Potassium maleate buffers for free chlorine analysis

Abstract

A kit is described that can be used to measure free chlorine in water. The kit comprises a buffering agent containing potassium maleate and an indicator. A method for measuring the concentration of free chlorine in an aqueous system comprises: adding an indicator and a buffer comprising potassium maleate to a water sample collected from the aqueous system; measuring the absorbance of the water sample after adding the indicator and the buffer to the water sample; and determining the concentration of free chlorine in the water system on the basis of the measured absorbance.

Description

Technical Field

[0001] The present disclosure generally relates to buffers for measuring free chlorine in water samples. Background Art

[0002] Chlorine is commonly used as a disinfectant in water systems and wastewater treatment processes. Measuring and monitoring free chlorine is important for ensuring safe drinking water and meeting environmental regulatory requirements for wastewater and industrial streams.

[0003] Colorimetric assays can be used to measure free chlorine in water, where an indicator and a buffer are added to the water sample. The indicator reacts with free chlorine in the sample to produce a color with an absorbance proportional to the concentration of free chlorine. The absorbance at a specific wavelength can be measured to determine the free chlorine concentration. The buffer adjusts the pH of the sample to an optimal range for free chlorine measurement.

[0004] A common buffer for free chlorine measurement is lithium maleate (C4H2Li2O4). However, the lithium in the buffer may interfere with the electrode for measuring the pH of the measurement system. Additionally, lithium may form insoluble particles with fluoride ions and phosphate groups, which are common elements in water and wastewater streams. Turbidity from the insoluble particles may interfere with accurate colorimetric measurement of free chlorine. Lithium is also a psychoactive drug that is not desired to be present in water streams. Furthermore, lithium hydroxide used to form lithium maleate is most commonly available in powder form, posing an inhalation hazard. Summary of the Invention

[0005] In view of the above, an object of the present disclosure is to provide a buffer that reduces or eliminates the common drawbacks of lithium maleate as a buffer for measuring free chlorine in water.

[0006] In one aspect, the present disclosure provides a kit comprising: a buffer comprising potassium maleate, and an indicator for measuring free chlorine in water.

[0007] In another aspect, the present disclosure provides a method for measuring the concentration of free chlorine in an aqueous system, the method comprising: adding an indicator and a buffer comprising potassium maleate to a water sample collected from the aqueous system; measuring the absorbance of the water sample after adding the indicator and the buffer to the water sample; and determining the concentration of free chlorine in the aqueous system based on the measured absorbance. Detailed Description

[0008] In the following description, numerous details are set forth to provide an understanding of the present disclosure. However, those skilled in the art will understand that the compositions and methods of the present disclosure can be practiced without these details, and various variations or alterations of the described embodiments may be feasible.

[0009] Broadly speaking, one embodiment of the present disclosure provides potassium maleate (C4H2K2O4 and / or C4H3KO4) as a buffer for measuring free chlorine in water samples, particularly for measuring free chlorine by colorimetry.

[0010] Regarding the present disclosure, it has been found that potassium maleate is an effective buffer in the determination of free chlorine and does not have the same drawbacks as lithium maleate. Potassium maleate has good solubility in water and effectively adjusts the pH of the water sample to the optimal range for free chlorine. Additionally, potassium maleate does not interfere with the electrode for measuring the pH of the water sample. Potassium maleate also does not form insoluble particles with common compounds such as fluoride and sulfate in the water stream, thus causing less interference to the colorimetric measurement of free chlorine. Potassium maleate is not a psychoactive drug. Furthermore, potassium hydroxide used to form potassium maleate can be obtained in pellet form, which eliminates any inhalation hazard.

[0011] Some embodiments of the present disclosure may include a kit. The kit may include: a buffer containing potassium maleate in a first container, and an indicator in a second container. Alternatively, the potassium maleate buffer and the indicator may be in the same container. The container may be a sealed container for containing the potassium maleate buffer and the indicator, such as a powder pillow, a sealed plastic bag, a vial, an ampoule, a sachet, a test tube, etc. The first container and the second container may be packaged together into a larger third container (e.g., a sealed bag, a box, etc.) and provided as a kit for measuring the free chlorine concentration in water.

[0012] The buffer may be an aqueous solution of potassium maleate. The concentration of potassium maleate in the solution may be in the range of 1 to 4 moles per liter (M), 2 to 3 M, or 2.4 to 2.8 M. In some embodiments, potassium maleate can remain soluble even at high concentrations within one or more of the above ranges and at temperatures in the range of 0 to 35 °C, 1 to 25 °C, 3 to 15 °C.

[0013] The pH of the potassium maleate buffer may be in the range of about 6.5 to 8, 7 to 8, or 7.3 to 7.6.

[0014] The indicator may include N,N-diethyl-p-phenylenediamine (DPD). The concentration of DPD may be in the range of 0.02 to 0.5 M, 0.05 to 0.25 M, or 0.1 to 0.2 M.

[0015] DPD can be provided in powder form in a container. The indicator solution can be provided in a different container. DPD and the indicator solution can be added together before use. The mixing of DPD and the indicator solution can be facilitated by manually or automatically swirling or shaking the container using a stir bar or any other suitable means. The indicator solution can contain deionized water. The deionized water can contain an acid to improve stability. The acid can be an inorganic acid or an organic acid having a concentration in the range of 0.05 to 1.5 M, 0.2 to 1 M, or 0.25 to 0.5 M. After adding DPD to the indicator solution, the concentration of the resulting DPD in the indicator solution can be in the range of 0.02 to 0.5 M, 0.05 to 0.25 M, or 0.1 to 0.2 M.

[0016] The present disclosure also relates to a method for measuring free chlorine by using an indicator and potassium malate as a buffer. The method includes adding the indicator and the buffer to water to measure the free chlorine concentration. For example, the indicator and the buffer can be added to a water sample collected from an aqueous system.

[0017] When adding the indicator to water for free chlorine measurement, at or near neutral pH, the free chlorine in the sample can immediately react (e.g., oxidize) with the indicator to form a colored compound (the active substance being a dye). The amount of the dye formed is generally proportional to the amount of free chlorine present in the water. For example, the free chlorine in water oxidizes DPD to form a pink / magenta Würster dye whose intensity is proportional to the free chlorine content.

[0018] The indicator can be added to water before or after adding the buffer. Alternatively, the indicator and the buffer can be added to water simultaneously. The indicator and the buffer can be added to water separately, or the indicator and the buffer can be combined and added together to water. After adding the indicator and / or the buffer to water, the mixing of water, the indicator, and / or the buffer can be facilitated by manually or automatically swirling or shaking the container using a stir bar or any other suitable means.

[0019] In some embodiments, the water sample can be at or near neutral pH before the addition of the indicator and buffer. For example, the initial pH of the water sample can be in the range of about 4 to 10, 7 to 8, or 7.3 to 7.6. The initial alkalinity of the water sample can be in the range of 0 to 1,000 ppm (as CaCO3), 0 to 500 ppm (as CaCO3), or 0 to 200 ppm (as CaCO3). Adding the indicator to the water sample typically reduces the pH of the sample beyond the acceptable range for accurately measuring free chlorine. For example, the equilibrium of chlorine species in the water sample can be pH-dependent. Additionally, too high or too low pH can cause interference with the measurement. The optimal pH range for measurement with the indicator can be about 5.5 to 7, 6 to 6.8, or 6.3 to 6.6. Potassium maleate buffer can stabilize the pH fluctuations.

[0020] When the indicator and potassium maleate buffer are added to the water sample, the pH of the resulting mixture can be in the range of about 5.5 to 7, 6 to 6.8, or 6.3 to 6.6.

[0021] In some embodiments of the present disclosure, a sample volume of about 12 milliliters (ml) of water to be measured can be used with about 10 microliters (µl) of the indicator and about 2 µl of potassium maleate buffer to measure free chlorine in the water sample. In some embodiments, a sample volume of about 2 ml of water to be measured can be used with 0.025 ml of the indicator and 0.025 ml of potassium maleate buffer. In some embodiments, the volume of the water sample to be measured can be in the range of 60 to 100 ml and can be used with 1 ml of the indicator and 1 ml of potassium maleate buffer.

[0022] For example, the amount of potassium maleate buffer to be added to about 12 ml of water sample can be in the range of 0.5 to 10 µl, 1 to 5 µl, or 1.5 to 4 µl. The volume of the indicator to be mixed with about 12 ml of water sample can be in the range of 1 to 50 µl, 4 to 30 µl, or 8 to 20 µl. The ranges of potassium maleate and the indicator can be scaled up proportionally with the volume of the water sample.

[0023] The method further includes measuring the absorbance of the water sample after adding the potassium maleate buffer and the indicator to the water sample. For example, the absorbance of the water sample can be measured in the range of 480 to 580 nm, 500 to 560 nm, or at a wavelength of 530 nm. The absorbance can be measured using, for example, a spectrophotometer, a colorimeter, or any other known instrument suitable for measuring absorbance. The concentration of free chlorine in the water sample collected from the water system can be determined from the measured absorbance.

[0024] For example, the concentration of free chlorine can be determined by comparing the measured absorbance with a calibration curve obtained in advance by a known method. For example, the calibration curve can be obtained by measuring the absorbance of a plurality of equal-volume standard solutions having different known free chlorine concentrations. Based on the measured absorbance values at different free chlorine concentrations, a curve of absorbance versus free chlorine concentration can be made to produce a calibration curve. Then the calibration curve can be used to determine the free chlorine concentration of a water sample by comparing the measured absorbance of the water sample containing an unknown amount of free chlorine with the calibration curve.

[0025] Alternatively, a spectrophotometer, a colorimeter, or other instrument for measuring absorbance can be programmed to automatically determine the free chlorine concentration based on the measured absorbance value using, for example, a calibration curve already stored in the memory of the instrument.

[0026] The present disclosure also relates to a method for preparing a potassium maleate buffer. The method can include forming maleic acid by reacting maleic anhydride with water. Then maleic acid can be reacted with potassium hydroxide to form dipotassium maleate and monopotassium maleate (potassium hydrogen maleate), collectively referred to as potassium maleate.

[0027] The foregoing is further illustrated by reference to the following examples, which are provided for illustrative purposes only and are not intended to limit the scope of the present disclosure.

[0028] Embodiment

[0029] Example 1 - Measurement of Free Chlorine in Water Samples

[0030] 0.025 ml of a 0.1 M DPD indicator solution stabilized with 0.5 M organic acid and 0.025 ml of a 2.7 M potassium maleate buffer were added to each of five samples to be measured, the samples containing 2 ml of water containing free chlorine. The samples were assayed with a CL17 analyzer (manufactured by Hach Company) to measure the free chlorine concentration based on the absorbance at 530 nm. The average results and the standard deviation of the results for each sample are shown in Table 1. Standard assays were also performed to measure the free chlorine concentration in each sample.

[0031] Table 1

[0032]

[0033] The results shown in Table 1 demonstrate that potassium maleate can be used as a buffer in a method for accurately measuring the free chlorine concentration using an indicator.

[0034] Although some embodiments of the present invention have been described in detail above, those skilled in the art will readily understand that various changes are feasible in the exemplary embodiments without materially departing from the disclosed embodiments. Accordingly, all such changes are intended to be included within the scope of the present disclosure as defined by the appended claims.

Claims

1. A kit, the kit comprising: A buffer containing potassium maleate; and An indicator for measuring free chlorine in water.

2. The kit according to claim 1, wherein the concentration of potassium maleate in the buffer is in the range of 1 to 4 moles per liter (M).

3. The kit according to claim 1, wherein the buffer comprises an aqueous solution of potassium maleate.

4. The kit according to claim 3, wherein potassium maleate is soluble in the aqueous solution at a temperature in the range of 0°C to 35°C.

5. The kit according to claim 1, wherein the pH of the buffer is in the range of 7 to 8.

6. The kit according to claim 1, wherein the indicator is N,N-diethyl-p-phenylenediamine (DPD).

7. A method for measuring the concentration of free chlorine in an aqueous system, the method comprising: Adding an indicator and a buffer containing potassium maleate to a water sample collected from the aqueous system; After adding the indicator and the buffer to the water sample, measuring the absorbance of the water sample; and Determining the concentration of free chlorine in the aqueous system based on the measured absorbance.

8. The method according to claim 7, wherein the pH of the water sample is in the range of 4 to 10 before adding the indicator and the buffer.

9. The method according to claim 7, wherein the pH of the water sample is in the range of 5.5 to 7 after adding the indicator and the buffer.

10. The method according to claim 7, wherein the buffer is prepared by reacting potassium hydroxide with maleic acid.

11. The method according to claim 10, wherein maleic acid is prepared by reacting maleic anhydride with water.

12. The method according to claim 7, wherein the buffer contains dipotassium maleate and monopotassium maleate.

13. The method according to claim 7, wherein the indicator is N,N-diethyl-p-phenylenediamine (DPD).

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