 and 35 \vt%. The number of films that are needed to cover a given alkalinity range if a weak acid such as a carboxylic acid is used is fewer than that if a strong acid is used. This is 'because the weak acid exhibils a flatter titration curve than the strong acid. However, weak acids usually form complexes with ions such as calcium ions that commonly exist in water samples. When a given amount of sample solution is deposited on the hydrogel films, alkaline species in the sample neutralize the acid added. Since an acid concentration gradient is created by addition of different amount of acid in the film series, a profile of color change is resulted in when the film series (multiple films) is (are) exposed to the sample solution, corresponding to different degree of neutralization. Average absorbance of all the films in the series is used for quantitative determination of sample alkalinity. In one aspect, such as for a phosphate sensor, the self-contained sensor includes a molybdate salt and a dye as the analyte-specific reagent and a sulfonic acid as the pH-modifier. The molybdate salt may be any of the various soluble salts commercially available and compatible with the other constituents. Examples of suitable molybdate salts that may be used include, but are not limited, to ammonium, sodium, potassium, calcium and lithium molybdates. In another aspect, ammonium heptamolybdate is used as a molybdate salt. The dye is a chromogenic indicator, which shows a change in the optical property of the sensor, after contacting the dye with the molybdate salt and the phosphate. Some examples of suitable dyes that may be employed in the analyte-specific reagents include azo dyes, oxazine dyes, thiazine dyes, triphenylmethane dyes, and any combinations thereof. In one aspect, the analyte-specific reagent includes thiazine or oxazine dyes. Some specific examples of thiazine and oxazine dyes that may be used include, but are not limited to, Azure A, Azure B, Basic Blue, Methylene Blue, and Brilliant Cresyl Blue. Thiazine and oxazine dyes are used because the main absorption band in the spectra of most thiazine and oxazine dyes in the range of 400 nm to 800 nm does not undergo any significant change when the test solution pH is adjusted from 3 to 0 5. This is in contrast to the triphenylmethane dye known in the art for phosphate analysis. The aq%eous solutions of the Tripnenylmethane dyes undergo a color transition in the pH range of 0 to 2, exhibiting an intense color with an absorption maximum ranged from 550 nm to 650nm at neutral pH and much less color or colorless at low pH. Because the absorption band of the triphenylmethane dye solution at neutral pH usually overlaps with that of the dye-HPA complex, pH of the test media for phosphate determination must be controlled below the transition pH of the dye in order to reveal the absorbance change due to formation of the dye-HPA complex. Thus, strong acids are required with triphenylmethane dyes and molybdate salts. Thiazine and oxazine dyes on the other hand do not require very strong acidic conditions 10 suppress dye color. In fact, low concentrations of low-acidity pH-modifiers are able !o bring about the color change in this case. As noted, a sulfonic acid may be used as a pH-modifier in the self-contained sensor described herein. Suitable sulfonic acids are selected such that the pH of the sensor formulation is in the range from about 0.5 to 3. In one aspect para-toluenesulfonic acid is used as a pH-modifier. The concentration of the sulfonic acid is selected such that the color transition of the dye occurs, or a change in absorbance occurs, on contacting with the molybdate salt and the phosphate. For example, when the thiazine and oxazine dyes are mixed '.vith mo.'ybdate :n an aqueous solution in which the hydrogen ion to molybdate concentration ratio •:, less than 30, a significant red shift of the main absorption band of the dyt> is observed. Upon addition of phosphate to the solution, the solution turns blue Cm the other hand, when the thiazine or oxazine dye is mixed with molybdate in an aqueous solution in which the hydrogen ion to molybdate concentration ratio is kept in the range between 30 and 120, the main absorption band of the dye remains the same and no red shift is observed. In this case, the main absorption band decreases upon addition of phosphate to the test solution. The decrease in absorbance is proportional to the phosphate concentration. In one aspect, the ratio of the hydrogen ion concentration to molybdate concentration is in the range from a'nnui 0.! to about 150, while in another aspect, the ratio of the hydrogen ion concentration to molybdate concentration is in the range from about 1 to about 120, and in a further aspect, the ratio of the hydrogen ion concentration to 'rrtWybdate concentration is~ifnh"e range from about 30 to about 120. In a further aspect, the self-contained sensor described herein, includes at least one additive from the group of polyethylene glycols, polypropylene glycols, polyoxyethylene alkyl ethers, polyvinyl alcohols, or any combinations thereof. The above additives facilitate the solubilization of the analyte-specific reagents and the dyes and also deter the formation of phosphomolybdate-dye aggregates. Thus, by addition of the above additives, precipitation of the phosphomo!ybdale-dye species resulting in signal loss may be prevented. Additionally, when the self-contained sensors are immobilized in a polymer matrix, the above compounds may function as plasticizers and may aid in enhancing the permeability of the polymer matrix to the analyte species (phosphate in this case). In one aspect, polyethylene glycol is used as an additive to the self-contained sensor. In one aspect, molecular weight of the polyethylene glycol additive is in the range from about 100 g/mol to about 10,000 g/mol, while in another aspect, molecular weight of the polyethylene glycol is in the range from about 20C g/raol to about 4000 g/mol, and in a further aspect, molecular weight of the polyethylene glycol is in the range from about 400 g/mol to about 600 g/mol. In one aspect, the weight fraction of the polyethylene glycol additive to the sensor formulation ir in the range from about O.I wt% to about 20 wt%, while in another aspect, the weight fractioti of the polyethylene glycol additive to the sensor formulation is in the range from about 0.5 wt% to about 10 wt%, and in a further aspect, the weight fraction of the polyethylene glycol additive to the sensor formulation is in the range from about 1 wt% to about 5 wt%. In a further aspect, the self-contained sensor described herein includes a signal enhancer. The signal enhancer may be formed of the same material as the pH-modifier or may be formed of a different material. Signal enhancers may be used to mask free isopolymolybdates that are to be distinguished from phosphomolybdate species. If not masked, the free isopulyrnolydbates may ion pair -with the dyes resulting in a higher background signal or reduced signal uue to phosphate alone. Examples of a suitable signal enhancer include, but are not limited to, oxalic acids, stltfonic acids, oxalates, sulfohafes, and any combinations thereof. In one aspect, the analyte-specific reagent includes a metal complex and a dye. The metal complex is selected such that it has high specificity to the analyte (phosphate in this case). Examples of suitable metal complexes that can be used include zinc complexes and cobalt complexes. The above metal complex further includes at least one ligand capable of coordinating with the metal cation. The metal ligand complex is chosen such that it provides some geometrical preferences resulting in selective binding of anions of a particular shape. Examples of suitable ligands include pyridines, amines and any other nitrogen containing ligands. In one embodiment, a dinuclear zinc complex of (2,6-Bis(bis(2-pyridylmethyl)aminomethyl)-4-methyl-phenol) ligand was employed as the analyte-specific reagent. Metalochromic dyes are used along with the metal complexes. Some examples of metalochromic dyes that can be used with the metal complexes include catechol dyes, triphenylmethane dyes, thiazine dyes, oxazine dyes, anthracene dyes, azo dyes, phthalocyanine dyes, and any combinations thereof. Some rpec:fic examples of metalochromic dyes include, but are not limited to, pyrocatechol violet, Murexide, Arsenazo I, Arsenazo III, Antipyrylazo III, Azol, Acid Chrome Dark Blue K, BATA (bis-aminopehnoxy tetracetic acid), Chromotropic acid, and XB-1 (3-[3-(2,4-dimethylphenylcarbamoyl)-2-hydroxynaphthalen]-I-yl-azo]-4-hydroxybenzene sulfonic acid, sodium salt. The pH-modifier for the analyte-specific reagent comprising a metal complex and a metalochromic dye is selected such that the pH of the sensor formulation is maintained at pH=7. Examples of suitable pH-modifiers include biological buffers such as Good's buffers or amines. An example of biological buffer which may be used includes, but is not limited to, HEPES (2-[4-(2-hydroxyethyI)-l-piperazinyl]ethanesulfonic acid). Examples of suitable amines include, but are not limited to, cycloamines or more specifically cyclohexylamines. The concentration of the pH-modifier is selected such that the color transition of the dye occurs, or a change in absorbance occurs, on contact with the metal complex and the dye. In one aspect, the self-contained sensor includes a metal complex, a dye and a , +, ifff sulfonic acid pH-modifier, which are dissolved in a non-aqueous solvent. In another aspect, the self-contained sensor includes a metal complex, a dye and an amine pH-modifier, which are immobilized in a polymer matrix to form a solid-state device The polymer matrix of the sensor film is permeable to selected analytes. The sensor film may be selectively permeable to analytes on the basis of size. i.e.. molecular weight; hydrophobic/hydrophilic properties; phase, i.e., whether the analyte is a liquid, gas or solid; solubility; ion charge; or, the ability to inhibit diffusion of colloidal or particulate material. In one aspect, additives such as polyethylene glycols, polypropylene glycols, polyoxyethylene alkyl ethers, polyvinyl alcohols, or any combinations thereof may be added to the self-contained sensors. These additives may aid in enhancing the permeability of the polymer matrix to the analyte species (phosphate in this case) by plasticizing the polymer matrix. The sensor film described herein may be self-standing or further disposed on a substrate such as glass, plastic, paper or metal. The sensor film may be applied or disposed on the substrate using any techniques known to those skilled in the art, for example, painting, spraying, spin-coating, dipping, screen-printing and the like. In one aspect, the polymer matrix is dissolved in a common solvent for the analyte-specific reagent atnd the pH-modifier and then dip-coated onto a clear plastic surface to form a thin layer which is then allowed to dry over a period of several hours in the dark. Alternatively, the analyte-specific reagent may be applied directly to a preformed polymer film. The concentration of the solution used to coat the surface of the substrate is kept low, for example, in the range from about 20 wt% solids to about 30 wt% solids, so as to not adversely affect the thickness of the film and its optical properties. In one aspect, the thickness of the film is the range from about 1 micron to about 60 microns, in another aspect, the thickness of the film is in the range from about 2 microns to about 40 microns, in another embodiment, the thickness of the film is in the range from about 5 microns to about 20 microns. In one aspect, the analyte-specific reagent is attached to or incorporated into a sensor film, which is then disposed on an optical media disc such as a CD or a DVD. In another aspect, the analyte-specific reagent on the sensor film forms sensor spots when applied to the optical storage media substrate. As used herein, "sensor spots" and "sensor regions" are used interchangeably to describe sensor materials placed on the surface, or in an indentation placed in the surface but not penetrating the region containing the digital information, of an optical storage media at predetermined spatial locations for sensing using an optical storage media drive. Depending on the application, the sensor spots are responsive to physical, chemical, biochemical, and other changes in the environment. In some aspects, the sensor film applied to the optical storage media may be subjected to treatment to form these sensor spots. Methods for such application are known to those skilled in the art anJ may include physical masking systems and both negative and positive photoresist applications. Alternatively, once the optical storage media has been coated with a polymer film, the analyte specific reagent and pH-modifier may be applied as sensor spots to the optical storage media article. The sensor is then used to qualitatively and quantitatively analyze the presence of the chemical species in an aqueous test sample. In one aspect, a method of determining the concentration in » test sample includes contacting a test sample with the self-contained sensor described herein, measuring a change in an optical property of the self-contained sensor produced by contacting the test sample with the self-contained phosphate-sensor, and converting the change in optical property to the concentration. Contacting of the sensor with the test sample may be carried out by any suitable mechanism or technique depending upon whether the sensor is in solution or in solid-state. Some examples by which contacting may occur include, but are not limited to, mixing a solution of the sensor with a test sample solution, by dipping a strip of the sensor in a test-sample solution, by spotting a sensor film with a test sample solution, by flowing a test sample through a testing device having a sensor, and the like. After contacting, a change in the optical property of the sensor is optically measured. The change in the optical p£operty may be simply qualitative such as a change in color of the sensor. Alternatively, the change may be quantitative, for example, change in elastic or inelastic scattering, absorption, luminescence intensity, luminescence lifetime or polarization state By way of example, when a sensor having ammonium molybdate, a thiazine dye such as Azure C, and para-toluenesulfonic acid is contacted with a sample, the color of the sensor changes from violet to blue and a change in the absorption peak at 650 nm occurs. By measuring the change (increase or decrease) in the absorption peak, the concentration can be determined. In one embodiment, measurements of optical response can be performed using an optical system that included a white light source (such as a Tungsten lamp available from Ocean Optics, Inc. of Dunedin, FL) and a portable spectrometer (such as Model ST2000 available from Ocean Optics, Inc. of Dunedin, FL). The spectrometer is equipped with a 600-grooves/mm grating blazed at 400 nm and a linear CCO-array detector. Desirably, the spectrometer covers the spectral range from 250 to 800 nm and to 1100 nm with efficiency greater than 30%. Light from the lamp is focused into one of the arms of a "six-around-one" bifurcated fiber-optic reflection probe (such as Model R400-7-UV/VIS available from Ocean Optics, Inc. of Dunedin, FL). The common arm of the probe illuminates the sensor material. The second arm of the probe is coupled to the spectrometer. For fluorescence measurements light from a source is prefiltered to select an excitation wavelength of interest. Fluorescence emission is collected with the same setup but including an emission long-pass filter. Other known methods of measuring the response may also be used. After measuring the change in the optical property, the concentration in the sample can be determined by converting the change in the optical property to the phosphate concentration. This converting may be carried out using a calibration curve. The calibration curve may be generated by measuring changes in an optical property of a sensor after contacting with test samples of known concentrations. After the calibration curve is generated, the concentration in an unknown test sample may be determined by using the calibration curve. In one aspect, the change in absorbance of the sensor after contacting with a test sample is directly proportional to the co dry over a period of several hours in the dark. The combined final film thickness was between 10 and 40 microns. FIG. 33 shows a typical set of spectra at different sulfite concentrations tor the described device. Sulfite concentrations were prepared in a highly alkaline (1000 mg/L M- alkalinity) and high pH (ph 12) water matrix. FIG. 34 shows a typical response curve for the described device. EXAMPLE 8. ALKALINITY SENSOR The acid used in this example is p-toluenesulfonic acid, and the indicator used is bromoscresol green (pKa = 4.9 in aqueous phase). Polymer solution compositions are listed in Table 2 Films were prepared by depositing an 8 ul polymer solution into wells on a glass slide. The wells (5.4 mm diameter and 0.32 mm deep) were created with a die-cut, adhesive-backed polymer mask layer. The mask tayer wa< not removed during the test. Average absorbance at 650 nm is used to quantify the sample total alkalinity. A calibration curve is shown in FIG. 35. Table 2. Polymer solution composition (Table Removed) EXAMPLE 9. CHLORINE SENSOR In one embodiment, Nafion polymer was added to the sensor formulation. The addition of Nafion resulted in the improvement of the relative response of the sensor film when sensor signal change is normalized by the remaining absorbance at highest tested concentration. Above and below this concentration, the effect was diminished. One suitable indicator is 2-[2-[3-[(l,3-Dihydro-3,3-dimethyl-l-propyl-2H-indol-2-ylidene)ethylidene]-2-phenoxy-1 -cyclohexen-1 -yl]ethenyl]-3,3-dimethyl-l -propylindolium perchlorate known as 1R 768 perchlorate.. FIG. 36 shows an improvement of sensitivity of response when measured at 566 nm upon addition of increasing amounts of Nafion solution to 2000 uL of dye formulation. This improvement can be plotted as the sensor signal upon exposure to 2 ppm of chlorine as shown in FIG. 37. This figure demonstrates an existence of a critical non-intuitive region of concentration of Nafion in PHEMA where a maximum sensor response is obtained. While the disclosure has been illustrated and described in typical embodiments, it is not intended to be limited to the details shown, since various modifications and substitutions can be made without departing in any way frorr the spirit of the present disclosure. As such, further modifications and equivalents of the disclosure herein disclosed may occur to persons skilled in the art using no more than routine experimentation, and all such modifications and equivalents are believed to be within the scope of the disclosure as defined by the following claims. What is claimed is: CLAIMS 1. A method of quantitatively measuring the concentration of a chemical species in a sample solution with a sensor film, said method comprising. 'contacting said sample with a self-contained sensor film; measuring a change in an optical property of said self-contained sensor film produced by contacting said sample with said self-contained sensor; and converting said change in optical property to said concentration. 2. The method of claim 1, wherein the sensor film is self-contained such that it does not need auxiliary reagents outside the film to measure the concentration of the chemical species and wherein the optical response is in the UV, visible, or near-]R spectral ranges with said change in optical property comprising a change in elastic scattering, inelastic scattering, absorption, luminescence intensity, luminescence lifetime or polarization state and said converting is conducted by using a calibration curve. 3. The method according to claim 1 wherein sensor film is prepared by coating a clear plastic surface with a thin layer of a chemical mixture and allowed to dry, said film having a thickness between 10 and 20 microns. 4. The method of claim 1, further comprising preparing a hydrogel sensor film having a chemical composition comprising an indicator that changes its optical properties upon being exposed to the chemical species in the sample solution and exposing the film to a fixed amount of the sample solution. 5. The method of claim 4, further comprising measuring the absoy bance of the film at a wavelength near the maximum absorbance peak (A.max) of the indicator using optical scanning equipment; and quantifying the concentration of the chemical species in the sample solution using the average absorbance measured from the sensor film. 6. The method according to claim 5 wherein chemical composition added in the hydrogel films comprises an organic salt or surfactant and an acid. 7. The method according to claim 5 wherein the sensor comprises at least one anajyte-specific reagent cqrnrjnsing a molybdate salt and a dye and a pH-modifier comprising at least one sulfonic acid. 8. The method according to claim 5 wherein the indicator is immobilized in the hydrogel film by forming an ion pair between the indicator and a quaternary ammonium, quaternary phosphonium, quaternary imidazolium, quaternary pyridium, quatemany pyrrolidinium, or quaternary sulfonium ion, wherein the concentration of quaternary ammonium, quaternary phosphonium, quaternary imidazolium, quaternary pyridium, quaternary pyrrolidinium, or quaternary sulfonium salt is substantially higher that the stoichiometric amount required to ion pair the indicator. 9. The method according to claim 8 wherein the concentration of quaternary ammonium salt, quaternary phosphonium salt, quaternary imidazolium quaternary pyridium, quatemany pyrrolidinium, or quaternary sulfonium salt is about 5-1000 times greater than stoichiometric amounts relative to the indicator. 10. The method according to claim 5 wherein the indicator is pH modified in the hydrogel film by using an acid selected from the group consisting of sulfonic acid, phosphonic acid, carboxylic acid and phenol, wherein the concentration of the acid is substantially higher that the stoichiometric amount required to ion pair the indicator. 11. The method according to claim 5 wherein the sensor film is selected from the group consisting of a phosphate sensor, a molybdate sensor film, a magnesium sensor film, a hardness sensor film, a calcium sensor film, a sulfite sensor film, an alkalinity sensor film, and a chlorine sensor film. 12. The method according to claim 1 wherein the sensor film is a molybdate sensor film prepared by adding Bromopyrogallol Red, calcium sensor film prepared by adding Chlorophosphonazo III, para-toluenesulfonic acid and benzyldimethyltetiradecylammonium chloride (Zephiramine) to pHEMA. 13. The method according to claim 1 wherein the sensor film is a magnesium sensnr film prepared by adding Xyiidyl Rlue 1, sodium salt and tetrabutylammonium bromide (TBAB) polyethylenimine, ethylene glycol-bis(aminoethylether)- N,N,N',N'-tetraacetic acid, tetrasodium salt (EGTA-Na4) to pHEMA. 14. The method according to claim 1 wherein the sensor film is a hardness sensor film prepared by adding oxazine, thiazine, azo, triphenylmethane, cyanine, carbocyanine or inoline dye, metal salt and polyalkylenimine to pHEMA. 15. The method according to claim 1 wherein the sensor film is a molybdate, calcium or sulfite sensor film prepared by an oxazine, thiazine, azo, triphenylmethane, cyanine, carbocyanine or inoline dye, a sulfonic acid, phosphonic acid, carboxylic acid or and phenol and a quaternary ammonium, quaternary phosphonium, quaternary pyridinium, quaternary pyrrolidinium, quaternary imidazolium, or sulfonium salt to pHEMA (MW 300,000). 16. The method according to claim 1 wherein the sensor film is a sulfite sensor film prepared by adding Brilliant Green and tetrabutylammonium bromide (TBAB) to pHEMA. 17. The method according to claim 1 wherein the sensor film is a chlorine sensor film prepared by adding Nation to an oxazine, thiazine, azo, triphenylmethane, cyanine, carbocyanine or inoline dye to pHEMA. 18. The method according to claim 1 wherein the sensor film is a phosphate sensor film prepared with at least one analyte-specific reagent comprising a metal complex and a dye, a pH-modifier comprising at least one sulfonic acid, and at least one non-aqueous solvent. 19. The method according to claim 1 wherein the sensor film is a two-layer sensor film prepared by overcoating a film containing a oxazine, thiazine, azo, triphenylmethane, cyanine, carbocyanine or inoline dye, a quaternary ammonium, quaternary phosphonium, quaternary pyridinium, quaternary pyrrolidinium, quaternary imidazolium, or sulfonium salt, and a metal phosphate salt in pHEMA with a second film containing quaternary ammonium, quaternary phosphonium, quaternary pyndinium, quaternary pyrrolidinium, quaternary imidazolium, or sulfonium salt, and metal phosphate salt in pHEMA. 20. The method according to claim 1 further comprising adding a polymer additive during the formation of the hydrogel sensor film, said additive being Nafion for creation of a sensor film for measurements of chlorine. 21. The method according to claim 1 wherein the sensor film is a chlorine sensor film and a polymer super acid additive such as Nafion and the indicator is 2-[2-[3- [(l,3-Dihydro-3,3-dimethyl-l-propyl-2H-indol-2-ylidene)ethylidene]-2-phenoxy-l- cyclohexen-l-yl]ethenyl]-3,3-dimethyl-l-propylindolium perchlorate. 22. A method of quantitatively measuring the concentration of a chemical species in a sample solution with a plurality of sensor film, said method comprising: preparing a plurality of hydrogel sensor films that change their optical property in the ultra-violet, visible or near-infrared spectral range upon being exposed to the chemical species in the sample solution, in the sample solution, wherein the chemical composition added in the hydrogel films comprises a pH indicator, a surfactant, and an acid; varying the acid concentration in each of the plurality of films by a predetermined pattern; exposing the films to a fixed amount of the sample solution; measuring the absorbance of the films at a wavelength near the maximum absorbance peak of the indicator using optical scanning equipment; and quantifying the concentration of the chemical species in the sample solution using the average absorbance measured from the sensor films. 23. The method according to claim 22 wherein the sensor films are alkalinity sensor films and wherein the concentration of acid in the film having the lowest concentration of acid is about 0.8 wt% relative to dry pHEMA (hydrogel), and the concentration of acid in the film having the highest concentration of acid is about 35 wt% relative to dry pHEMA. 24. A self-contained sensor comprising: *af1east one analyte-specific reagent comprising a molybdate salt and a dye; and a pH-modifier comprising at least one sulfonic acid. 25. The self-contained sensor of claim 24, wherein said dye comprises at least one from the group consisting of azo dyes, oxazine dyes, thiazine dyes, triphenylmethane dyes, and any combinations thereof. 26. The self-contained sensor of claim 24 further comprising at least one additive from the group consisting of polyethylene glycols, polypropylene glycols, polyoxyethylene alkyl ethers, polyvinyl alcohols, and any combinations thereof. 27. The self-contained sensor of claim 24 further comprising a signal enhancer comprising at least one from the group consisting of oxalic acids, sulfonic acids, oxalates, sulfonates, and any combinations thereof. 28. The self-contained sensor of claim 27, wherein said signal enhancer and said pH-modifer are formed of the same material. 29. The self-contained sensor of claim 24, further comprising a polymer matrix. 30. The self-contained sensor of claim 29, wherein said polymer matrix comprises at least one hydrogel f.om the group consisting of poly(hydroxyethylmethacrylates), poly(methylmethacrylates), poly(acrylic acids), poly(methacrylic acids), poly(glyceryl methacrylate), poly(vinyl alcohols), poly(ethylene oxides), poly(acrylamides), poly(N-acrylamidcs), poly(N,N-dimethylaminopropyl-N'-acrylamide), poly(ethylene imines), sodium/potassium poly(acrylates), polysaccharides, poly(vinyl pyrrolidone). and copolymers thereof. 31. A self-contained phosphate sensor comprising: at least one analyte-specific reagent comprising a metal complex and a dye; a pH-modifier comprising at least one sulfonic acid; and at least one non-aqueous solvent. 32. The self-contained phosphate sensor of claim 31, wherein said metal complex comprises at least one from the group consisting of zinc metal complexes, copper metal complexes, and any combinations thereof. 33. The self-contained phosphate sensor of claim 31, wherein said dye comprises at least one from the group consisting of catechol dyes, triphenylmethane dyes, thiazine dyes, oxazine dyes, anthracene dyes, azo dyes, phthalocyanine dyes, and any combinations thereof. 34. The self-contained phosphate sensor of claim 31, further comprising a polymer matrix. 35. The self-contained phosphate sensor of claim 34, wherein said polymer matrix comprises at least one hydrogel from the group consisting of poly(hydroxyethylmethacrylates), poly(methylmethacrylates), poly(acrylic acids), poly(methacrylic acids), poly(glyceryl methacrylate), poly(vinyl alcohols), poly(ethylene oxides), poly(acrylamides), poly(N-acrylamides), poly(N.N- dimethylaminopropyl-N'-acrylamide), poly(ethylene imines), sodium/potassium poly(acrylates). polysaccharides, poly(vinyl pyrrolidone), and copolymers thereof. 36. A sensor used in determining the concentration of chemical species in a sample at trace concentrations, the sensor comprising a hydrogel sensor film comprising a quaternary ammonium, quaternary phosphonium, quaternary imidazolium, quaternary pyridium, quatemany pyrrolidinium, or quaternary sulfonium salt, and an indicator, dye, pigment or reagent and/or its combinations, wherein the indicator, dye, pigment or reagent and/or its combinations changes its optical property in the ultra-violet, visible or near-infrared spectral range upon being exposed to the chemical species in the sample solution, and wherein the indicator is immobilized in the hydrogel film by forming an ion pair with the quaternary ammonium, quaternary phosphonium, quaternary imidazolium, quaternary pyridium, quaternany pyrrolidinium, or quaternary suifouium salt, wherein the concentration of quaternary ammonium salt is substantially higher that the sioichiometric amount required to ion pair formation.