Powder composition, kit and method for determining orthophosphate concentration

Abstract

Kits and powder compositions are described that can be used to determine orthophosphate concentrations in an aqueous system. The kit comprises: a first powder composition comprising a molybdate; and a second powder composition comprising ascorbic acid. Also described is a powder composition comprising molybdate, pyrosulfate, and less than 1% by weight ascorbic acid. A method for determining the orthophosphate concentration in a water system using the kit comprises: adding the first powder composition and the second powder composition to a water sample collected from the water system, measuring the absorbance of the water sample after adding the first powder composition and the second powder composition to the water sample, and determining the orthophosphate concentration in the water system based on the measured absorbance.

Description

Technical Field

[0001] The present disclosure relates to kits and powder compositions for determining orthophosphate concentration in an aqueous system and methods of using the kits to determine orthophosphate concentration. Background Art

[0002] Phosphates, including orthophosphates, are widely used as corrosion inhibitors in a variety of water systems, such as wastewater streams and process streams in various industrial processes. However, when the concentration of orthophosphate is too high, it may cause scaling, pollution and environmental problems. Therefore, it is important to accurately measure and control the orthophosphate concentration within an appropriate range.

[0003] A known method for measuring orthophosphate in water systems is the molybdenum blue method (Standard Method 4500-PE, 1997), in which a reagent is added to a water sample to react with orthophosphate in the water sample and produce a blue color whose absorbance is proportional to the orthophosphate concentration. A powder composition based on the molybdenum blue method comprises a molybdate and a reducing agent such as ascorbic acid. The molybdate reacts with the orthophosphate in the water to produce a heteropolyacid complex, and the reducing agent reduces the complex to produce a blue color. The absorbance of the water sample can then be measured to determine the orthophosphate concentration.

[0004] Although the powder composition was originally developed as a stable alternative to reagent solutions, it has long been recognized that the powder composition may suffer from discoloration during storage, which can result in loss of chemical function and significantly reduce the accuracy of the measured orthophosphate concentration, as well as causing poor perception of the powder composition. This problem has plagued the water treatment industry for more than two decades, and no cause or solution is known. Although there have been speculations as to the source of the problem, such as that it is caused during the manufacture of one or more powder reagents, it is difficult to determine the source of the problem, which in turn makes it difficult to find an effective solution.

[0005] There has long been a need for an improved powder composition that is storage stable without suffering from discoloration problems and that can be used to accurately measure orthophosphate concentration in aqueous systems. Summary of the invention

[0006] In view of the above, one object of the present disclosure is to provide stable powder compositions and kits that can be used to accurately determine phosphate concentration.

[0007] In one aspect, the present disclosure provides a kit comprising: a first powder composition comprising a molybdate; and a second powder composition comprising ascorbic acid.

[0008] In another aspect, the present disclosure provides a powder composition comprising molybdate, pyrosulfate, and less than 1 wt % ascorbic acid.

[0009] In another aspect, the present disclosure provides a method for determining the orthophosphate concentration in a water system using the kit. The method comprises: adding the first powder composition to a water sample collected from the water system, adding the second powder composition to the water sample, measuring the absorbance of the water sample after adding the first powder composition and the second powder composition to the water sample, and determining the orthophosphate concentration in the water system based on the measured absorbance. DETAILED DESCRIPTION

[0010] In the following description, numerous details are set forth to provide an understanding of the present disclosure. However, those skilled in the art will appreciate that the methods and systems of the present disclosure may be implemented without these details, and that various variations or modifications of the described embodiments may be feasible.

[0011] After studying the discoloration problem in depth, the inventors unexpectedly found that the discoloration of the powder reagent composition for measuring the orthophosphate concentration may be caused by one or more reactions between the reagents under humid conditions. In particular, as shown in the examples, the inventors found that ascorbic acid can react with molybdate under humid conditions to produce discoloration in the powder composition. The reaction between ascorbic acid and molybdate produces water-insoluble dark blue or black particles, and may significantly reduce the accuracy of orthophosphate concentration measurement. The reaction between ascorbic acid and molybdate in the presence of pyrosulfate produces orange or brown particles, which may also interfere with orthophosphate measurement, and cause poor perception of the powder reagent composition. The inventors found that discoloration can occur in a powder composition comprising ascorbic acid and molybdate and in a powder composition also comprising pyrosulfate under humid conditions (e.g., at least 50% relative humidity).

[0012] The kits and powder compositions disclosed herein can solve the discoloration problem and provide a stable powder composition for accurately measuring the orthophosphate concentration in an aqueous system. For example, in the kit disclosed herein, ascorbic acid can be provided in a different powder composition from molybdate. By providing ascorbic acid separately from molybdate, discoloration can be eliminated or at least greatly reduced even when stored under humid conditions. In another embodiment, a powder composition is provided, comprising molybdate, pyrosulfate, and less than 1% by weight of ascorbic acid. By using a lower amount of ascorbic acid in a powder composition containing molybdate and pyrosulfate, discoloration can be greatly reduced or eliminated even when the powder composition is stored under humid conditions.

[0013] The compositions disclosed herein are in solid form, such as in the form of a dry powder under ambient conditions. Ambient conditions may include a temperature in the range of 16 to 35°C and a pressure of 1 atm.

[0014] The kit comprises: a first powder composition comprising at least molybdate, and a second powder composition comprising ascorbic acid. The first powder composition and the second powder composition can be separated from each other in the kit. For example, the first powder composition can be provided in a first container, and the second powder composition can be provided in a second container. The first container and the second container can be sealed containers that respectively accommodate the first powder composition and the second powder composition, such as a powder pillow, a sealed plastic bag, a vial, a test tube, etc. The first container and the second container can be packaged together in a larger third container (e.g., a sealed bag, a box, etc.) and provided as a kit for measuring the concentration of orthophosphate in water.

[0015] Molybdate can be a water-soluble molybdate that can react with orthophosphate to form a phosphomolybdic acid complex under acidic conditions. For example, molybdate can be sodium molybdate, potassium molybdate, ammonium molybdate or a mixture thereof. Molybdate is in solid form, such as in the form of a dry powder. In some embodiments, based on the gross weight of the first powder composition and the second powder composition, molybdate can be present in the first powder composition in an amount ranging from 0.5 wt % to 20 wt %, from 0.8 wt % to 10 wt % or from 1 wt % to 5 wt %. Unless otherwise stated, all concentrations (wt %) disclosed herein about the kit are based on the gross weight of the first powder composition and the second powder composition.

[0016] The first powder composition can also include pyrosulfate. Pyrosulfate can be a water-soluble pyrosulfate capable of acidifying a water sample. Pyrosulfate can be capable of reducing the pH of a water sample to any suitable acidic pH. For example, the pH of a water sample can be reduced to a pH in the range of 0 to 3, a pH in the range of 0.5 to 2.5, or a pH below about 2. Pyrosulfate can be potassium pyrosulfate, sodium pyrosulfate, ammonium pyrosulfate, or a mixture thereof. Pyrosulfate is in a solid form, such as in the form of a dry powder. Based on the gross weight of the first powder composition and the second powder composition, pyrosulfate can be present in the first powder composition in an amount in the range of 50% by weight to 99% by weight, 70% by weight to 95% by weight, or 75% by weight to 85% by weight.

[0017] In addition to pyrosulfate or in place of pyrosulfate, the first powder composition may include an acid different from pyrosulfate. The acid may contribute to lowering the pH of the water sample. The acid may include, for example, a powdered form of sulfamic acid, benzenesulfonic acid, trichloroacetic acid or a mixture thereof. When present in the first powder composition, based on the gross weight of the first powder composition and the second powder composition, pyrosulfate and one or more other acids may be in the range of 50 wt % to 99 wt %, 70 wt % to 95 wt % or 75 wt % to 85 wt %. Alternatively, pyrosulfate and / or an acid different from pyrosulfate may be present in the second powder composition, as long as it does not interfere with the performance of other compounds in the second powder composition.

[0018] The first powder composition may contain no ascorbic acid, contain substantially no ascorbic acid, or contain less than 1 wt % ascorbic acid based on the total weight of the first powder composition and the second powder composition. As used herein with respect to the kit, "substantially containing no ascorbic acid" means an ascorbic acid concentration of less than 0.01 wt % based on the total weight of the first powder composition and the second powder composition.

[0019] Ascorbic acid is provided in a second powder composition different from the first powder composition comprising molybdate. The ascorbic acid is in solid form, such as in the form of a dry powder, and may be present in the second powder composition in an amount ranging from 5 wt % to 40 wt %, from 8 wt % to 30 wt %, or from 10 wt % to 20 wt %, based on the total weight of the first powder composition and the second powder composition.

[0020] The second powder composition may contain no molybdate, substantially no molybdate, or less than 0.5 wt % molybdate based on the total weight of the first powder composition and the second powder composition. "Substantially no molybdate" as used herein means less than 0.01 wt % molybdate based on the total weight of the first powder composition and the second powder composition.

[0021] The first powder composition and / or the second powder composition may also include antimony. Antimony may be water-soluble antimony. Antimony may be able to accelerate the color-forming reaction between molybdate and the orthophosphate present in the water sample. For example, antimony may be an alkali metal salt of antimonyl carboxylate, such as one or more salts of antimonyl tartrate, antimonyl glycolate, antimonyl citrate, antimonyl lactate and antimonyl malate. The alkali metal may be sodium, potassium, rubidium, cesium, lithium or francium. In one embodiment, antimony may be potassium antimonyl tartrate. Antimony is in solid form, such as in the form of a dry powder. Based on the gross weight of the first powder composition and the second powder composition, antimony may be present in an amount ranging from 0.001% by weight to 5% by weight, from 0.01% by weight to 2% by weight or from 0.05% by weight to 0.5% by weight.

[0022] The first powder composition and / or the second powder composition can also include a chelating agent. The chelating agent can chelate metals that cause ascorbic acid to be unstable or otherwise incompatible with other elements present in the kit disclosed herein. For example, the chelating agent can be combined with heavy metals that can catalyze the decomposition of ascorbic acid. The chelating agent can be hydroxycarboxylates and aminocarboxylates, citric acid, phosphonic acid butane tricarboxylic acid, ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid. EDTA can be used in any of its various salt forms, for example, disodium EDTA, tetrasodium EDTA, sodium edetate, disodium edetate, trisodium edetate and disodium calcium edetate. The chelating agent is a solid form, such as a dry powder form. Based on the gross weight of the first powder composition and the second powder composition, the chelating agent can be present in an amount in the range of 0.01 wt % to 5 wt %, 0.03 wt % to 2 wt % or 0.1 wt % to 1 wt %.

[0023] In one aspect, the first powder composition comprises molybdate, pyrosulfate, a chelating agent (such as EDTA), and antimony, and the second powder composition comprises ascorbic acid. By separating the ascorbic acid from the molybdate, the reaction between the ascorbic acid and the molybdate (alone or in the presence of pyrosulfate) under humid conditions can be prevented, thereby preventing discoloration and loss of chemical function of the first and second powder compositions. As a result, as discussed below, the first and second powder compositions can be used to accurately measure orthophosphate concentrations in water samples collected from water systems.

[0024] In another embodiment, a powder composition is provided. The powder composition comprises molybdate, pyrosulfate, and less than 1% by weight of ascorbic acid. The powder composition may comprise less than 0.5% by weight of ascorbic acid, substantially no ascorbic acid, or no ascorbic acid. As used herein with respect to the powder composition, "substantially no ascorbic acid" means that the composition contains less than 0.01% by weight of ascorbic acid based on the total weight of the powder composition. By using lower amounts of ascorbic acid in the powder composition containing molybdate and pyrosulfate, discoloration can be substantially reduced or eliminated even when the powder composition is stored under humid conditions.

[0025] The molybdate and pyrosulfate can be any suitable molybdate and pyrosulfate, such as those discussed above with respect to the kit. Based on the gross weight of the powder composition, the molybdate can be present in the powder composition in an amount ranging from 0.5 wt % to 20 wt %, from 1 wt % to 10 wt %, or from 1.5 wt % to 5 wt %. Based on the gross weight of the powder composition, the pyrosulfate can be present in the powder composition in an amount ranging from 70 wt % to 99.5 wt %, from 80 wt % to 98.5 wt %, or from 90 wt % to 98 wt %.

[0026] The powder composition can also include antimony and / or a chelating agent in the same manner as discussed above with respect to the kit. Based on the gross weight of the powder composition, antimony can be present in the powder composition in an amount in the range of 0.001 wt % to 5 wt %, 0.01 wt % to 2 wt % or 0.05 wt % to 0.5 wt %. Based on the gross weight of the powder composition, a chelating agent can be present in the powder composition in an amount in the range of 0.01 wt % to 5 wt %, 0.03 wt % to 2 wt % or 0.1 wt % to 1 wt %.

[0027] The powder composition may contain less than 0.1 wt % vanadate, substantially no vanadate, or no vanadate. As used herein, "substantially no vanadate" means less than 0.01 wt % vanadate based on the total weight of the powder composition.

[0028] The kits and powder compositions disclosed herein can be used to quickly and accurately determine the concentration of orthophosphate contained in an aqueous system. The powder compositions are stable for long-term storage and do not suffer from discoloration and loss of chemical function even when stored under humid conditions. For example, the powder compositions can be shown to be substantially non-discolored when stored under humid conditions such as above 50% relative humidity, above 70% relative humidity, or above 90% relative humidity for a period of at least 1 month, at least 4 months, at least 8 months, or at least 12 months. Thus, the powder compositions can be stored under various conditions for these time periods without any observable discoloration.

[0029] In another embodiment, a method for determining the orthophosphate concentration in an aqueous system using the kit is provided. The method may include collecting a water sample from the aqueous system, and adding a first powder composition and a second powder composition to the water sample. For example, a water sample in an amount ranging from 1 to 50 ml, 2 to 20 ml, or 5 to 10 ml may be collected in a test tube or vial, and the first powder composition and the second powder composition may be added to the sample. The first powder composition and the second powder composition may be added in any order. For example, the first powder composition may be added to the water sample first, followed by the second powder composition, or vice versa. Alternatively, the first powder composition and the second powder composition may be added to the water sample simultaneously. Once the first powder composition and the second powder composition are added, the test tube or vial may be shaken or otherwise mixed for at least 10 seconds, at least 20 seconds, at least 1 minute, and not more than 5 minutes, or not more than 2 minutes.

[0030] The first powder composition can react with orthophosphate in the water sample to produce a phosphomolybdic acid complex, and the phosphomolybdic acid complex can be reduced to molybdenum blue by the second powder composition. In particular, molybdate reacts with orthophosphate in the water sample under acidic conditions produced by pyrosulfate and / or other acids to produce a phosphomolybdic acid complex. When present, the complex may also contain antimony. Antimony is believed to accelerate the reaction between molybdate and orthophosphate in the water sample. The phosphomolybdic acid complex is then reduced to brightly colored molybdenum blue by ascorbic acid in the second powder composition. The blue water sample has an absorbance proportional to the orthophosphate concentration.

[0031] The method further includes: after adding the first powder composition and the second powder composition to the water sample, measuring the absorbance of the water sample. For example, the absorbance of the water sample can be measured at a wavelength in the range of 600 to 900 nm, 610 to 880 nm, or 625 to 750 nm. The absorbance can be measured using, for example, a spectrophotometer, a colorimeter, or any other known instrument suitable for measuring absorbance. The orthophosphate concentration in the water system can be determined by colorimetry from the measured absorbance.

[0032] For example, the orthophosphate concentration can be determined by comparing the measured absorbance with a calibration curve obtained in advance using a known method. For example, the calibration curve can be obtained by measuring the absorbance of multiple equal volumes of standard solutions having different known concentrations of orthophosphate. Based on the measured absorbance values ​​at different orthophosphate concentrations, a curve of absorbance relative to orthophosphate concentration can be made to produce a calibration curve. The calibration curve can then be used to determine the orthophosphate concentration of a water sample by comparing the measured absorbance of a water sample containing an unknown amount of orthophosphate with the calibration curve.

[0033] Alternatively, a spectrophotometer, colorimeter, or other instrument for measuring absorbance may be programmed to automatically determine the orthophosphate concentration based on the measured absorbance values ​​using, for example, a calibration curve that has been stored in the instrument's memory.

[0034] The water system tested using the kits and powder compositions disclosed herein can be a water system containing less than 20 mg / L orthophosphate or having an orthophosphate concentration ranging from 0.001 mg / L to 10 mg / L, 0.01 to 7 mg / L, or 0.06 to 5 mg / L.

[0035] The foregoing is further illustrated by reference to the following examples, which are provided for the purpose of illustration and are not intended to limit the scope of the present disclosure.

[0036] Example

[0037] Example 1 - Reaction between ascorbic acid and molybdate under humid conditions

[0038] Four kinds of powder compositions (samples A, B, C and D) are prepared as shown in Table 1. Samples A and B have the same powder composition comprising sodium molybdate and ascorbic acid. Sample A is placed in 100% relative humidity for one day, while sample B is stored in room humidity (~35%) for one day. Sample C comprises molybdate, pyrosulfate, antimony and EDTA, but does not comprise ascorbic acid. Sample D comprises the same powder composition as sample C except also comprising ascorbic acid. Samples C and D are all placed in 100% humidity for one day. Before storage, each composition in samples A, B, C and D has a white powder appearance.

[0039] Table 1

[0040]

[0041] As shown in Table 1, as in samples A and D, when the powder composition comprises molybdate and ascorbic acid simultaneously and is placed in 100% relative humidity for one day, water-insoluble dark blue / black particles are formed. In the sample A that only comprises ascorbic acid and molybdate, the whole powder becomes dark blue / black. In the sample D that additionally comprises pyrosulfate, tartrate and EDTA, dark blue / black spots are produced in the white powder. On the other hand, when the molybdate and ascorbic acid in the sample B are stored in room humidity for one day, there is no discoloration. Similarly, when the powder composition of the sample C that will comprise molybdate but does not comprise ascorbic acid is placed in 100% relative humidity for one day, there is no discoloration. The above examples show that ascorbic acid reacts with molybdate under humid conditions to produce dark blue / black insoluble powder.

[0042] Example 2 - Reaction between ascorbic acid, molybdate and pyrosulfate under humid conditions

[0043] Four pairs of samples were prepared as shown in Table 2 below. Two samples (1) and (2) in each pairing A, B, C and D contained the same powder composition. Each pairing A, B, C and D contained potassium pyrosulfate and sodium molybdate. Pairing A, B and C also contained: (A) potassium antimony tartrate, (B) ascorbic acid, and (C) EDTA. Pairing D was a control and contained only potassium pyrosulfate and sodium molybdate. Each paired sample (1) in the four pairs A, B, C and D was left to stand for three days under room humidity (about 35% relative humidity), and each paired sample (2) in the four pairs A, B, C and D was placed in a 100% humidity chamber for three days. Before storage, the composition in each sample of pairing A, B, C and D had a white powder appearance.

[0044] Table 2

[0045]

[0046] As shown in Table 2, the powder compositions of pairings A, C and D all had the same appearance after storage for three days at room humidity or at 100% relative humidity. However, in pairing B, the powder composition comprising sodium molybdate, potassium pyrosulfate and ascorbic acid turned into brown / dark sand with some black spots after storage for three days at 100% relative humidity. The same powder composition in the first sample in pairing B showed no change in appearance after storage at room humidity. The above examples show that ascorbic acid reacts with sodium molybdate in the presence of pyrosulfate under humid conditions, causing discoloration to brown / dark sand including black spots.

[0047] Example 3 - Two powder composition systems do not show discoloration

[0048] As shown in Table 3 below, a first powder composition comprising potassium pyrosulfate, potassium antimony tartrate, EDTA, and sodium molybdate, and a second powder composition comprising ascorbic acid were prepared.

[0049] The first powder composition and the second powder composition were sealed in different containers called powder pillows. Pinholes were punched in each powder pillow, and the powder pillows were stored at 97% relative humidity for five days. As shown in Table 3, the first powder composition and the second powder composition remained white and did not show any discoloration after being stored at 97% relative humidity for five days. This example shows that discoloration can be avoided by separating ascorbic acid from molybdate.

[0050] Comparative Example 1 - Single Powder Composition Showing Discoloration

[0051] A single powder composition containing the same amounts of potassium pyrosulfate, potassium antimony tartrate, EDTA, sodium molybdate, and ascorbic acid as in the two powder compositions of Example 3 was prepared and stored in a single powder pillow. Pinholes were punched in the powder pillow and stored at 97% relative humidity for five days.

[0052] As shown in Table 3 below, compared to Example 3, the single powder composition of Comparative Example 1 turned black with brown spots after storage at 97% relative humidity.

[0053] Example 3 and Comparative Example 1 demonstrate that the reaction between ascorbic acid and molybdate (alone or in the presence of pyrosulfate) under humid conditions and the resulting discoloration and loss of chemical functionality that occurs in a single powder system can be prevented by using the dual powder system disclosed herein.

[0054] Table 3

[0055]

[0056] Example 4 - Measurement of orthophosphate concentration using a dual powder system

[0057] The first powder composition and the second powder composition of Example 3 were used to determine the orthophosphate concentration in a standard solution containing 5 mg / L orthophosphate. 5 ml of 5 mg / L PO4 3- The standard solution was added to a test tube. The first powder composition and the second powder composition of Example 3 were added to the standard solution in the test tube. Then, the lid was fixed to the test tube and the vial was shaken for at least 20 seconds, after which the solution turned blue. After about two minutes, the vial was inserted into a spectrophotometer and the absorbance at a wavelength of 880 nm was measured to correlate the measured absorbance with the orthophosphate concentration using a pre-programmed method to determine the orthophosphate concentration. This process was repeated multiple times. The results are shown in Table 4 below.

[0058] Reference Example 1 - Measuring orthophosphate concentration using a single powder system

[0059] A PhosVer® 3 reagent powder pillow (manufactured by Hach Company, known as a composition for measuring orthophosphate concentration by the ascorbic acid method) was obtained and used as a control. The single powder composition of the PhosVer® 3 reagent powder pillow was added to a 5 ml 5 mg / L PO4 3- The standard solution was placed in a test tube and used to measure the orthophosphate concentration in the same manner as described above in Example 4. The results of Example 4 and Reference Example 1 are shown in Table 4 below.

[0060] Table 4

[0061]

[0062] As shown above in Table 4, the dual powder system obtains substantially the same orthophosphate measurement results as the known single powder system. Therefore, the dual powder system can be used to accurately measure orthophosphate concentration.

[0063] The dual powder system of Example 4 and the single powder system of Reference Example 1 were then stored at room humidity (~35%) for four months. At different times during the four months, the powder compositions of Example 4 and Reference Example 1 were used to determine the concentration of 5 mg / L PO4 in the same manner as described above. 3- The results are shown in Table 5 below.

[0064] Table 5

[0065]

[0066] As shown in Table 5 above, the dual powder system was able to be stored stably for four months without loss of chemical functionality and performed equally well as the current single powder compositions.

[0067] The above examples demonstrate that the dual powder system disclosed herein can be used to accurately measure orthophosphate concentration and, unlike current single powder systems, can be stably stored—even under humid conditions—without suffering from discoloration and loss of chemical functionality.

[0068] Comparative Example 2

[0069] Then, the single powder composition of Comparative Example 1 which turned black with brown spots after storage at 97% relative humidity was used to measure 5 mg / L PO4 in the same manner as described in Example 4 above. 3- Orthophosphate concentration in standard solution. Color-changing powder composition measured 5 mg / L PO4 3- The orthophosphate concentration in the standard solution was only 3.5 mg / L. This example shows that the reaction under humid conditions not only causes discoloration, but also causes a loss of chemical function, thereby making the powder composition unable to accurately measure the orthophosphate concentration.

[0070] Comparative Example 3

[0071] As shown in Table 6 below, single powder compositions (Samples 1-4) containing the same compound as Comparative Example 1 but with decreasing amounts of ascorbic acid were prepared. As shown in Table 6 below, the powder composition of Sample 1 has the same composition as Comparative Example 1, such that the amount of ascorbic acid in Sample 1 (approximately 17% by weight) is equivalent to 100% of the ascorbic acid formulation percentage, and Samples 2-4 contain decreasing amounts of ascorbic acid. The samples were placed in 100% relative humidity for 48 hours.

[0072] Each sample was then spiked with 5 ppm PO4 3- The samples were placed in a standard solution and the orthophosphate concentration was measured in the same manner as described in Example 4 above. As shown in Table 6 below, all samples showed discoloration and loss of chemical function after storage at 100% relative humidity for 48 hours. None of the samples were able to accurately measure 5 ppm PO4 3- Orthophosphate concentration in the standard solution.

[0073] Table 6

[0074]

[0075] The above examples show that even small amounts of ascorbic acid in combination with molybdate in the presence of pyrosulfate can react when exposed to moisture, resulting in discoloration and loss of chemical functionality, rendering the powder composition incapable of accurately measuring orthophosphate concentration.

[0076] Although some embodiments of the present invention are described in detail above, it will be readily appreciated by those skilled in the art that, without materially departing from the disclosed embodiments, various changes are feasible in the exemplary embodiments. Therefore, all such changes are intended to be included within the scope of the present disclosure defined in the appended claims.

Claims

1. A kit, comprising: a first powder composition comprising a molybdate; as well as A second powder composition comprises ascorbic acid. 2 . The kit according to claim 1 , wherein the first powder composition comprises the molybdate in an amount ranging from 0.5 wt % to 20 wt % based on the total weight of the first powder composition and the second powder composition.

3. The kit of claim 1, wherein the molybdate is selected from the group consisting of sodium molybdate, potassium molybdate, ammonium molybdate, and various mixtures thereof.

4. The kit of claim 1, wherein the first powder composition further comprises a pyrosulfate salt.

5. The kit of claim 4, wherein the pyrosulfate salt is selected from the group consisting of potassium pyrosulfate, sodium pyrosulfate, ammonium pyrosulfate, and various mixtures thereof.

6. The kit according to claim 4, wherein the pyrosulfate salt is present in the first powder composition in an amount ranging from 50 wt% to 95 wt% based on the total weight of the first powder composition and the second powder composition.

7. The kit of claim 1, wherein the first powder composition further comprises antimony.

8. The kit according to claim 7, wherein the antimony is potassium antimony tartrate.

9. The kit of claim 1, wherein the first powder composition further comprises a chelating agent.

10. The kit according to claim 1, wherein the first powder composition comprises, based on the total weight of the first powder composition and the second powder composition: pyrosulfate in an amount ranging from 50% to 95% by weight; a molybdate in an amount ranging from 0.5 wt % to 20 wt %; Antimony in an amount ranging from 0.01 wt % to 5 wt %; and Chelating agents in amounts ranging from 0.03 wt % to 5 wt %.

11. The kit according to claim 1, wherein the ascorbic acid is present in the second powder composition in an amount ranging from 5 wt% to 40 wt% based on the total weight of the first powder composition and the second powder composition.

12. The kit of claim 1, wherein the first powder composition and the second powder composition are separated from each other.

13. The kit of claim 1, wherein the first powder composition is in a first container and the second powder composition is in a second container.

14. A powder composition, comprising: Molybdate; Pyrosulfate; and Less than 1 wt% ascorbic acid.

15. The powder composition of claim 14, wherein the powder composition comprises substantially no ascorbic acid.

16. The powder composition of claim 14, further comprising antimony.

17. The powder composition of claim 14, wherein the powder composition exhibits substantially no discoloration when stored above 50% humidity for at least 4 months.

18. A method for determining the orthophosphate concentration in an aqueous system using the kit according to claim 1, the method comprising: adding the first powder composition to a water sample collected from the water system; adding the second powder composition to the water sample; after adding the first powder composition and the second powder composition to the water sample, measuring the absorbance of the water sample; and The orthophosphate concentration in the water system is determined based on the measured absorbance.

19. The method of claim 18, wherein the first powder composition reacts with orthophosphate in the water sample to produce a phosphomolybdic acid complex, and the phosphomolybdic acid complex is reduced to molybdenum blue by the second powder composition.

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