SIMULTANEOUS DETERMINATION OF MULTIPLE ANALYTES IN
INDUSTRIAL WATER SYSTEM
BACKGROUND OF THE INVENTION
Field of the Invention
[0001] This invention relates generally to a method and reagent composition for
simultaneous determination of at least two analytes in water samples with a single stable reagent
composition containing at least two colorimetric reagents and auxiliary reagents such as buffer
and masking reagent. The reagent composition can be formulated to have multiple functions
including (1) clearing the deposition in the fluidic system, (2) pH buffering, and (3) acting as an
auxiliary reagent to assist other analyte analysis that uses other reagents delivered within the
same fluidic system.
Description of Related Art
[0002] Water is used in a number of industrial water systems such as cooling and boiler
water systems. Municipal or untreated water contains impurities that can affect heat transfer,
fluid flow, or cause corrosion of system equipment. For example, metal cations such as calcium,
magnesium, barium and sodium are often present in untreated water. When the water contains
an excess of these impurities, precipitates can form on equipment surfaces in the form of scales
or deposits. The presence of these scales or deposits adversely affects the rate of heat transfer,
and therefore the efficiency of the system. Furthermore, the cleaning or removal of such scales
or deposits is expensive and burdensome because it typically requires a shutdown of the system.
Accordingly, before the water is utilized for cooling or steam purposes, it is desirably treated
with appropriate chemicals in order to inhibit scale formation.
[0003] It is known, for example, to add anionic water-soluble polymers to the water.
One particularly useful water-soluble polymer is HPS-I, although other water-soluble polymers
such as AEC and APES are in use as well. However, the employment of water-soluble polymers
in industrial water systems presents its own set of problems because the concentration of the
polymers in the water must be carefully monitored. For example, if too little of the polymer is
employed, scaling and deposition will occur. In contrast, if too high a concentration of the
polymer is employed, the cost/performance efficiency of the system is adversely affected.
Additionally, chlorine is used to prevent biofilm formation in the tower. As with other methods
of chemically treating aqueous systems, there is an optimal concentration of treatment chemicals
that should be maintained. Monitoring and control of the industrial water chemistry is essential
for maintaining proper performance and prolonging the life of the cooling tower and associated
equipment.
[0004] Thus, it is understood that it is necessary to measure concentrations of multiple
chemical and biological species in the industrial water, such as free chlorine, total chlorine, HPSI,
and phosphate, Additionally, the pH of the water is controlled. Typically, operators rely on
colorimetric analyzers based on known wet chemical analysis methods to monitor and control
the water chemistry. For example, there are several colorimetric methods for determination of
polyelectrolytes using dyes. One example is U.S. Patent No. 6,214,627 issued to Ciota et al. In
addition, there is a Hach polyacrylic acid method that uses iron thiocyanate chelation to detect
calibration based on polyacrylic acid. Generally, these methods require a complicated, multistep
operation procedure and are difficult to carry out in the field. Other methods, such as the
one disclosed in U.S. Patent No. 5,958,778 issued to Johnson et al., use luminol-tagged polymers
in combination with fluorescent or chemiluminescent detection techniques to monitor the
industrial waters.
[0005] U.S. Patent No. 5,972,713 discloses a method for determining total chlorine in a
water sample using N-sulfoalkyl 3,3',5,5'-tetramethylbenzidine (TMB-NSA). The total chlorine
concentration is visually determined by changes in the color and hue of the test solution. U.S.
Patent Application Serial No. 11/523,021 discloses a method for determining residual chlorine in
a water sample by a kit using TMB-NSA in a composition containing 3.5% sulfuric acid and
alcohol to prevent the TMB-NSA from being crystallized and separating from the solution. In
other colorimetric analyzers, a reagent is used for the determination of a single analyte, such as
in the vanadomolybdate-based method for phosphate detection. It is seen that in most
commercially available analyzers, at least one separate reagent is usually required for the
determination of each analyte being measured. Previously, analyzers that use only a single
reagent to determine multiple-analyte concentrations in the industrial environment have not been
available.
[0006] One advantage of using a single reagent to determine multiple analyte
concentrations is that the cost associated with analyzing two species is essentially the same as for
a single species. Reducing the number of reagents typically required by the conventional online
colorimetric analyzer also would improve instrument reliability because the number of reagent
pumps and other fluidic components could be reduced. Moreover, the cost associated with
reagent production, storage, transportation, and service may be reduced. Since the auxiliary
reagents such as buffer are shared, liquid waste generated by an analyte using a combined
reagent formulation of multiple analytes is also reduced.
[0007] Thus, there exists a strong need for simplified sensors and test methods that can
easily be used to determine the concentration of multiple analytes, such as anionic polymer,
phosphate, free chlorine, and total chlorine, and measure pH, using a single reagent in aqueous
systems with high reproducibility, decreased response to interferences, and enhanced stability.
SUMMARY OF THE INVENTION
[0008] One aspect of the invention is directed to a multi-purpose reagent composition for
simultaneously determining concentrations of at least two analytes in a water sample. The two
analytes are selected from the group consisting of free chlorine, pH, anionic polymer, and
phosphates. In one embodiment, the multi-purpose reagent composition is for simultaneously
determining the concentration of free chlorine and pH and comprises a free chlorine sensitive
dye and pH indicator. In another embodiment, the reagent composition is for simultaneously
determining of free chlorine and anionic polymer and includes a free chlorine sensitive dye, a pH
buffer, cationic dye, and organic co-solvent. Thus, the pairs of free chlorine and pH or free
chlorine and anionic polymer are determined using a single reagent composition.
[0009] Another aspect of the invention is directed toward reagent sharing in which a
reagent composition has at least two different functions. The reagent composition is the main
reagent for one analyte analysis when it is used alone. The reagent composition also functions as
an ancillary reagent when it is combined with a second reagent composition for the
determination of another analyte. The method and reagent formulations are not only ideally
suitable for online analyzer applications but also for offline, discrete, manual analysis of multiple
analytes in water samples.
[0010] Another aspect of the invention is directed toward a method of simultaneously
determining the concentrations of at least two analytes in a water sample using a multi-purpose
reagent composition. The method includes adding a multi-purpose reagent composition to an
aqueous sample and determining the concentration of at least two analytes in the aqueous sample,
wherein the two analytes are selected from the group consisting of free chlorine, pH, anionic
polymer, and phosphates. In one embodiment, the method simultaneously determines the
concentration of free chlorine and pH using a single reagent composition that includes a free
chlorine sensitive dye and pH indicator. In another embodiment, the method simultaneously
determines free chlorine and anionic polymer using a single reagent that includes a free chlorine
sensitive dye, a pH buffer, cationic dye, and organic co-solvent.
[0011] 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
[0012] 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:
[0013] FIG. 1 is a graph showing free and total chlorine concentrations measured
according to an embodiment of the invention;
[0014] FIG. 2 is a graph showing free chlorine decay in the presence of ammonium
chloride;
[0015] FIG. 3 is a graph showing spectral response of the multi-purpose reagent to
anionic polymer;
[0016] FIG. 4 is a graph showing spectral response of the multi-purpose reagent to free
chlorine;
[0017] FIG. 5 is a graph showing spectral response of the multi-purpose reagent to
chlorine/HPSI mixtures;
[0018] FIG. 6 is a graph showing the multivariate calibration equation representing the
HPS-I concentration;
[0019] FIG. 7 is a graph showing absorbance at 467 nm as a function of chlorine
concentration;
[0020] FIG. 8 is a graph showing spectral response of the multi-purpose reagent to free
chlorine at pH around 7.6;
[0021] FIG. 9 is a graph showing spectral response of the multi-purpose reagent to free
chlorine at pH around 8.0;
[0022] FIG. 10 is a graph showing spectral response of the multi-purpose reagent to free
chlorine at pH around 9.0;
[0023] FIG. 1 is a graph showing the multivariate calibration equation representing free
chlorine concentration;
[0024] FIG. 12 is a graph showing the multivariate calibration equation representing pH;
[0025] FIG. 13 is a graph showing a calibration curve of a vanadomolybdate method for
phosphate determination;
[0026] FIG. 14 is a graph showing results obtained from mixing a sample containing
phosphate, HPSI, vanadomolybdate and multi-purpose reagent for measuring HPSI; and
[0027] FIG. 15 is a graph showing measurements of lower range phosphate.
[0028] Corresponding reference characters indicate corresponding parts throughout the
views of the drawings.
DETAILED DESCRIPTION OF THE INVENTION
[0029] 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. To the contrary, the invention includes numerous alternatives, modifications and
equivalents as will become apparent from consideration of the following detailed description.
[0030] Approximating language, as used herein throughout the specification and claims,
may be applied to modify any quantitative representation that could permissibly vary without
resulting in a change in the basic function to which it is related. Accordingly, a value modified
by a term or terms, such as "about", is not limited to the precise value specified. In at least some
instances, the approximating language may correspond to the precision of an instrument for
measuring the value. Range limitations may be combined and/or interchanged, and such ranges
are identified and include all the sub-ranges included herein unless context or language indicates
otherwise. Other than in the operating examples or where otherwise indicated, all numbers or
expressions referring to quantities of ingredients, reaction conditions and the like, used in the
specification and the claims, are to be understood as modified in all instances by the term
"about".
[0031] "Optional" or "optionally" means that the subsequently described event or
circumstance may or may not occur, or that the subsequently identified material may or may not
be present, and that the description includes instances where the event or circumstance occurs or
where the material is present, and instances where the event or circumstance does not occur or
the material is not present.
[0032] As used herein, the terms "comprises," "comprising," "includes," "including,"
"has," "having" or any other variation thereof, are intended to cover a non-exclusive inclusion.
For example, a process, method, article or apparatus that comprises a list of elements is not
necessarily limited to only those elements, but may include other elements not expressly listed or
inherent to such process, method article or apparatus. The singular forms "a," "an" and "the"
include plural referents unless the context clearly dictates otherwise.
[0033] Embodiments of the present invention are directed to improved methods for
determining the concentration of multiple analytes in a water sample. According to one aspect of
the invention, multiple analytes in a water sample can be determined using a single multi¬
purpose reagent composition. For example, in one embodiment, the pairs of free chlorine and
pH or free chlorine and anionic polymer are determined using a single reagent composition. In
another embodiment, the reagent composition performs at least two different functions. The
reagent composition is the main reagent for one analyte analysis when it is used alone. The
reagent composition also functions as an ancillary reagent when it is combined with a second
reagent composition for the determination of another analyte. Accordingly, varying the reagents
and sample mixing sequence enables multiple analytes to be measured simultaneously or
sequentially. The disclosed embodiments are particularly well suited for quickly and accurately
determining the concentration of analytes in aqueous systems, including but not limited to boilers,
cooling towers, evaporators, gas scrubbers, kilns and desalination units. The methods and
reagents compositions described herein are ideally suitable for online analyzer applications.
However, they are also suitable for offline, discrete, manual analysis of multiple analytes in
water samples.
Free chlorine + polymer
[0034] In one embodiment, the multi-purpose reagent composition is used to determine
both the concentration of free chlorine and anionic polymer corrosion or scale inhibitors in an
aqueous sample of industrial water. The reagent composition includes a free chlorine sensitive
dye, a pH buffer, a cationic dye, and an organic co-solvent in a single reagent solution.
Desirably, the reagent composition does not contain alcohol or a high concentration sulfuric acid.
This mild condition makes it possible to blend the composition so the reagent can respond to
multiple analytes in the sample.
[0035] The free chlorine sensitive dye desirably includes one or more coloring reagents
selected from the group consisting of tetraalkyl benzidine compounds. The tetraalkyl benzidine
compound is an oxidative chromogenic coloring reagent which has an absorption peak at a
wavelength of about 450 to 470 nm in a reaction with free chlorine in an acidic sample, and
which colors to a hue within a range of yellow to blue green. U.S. Patent Application No.
11/523,021 entitled "Composition for Measuring Residual Chlorine Concentration" discloses
suitable tetraalkyl benzidine compounds, including preferred examples N-(2-sulfoethyl)-3,3',5,5'-
tetramethylbenzidine; N-(3-sulfopropyl)-3,3',5,5'-tetramethylbenzidine; N-(4-sulfobutyl)-
3,3',5,5'-tetramethylbenzidine; N-(2-hydroxy-2-sulfoethyl)-3,3',5,5'-tetramethylbenzidine; N-(2-
hydroxy-3-sulfopropyl)-3,3',5,5'-tetramethylbenzidine; N-(2,4-disulfobenzyl)-3,3',5,5'-
tetramethylbenzidine; N,N'-bis(2-sulfoethyl)-3,3', 5,5'-tetramethylbenzidine; N,N'-bis(3-
sulfopropyl)-3,3',5,5'-tetramethylbenzidine; N,N'-bis(4-sulfobutyl)-3,3',5,5'-tetramethylbenzidine;
N,N'-bis(2-hydroxy-2-sulfoethyl)-3,3',5,5'-tetramethylbenzidine; N,N'-bis(2-hydroxy-3-
sulfopropyl)-3,3',5,5'-tetramethylbenzidine; N,N'-bis(2,4-disulfobenzyl)-3,3',5,5'-
tetramethylbenzidine; and alkali metal salts thereof. Of the compounds, a compound in the form
of a sodium salt is particularly preferably used because the compound has high water solubility
and is hardly crystallized at normal temperatures.
[0036] In one embodiment, the free chlorine sensitive dye is N-(3-sulfopropyl)-3,3',5,5'-
tetramethylbenzidine (TMB-PS). The TMB-PS selectively reacts with free chlorine in the
presence of chloramines to produce a yellow species with max = 467 n .
[0037] The reagent also includes a cationic dye for the determination of an anionic
polymer and/or oligomers in the sample. Polymers capable of being detected by the methods
disclosed herein include, but are not limited to, water-soluble anionic polymers that contain
anionic groups, such as carboxylate, sulfonate, sulfate, phosphonate, phosphate. Examples of the
same are polyacrylic acid moiety polymers, polysufonated polymers, and maleic anhydride
polymers. Some specific examples of contemplated anionic polymers are HPS-1, AEC and
APES (available from GE Water & Process Technologies of Trevose, Pa.). The cationic dye is
chosen from a group of metachromatic dyes, which are dyes that undergo a color change upon
interaction with polyionic compounds. The group of dyes from which the cationic dyes are
chosen includes, but is not limited to, Dimethyl Methylene Blue (DMMB), Basic Blue 17, New
Methylene Blue (NMB), and combinations thereof. One embodiment of the invention calls for
the use of 1,9-dimethyl methylene blue (DMMB) as the cationic dye. The cationic dye is added
in an effective amount, which amount is generally from about 0.5 to about 3.0 times the molar
concentration of the polymer in the assay. In one embodiment, DMMB is added in an amount to
obtain 100 ppm of DMMB.
[0038] As is known in the art, one factor that needs to be evaluated for each particular
polymer is its degree of interaction between that polymer and the dye. This factor can be
determined by mapping the absorbance change of a dye as a function of a particular polymer. In
order to determine the change in absorbance of a dye composition, an initial absorbance of the
dye composition is determined at a set time after mixing the composition at any wavelength in
the visible spectrum of 300 to 700 nm. It is possible to quantify the degree of interaction
between a particular dye and any polymer, such as, but not limited to, HPS-I.
d
[0039] The reagent composition also includes a pH buffer that does not have any freechlorine
demand. Desirably, weak acids are used as pH buffers to obtain a pH in the range of
2.85 to 4.5. Suitable weak acids are acetic acid and citric acid; however, other acids such as
phthalic acid, oxalic acid, succinic acid may also be used. For example, DMMB precipitates
quickly from the assay solution upon addition of a polymer. However, it has been found that
DMMB is stable in acetate and citric buffer in the pH range of 2.85 to 4.5, even if a stabilizing
agent is not added into the DMMB-buffer mixture. Therefore, addition of a stabilizing agent is
not necessary for online application where the reagent and sample are brought to mix by pumps
and the distance and time between the mixing point and the measurement point are controlled
and reproducible.
[0040] The reagent may also contain a masking agent for the masking of surfactants or
other chemical species that may interfere with the results that may be present with the polymers
and/or oligomers. It is known to add stabilizing agents to the buffer to assists with solving the
issue of precipitation and assay instability. Anionic surfactants or co-existing polymer can be
effectively masked by including a masking agent in the multifunctional buffer. By including
such a masking agent, the reading of the polymer concentration is more accurate. An example of
an anionic surfactant that can be masked by adding a cationic surfactant to the aqueous system is
dodecylbenzene sulfonate. The masking agent is added in an amount of from 20 ppm to about
3000 ppm. Masking agents include, but are not limited to, bivalent manganese salt, ferrous salt,
calcium salts, zinc salts, quaternary amine surfactant, or combinations thereof. In one
embodiment, the masking agent is 2500 ppm calcium chloride dehydrate.
[0041] The reagent composition also includes an organic co-solvent. The organic cosolvent
is desirable selected from the group of water-soluble ketones, acetone, NMP, ethylene
glycol, and methanol. When TMB-PS and DMMB were mixed in a pH = 3.4 citric buffer, a
cloudy solution was formed and filtering the cloudy mixture through a 0.2 mi filter resulted a
solution essentially free of DMMB and TMB-PS. This is presumably because the sulfonic acid
group from TMB-PS forms an ion pair with the positive quaternary amine cation from DMMB.
It was found that adding 1 ml acetone into 3 ml of the cloudy mixture produces a homogenous
solution. Filtering through the 0.2 mhi filter confirmed that the new composition containing 25%
acetone is essentially free of particulates.
[0042] Absorbance, as used herein, is defined according to the Lambert-Beer Law in
Equation 1 as follows:
A=abc (Eq. 1)
where:
A=absorbance;
a=absorbtivity of the dye;
b=light path length; and
c=concentration of the colored substance.
[0043] Each cationic dye used will have a maximum absorbance within the 300 to 1000
nm range, and it is desirable to measure absorbance at a wavelength within the range of
maximum absorbance. Absorbance may be measured using any suitable device known in the art
to measure absorbance. Such suitable devices include, but are not limited to, calorimeters,
spectrophotometers, color-wheels, and other types of known color-comparative measuring tools.
One embodiment provides for measurements of optical response performed using an optical
system that includes a white light source (such as a Tungsten lamp available from Ocean Optics,
Inc. of Dunedin, Fla.) and a portable spectrometer (such as Model ST2000 available from Ocean
Optics, Inc. of Dunedin, Fla.). Desirably, the spectrometer used covers the spectral range from
about 250 nm to about 1100 nm.
[0044] The method includes the use of predetermined calibration curves for optimal
efficiency and effectiveness. In order to determine the concentration or amount of available
anionic polymer in an industrial water system, it is first necessary to generate a calibration curve
for each polymer of interest. Calibration curves are generated by preparing various samples of
water containing known amounts of polymer, making an appropriate reagent composition and
measuring the absorbance of the sample using the reagent composition. In one embodiment of
this invention, absorbance is being reported as absorbance difference. Absorbance difference is
the difference between the absorbance of the reagent composition by itself and the absorbance of
the mixture of reagent composition and the sample of water being tested. The calibration curve
is then a plot of this absorbance difference versus the known concentration of the polymer in the
sample. Once created, the calibration curve can be used to tell how much polymer is present in a
sample by comparing the measured absorbance difference of the sample with the curve and
reading the amount of polymer present off the curve.
Example 1
[0045] Free chlorine concentration in tap water is determined using a TMB-PS solution
as shown in Fig. 1. The TMB-PS solution used contained 175 ppm TMB-PS and 0.1M, pH = 3.4
citric acid/sodium citrate buffer. Total chlorine concentration determined using a TMB solution
is also included in Fig. 1. During the test, the tap water contained 0.05 to 0.15 ppm chlorine and
1.5 to 1.9 ppm total chlorine, both confirmed by independent free chlorine analysis methods.
The total chlorine is presumably monochlorame. This example demonstrates that TMB-PS
responds selectively to free chlorine even at a large excess of monochlorame.
Example 2
[0046] A 1.57 ppm chlorine solution was prepared by adding 5 m 63.4 ppm chlorine
standard solution purchased from Hach into a 200 ml volumetric flask. 0.5 ml 100 ppm
ammonium chloride solution was added to the flask and deionized water was added to the 200 ml
mark. Free chlorine concentration in this solution was analyzed immediately. The free chlorine
decay caused by ammonium ion was monitored and is shown in Fig. 2 along with the free
chlorine concentration measured by the Hach DPD method. This example shows that TMB-PS
in pH 3.4 citric buffer responds selectively to free chlorine, and chloramine does not interfere
with the free chlorine analysis.
Example 3
[0047] A 75 g solution containing 175 ppm TMB-PS and 160 ppm DMMB was prepared
in 0.1 M, pH = 3.4 citric buffer. The solution was cloudy. The final all-in-one reagent was
prepared by adding 25 g acetone to the cloudy solution.
[0048] Figs. 3 and 4 show the response of the all-in-one reagent to free chlorine and
HPSI respectively. It is clear that the response of TMB-PS to free chlorine is not affected by the
presence of DMMB, and the response of DMMB to HPSI is not affected by TMB-PS. Fig. 5
shows the response of the all-in-one reagent to solutions containing 10 ppm HPSI and 0.0 to 1.26
ppm free chlorine. It is apparent that the 468 nm peak of TMB-PS is essential free of
interference from the 520 nm band of the DMMB-HPSI complex. The spectral region between
535 nm and 700 nm of the DMMB-HPSI complex is not affected by TMB-PS's 468 nm peak.
However, the 468 nm peak is overlapped with the 520 nm shoulder of the DMMB-HPSI
complex. We found that a multivariate calibration equation using absorbance values at 468 nm,
525 nm, and 634 nm is adequate to predict HPSI concentration in a free chlorine/HPSI mixture.
[0049] Fig. 6 shows the predicted HPSI concentration from Equation 2 as follows:
HPSI/ppm = 17.2 -1.18 A467 + 19.4 A525 - 12.8 A634 (Eq. 2)
where A52 , and A are absorbance value at 467, 525, and 634 nm respectively.
[0050] Since the TM -PS response at 468 nm is essentially free of interference from
DMMB-HPSI complex, absorbance value at 460 nm alone can be used to calibrate TMB-PS's
response to free chlorine, as shown in Fig. 7.
[0051] Absorbance values measured at 467 nm, 525 nm, 580 nm, and 634 nm are listed
in Table 1.
Table 1
Chlorine/ppm HPSI/ppm 467 nm 525nm 580nm 634nm
0.00 0.00 0.1678 0.4641 2.2928 2.0549
0.00 5.00 0.1689 0.621 5 2.0681 1.8737
0.00 10.00 0.1804 0.7654 1.8391 1.6877
0.00 15.00 0 .1897 0.8895 1.6542 1.5266
0.32 0.00 0.2960 0.4565 2.2382 2.01 95
0.63 0.00 0.491 0 0.4857 2.2748 2.0393
0.94 0.00 0.7068 0.5120 2.2368 2.0279
1.26 0.00 0.8866 0.5084 2.2584 2.0380
0.00 10.00 0.1804 0.7654 1.8391 1.6877
0.32 10.00 0.3177 0.7649 1.81 8 1 1.7020
0.63 10.00 0.51 2 1 0.7801 1.8044 1.6953
1.26 10.00 0.91 14 0.8125 1.81 87 1.7220
1.26 5.00 0.8961 0.6746 2.0474 1.9009
[0052] Fig. 6 shows that the multivariate calibration equation accurately represents the
HPSI concentration regardless of chlorine concentration in the standard solution. Note that five
points are included for 0 and 10 ppm HPSI standard solutions, which contain five different
chlorine concentrations.
Free Chlorine + pH
[0053] Another embodiment of the invention is a reagent composition and method for
simultaneously determining free chlorine and pH in a sample. This reagent composition
comprises a free chlorine sensitive dye and pH indicator.
[0054] The free chlorine sensitive dye is selected from the group of infrared dyes. The
pH indicator is a colorimetric pH indicator. Desirably, the pH indicator is a fluorescence pH
indicator. Several IR dyes at the pH range of 6.5-10.5, such as IR-783, selectively react with free
chlorine. The free chlorine concentration is proportional to the decrease of absorbance at near
infrared wavelength 780 nm. The extremely high molar absorbability (around 2 l05 M cm 1)
of IR dyes enables free chlorine to be detected at the ultra-low range. The response of IR dyes to
free chlorine is dependent on pH. Traditional free chlorine detection methods usually use a pH
buffer to control sample pH.
[0055] Unlike the traditional methods, this embodiment uses a reagent composition
containing a free chlorine tolerant pH indicator and an IR dye in a solvent mixture. In one
embodiment, the solvent mixture contains 25% (w/w) methanol and 75% (w/w) ethylene glycol;
however, any solvent, and/or solvent mixture that dissolves the free chlorine sensitive dye and
the pH indicator could be used. Desirably, the solvent should not react with free chlorine and
affect pH sample. Therefore, amine or an acid solvent should not be used. The solvent should
dissolve the dyes well without substantial precipitations. It is desirable to have a solvent to
dissolve both dyes and also stabilize the dye mixture, thus increasing the shelf-life of the dye
mixture. It has been found that the dye mixture of IR-783 and cresol red is much more stable in
ethylene glycol than in water. The solvent mixture of methanol and ethylene glycol is desirable
because it decreases the viscosity of the dye mixture enabling the mixture to be easily and
accurately pumped. Suitable IR dyes include IR-783, IR-780, IR-775, IR-746, Pinacyanol
chloride (CAS: 2768-90-3). Suitable free chlorine tolerant pH indicators include cresol red and
phenol red, phenolphthalein, thymol blue, and O-cresolphthalein. This composite reagent
formulation of these R dyes and pH indicators allows the mixed reagent to be very stable and
reduces cross interference during pH and free chlorine detection, thereby enabling free chorine
and pH to be measured simultaneously. A multivariate calibration equation is used to determine
the free chlorine concentration. Since the pH response at 575 nm is essentially free of
interference from IR dyes, absorbance value at 575 nm alone can be used to calibrate pH
response as described below.
Example 4
[0056] The composite reagent composition for free chlorine and pH measurement was
made by mixing of 10 mg IR-783 and 10 mg cresol red sodium salt in a solvent mixture
containing 25% (w/w) methanol and 75% (w/w) ethylene glycol. Calibration solutions were
made from synthetic cooling water with different pH and free chlorine concentration. The pH
and free chlorine concentration were analyzed by standard methods. A 2 ml standard solution
was mixed with 50 mΐ reagent composition for 20 seconds; the spectra were recorded by using a
UV-Vis spectrophotometer.
[0057] Figs. 8, 9, and 10 show the response of the reagent to 0.0 to 1.0 ppm free chlorine
at pH 7.6, 8.0, and 9.0. It is clearly that the response of free chlorine is not affected by the
presence of pH indicator cresol red sodium salt and the response of pH is not significant affected
by the presence of IR 783.
[0058] It was found that a multivariate calibration equation using absorbance values at
575 nm and 775 nm is adequate to predict free chlorine concentration. Fig. 11 shows the
predicted free chlorine concentration from the following equation:
F-Cl/ppm = 1.71 + 0.180*A m - 1.00*A 5 m (Eq. 3)
[0059] Absorbance values measured at 575 nm, 775 nm, plus actual pH, F-Cl/ppm and
calculated pH, F-Cl/ppm were listed in Table 2.
Table 2
575nm 775nm F-CI F-CI Cal. pH Cal.
1.38 1.86 0.00 8.98 0.10 8.95
1.39 1.71 0.24 8.97 0.25 8.96
1.37 1.46 0.50 8.99 0.50 8.94
1.39 1.00 1.04 8.98 0.96 8.96
0.92 1.87 0.00 8.06 0.01 8.32
0.69 1.40 0.41 8.11 0.44 8.01
0.68 0.51 1.23 8.07 1.32 7.99
0.68 0.90 0.95 8.07 0.93 8.00
0.82 1.90 0.00 8.06 0.00 8 .19
0.77 1.67 0.21 8.16 0.17 8 .11
0.29 1.75 0.00 7.36 0.01 7.45
0.36 0.76 1.04 7.62 1.01 7.55
0.37 1.36 0.43 7.60 0.42 7.56
0.46 1.76 0.00 7.67 0.03 7.68
[0060] Fig. 11 shows that the multivariate calibration equation accurately represents the
free chlorine concentration in the standard solutions. Since the pH response at 575 nm is
essentially free of interference from IR dyes, absorbance value at 575 nm alone can be used to
calibrate pH response as shown in Fig. 12 according to the following equation:
pH = 7.06 + 1.37*A575 nm (Eq. 3)
HPSI + PQ
[0061] Another embodiment of the invention is a reagent composition and method of
determining phosphate concentration and anionic polymer concentration in a water sample. In
one embodiment, the anionic polymer is measured by mixing the sample with the polymermeasuring
reagent as described above. High range phosphate is measured by mixing the sample
with a phosphate reagent alone, while the lower range phosphate is measured by mixing the
sample with the phosphate reagent and the reagent used to measure the anionic polymer mixed
together as will be described below. This mixing sequence can be easily recognized in any
online flow analysis system.
[0062] Vanadomolybdate has been widely used for the determination of phosphate
concentration in water samples. The advantage of this method is that only a single reagent
composition is needed to cover the phosphate concentration in a large range (0 to 50 ppm). The
disadvantages include high strong acid concentration, low sensitivity, and the need of using near
UV light sources, which make the method sensitive to interference of yellow background color,
which is common for nature water samples. In Example 2 above, it was demonstrated that an
anionic polymer such as HPS-I from GE Water & Process Technologies could be determined
from a single reagent composition containing both the buffer and the dye. With a careful
selection of the reagent pH and concentration, the vanadomolybdate reagent and the cationic dye
in the reagent designed for anionic polymer determination can be used to analyze phosphate at
the range of 1 to 3 ppm. Accordingly, an online wet chemistry procedure can be designed to
measure anionic polymer and phosphate in the range of 0 to 40 ppm with great accuracy for the 0
to 3 ppm range.
Example 5
[0063] Fig. 13 shows a calibration curve of the vanadomolybdate method for phosphate
determination. The vanadomolybdate reagent pH is adjusted using sulfuric acid and sodium
hydroxide to pH 1.4. The reagent composition contains enough vanadomolybdate for 40 ppm
orthophosphate when the reagent to sample ratio is 3/1. The absorbance shown in Fig. 13 was
measured at 365 nm from a 10 mm quartz cell. From the data presented in Fig. 13, the
sensitivity of the method at 365 nm is 0.05 au/ppm P0 4.
[0064] Fig. 14 shows the results obtained from mixing three parts of sample containing
phosphate and HPS-I, one part of the vanadomolybdate that is used for the results shown in Fig.
13, and one part of the reagent used to determine HPS-I. From the results shown in Fig. 14, it
can be seen that the concentration of HPS-I does not interfere with the phosphate determination.
Additionally, it was determined that the sensitivity is at least seven times of that of the traditional
vanadomolybdate method as shown in Fig. 13. Therefore, HPS-I is measure by mixing the
sample with the multi-purpose reagent alone, the high range phosphate is measured by mixing
the sample with the vanadomolybdate alone, and the lower range phosphate is measured by
mixing the sample with the vanadomolybdate and the all-in-one together. This mixing sequence
can be easily recognized in any online flow analysis system.
pH Reagent Composition as a Phosphate Standard
[0065] Another embodiment of the invention is a multi-functional reagent composition
that can be used for determining at least one analyte when it is used alone and has other
functionality to assist the determination of other analytes in an online analyzer system
comprising an analyte-sensitive reagent, solvent, and ancillary reagent. In one embodiment, the
reagent composition functions to act as a reagent for the determination of another analyte along
with a second reagent composition. In another embodiment, the reagent composition functions
to act as a buffer for the determination of another analyte along with a second reagent
composition. In another embodiment, the reagent composition functions to act as a cleaning
agent to clean the residual left from the analysis of other analytes. In another embodiment, the
reagent composition functions to act as a calibration solution that can be to calibrate the reagents
in said online system. The ancillary reagent is desirably selected from the group including an
acid, a surfactant, a buffer, or a complexing agent such as EDTA.
Example 6
[0066] A 2 ml phosphate standard solution was mixed with 1 ml vanadomolybdate
solution in a 10 mm quartz cuvette. Absorbance at 365 nm was measured. A calibration curve
was produced. A second set of absorbance values at 355 nm were measured from five phosphate
standard solutions. The phosphate concentrations of these five solutions were calculated
according to the calibration obtained at 365 nm. The deviations of calculated values from the
theoretical values are listed below.
Table 3
[0067] It is clear that if a photometer has some error in wavelength, the phosphate
concentration measured based on a global calibration curve is subject to error. We demonstrate
in this example that this type of error can be reduced by online calibration from a reagent
composition containing phosphate.
[0068] A cresol red solution containing 5 ppm phosphate was prepared. Because
phosphate concentration in the cresol red is insignificant, it can be used to measure pH.
Absorbance values at 360, 362, 365, and 367 nm were measured after 2 ml phosphate standard
solution and 1 ml vanadomolybdate, and 1 ml cresol red solution are mixed in a 10 mm cuvette.
The measurement was carried out for each phosphate standard solution. Absorbance values at
these wavelengths were also measured in the absence of cresol right solution. Absorbance
values from the above measurements were fitted to a two-variable equation:
P0 = 26.84 -78.06 Acreso + 96.21 A
where:
Acresoi absorbance measured when the sample, vanadomolybdate, and cresol red
solutions are mixed, and
A is absorbance measured when sample and vanadomolybdate were mixed.
[0069] Note that wavelength information is absent from the calibration equation.
Therefore, absorbance values can be measured with the same optical and fluidic setup with and
without cresol red addition. Phosphate concentrations calculated from the above calibration
equation are listed in Table 4. It is clear that absorbance measurement from the phosphate
containing pH reagent can be used to reduce variations caused by variation in photometer
wavelength. To those skilled in the art, this method demonstrated in this example can be used to
correct other types of errors in the photometric measurement.
Table 4
[0070] It can be seen that varying reagents/sample mixing sequence enables multiple
analytes to be measured simultaneously or sequentially. With the understanding of the
discovered underline principles, we have demonstrated that the following combinations of
analytes, which are significant to cooling tower monitoring and control, can be determined by
two reagent compositions delivered by two pumps: Free chlorine, total chlorine, and HPSI; Free
chlorine, total chlorine, and pH; Free chlorine, pH, and phosphate (high range, 0 to 40 ppm);
Free chlorine, anionic polymer, and phosphate (high range, 0 to 40 ppm); anionic polymer,
phosphate (low range, 0 to 4 ppm), and phosphate (high range 0 to 40 ppm). When two reagent
compositions are used in an online system, the first reagent composition that is designed for the
first analyte can be used as an ancillary reagent of the second reagent composition for the
determination of the second analyte. The functions of the ancillary solution include cleaning,
providing buffer for the main reagent, and acting as a standard solution. For example, phosphate
can be added into a reagent composition containing pH indicator for pH determination. Thus,
the pH reagent composition is for pH analysis when it is used alone. It can be used as a
phosphate standard to calibration a phosphate method.
[0071] 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 from 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.

CLAIMS
1. A multi-purpose reagent composition for simultaneously determining
concentrations of at least two analytes in a water sample, wherein the two analytes are selected
from the group consisting of free chlorine, pH, anionic polymer, and phosphates.
2. The multi-purpose reagent composition of claim 1 wherein the reagent
composition is for simultaneously determining the concentration of free chlorine and pH and
comprises a free chlorine sensitive dye and pH indicator.
3. The composition of claim 2 where said co-solvent is ethylene glycol and methanol.
4. The multi-purpose reagent composition of claim 2 wherein said free chlorine
sensitive dye includes one or more coloring reagents selected from the group consisting of
infrared dyes.
5. The multi-purpose reagent composition of claim 1 wherein said pH indicator is a
colorimetric pH indicator.
6. The multi-purpose reagent composition of claim 1 wherein the reagent
composition is for simultaneously determining of free chlorine and anionic polymer and
comprises a free chlorine sensitive dye, cationic dye, organic co-solvent and a pH buffer to
obtain a pH in the range of 2.85 to 4.5.
7. The composition of claim 6 wherein said free chlorine sensitive dye is a tetraalkyl
benzidine compound.
8. The composition of claim 6 wherein said free chlorine sensitive dye is N-(3-
sulfopropyl)-3,3',5,5'-tetramethylbenzidine (TMB-PS).
9. The composition of claim 6 wherein said cationic dye is selected from the group
consisting of Dimethyl Methylene Blue (DMMB), Basic Blue 17, and New Methylene Blue
(NMB).
10. The composition of claim 6 wherein said co-solvent is a water-soluble ketone.
1 . A method of simultaneously determining the concentrations of at least two
analytes in a water sample using a multi-purpose reagent composition, the method comprising:
adding a multi-purpose reagent composition to an aqueous sample;
determining the concentration of at least two analytes in the aqueous sample, wherein the
two analytes are selected from the group consisting of free chlorine, pH, anionic polymer, and
phosphates.
12. The method of claim 1 wherein the reagent composition is for simultaneously
determining the concentration of free chlorine and pH and comprises a free chlorine sensitive
dye and pH indicator.
13. The method of claim 11 wherein the reagent composition is for simultaneously
determining of free chlorine and anionic polymer and a free chlorine sensitive dye, cationic dye,
organic co-solvent and a pH buffer to obtain a pH in the range of 2.85 to 4.5.
14. The method of claim 13 wherein said free chlorine sensitive dye is a tetraalkyl
benzidine compound.
15. The method of claim 14 wherein said free chlorine sensitive dye is N-(3-
sulfopropyl)-3,3 ' ,5,5 ' -tetramethylbenzidine (TMB-PS).
16. The method of claim wherein said cationic dye is selected from the group
consisting of Dimethyl Methylene Blue (DMMB), Basic Blue 17, and New Methylene Blue
(NMB).
17. The method of claim 14 wherein said co-solvent is ethylene glycol and methanol.
18. A multi-functional reagent composition that can be used for determining at least
one analyte when it is used alone and has other functionality to assist the determination of other
analytes in an online analyzer system comprising an analyte-sensitive reagent, solvent, and
ancillary reagent.
19. The composition of claim 18 wherein said functionality is to act as a reagent for
the determination of another analyte along with a second reagent composition.
20. The composition of claim 1 wherein said functionality is to act as a buffer for the
determination of another analyte along with a second reagent composition.
21. The composition of claim 18 wherein said functionality is to act as a cleaning
agent to clean the residual left from the analysis of other analytes.
22. The composition of claim 8 wherein said functionality is to act as a calibration
solution that can be to calibrate the reagents in said online system.