Method for determining phosphate radical concentration in water sample

Abstract

The invention discloses a method for determining the concentration of phosphate radicals in a water sample. The method includes providing a water sample, determining a first phosphate concentration of the water sample at a first temperature by molybdenum yellow method or by a variation of molybdenum yellow method, and determining a corrected phosphate concentration of the water sample at a second temperature by molybdenum yellow method or by a variation of molybdenum yellow method if the first phosphate concentration is within a specified range.

Description

[0001] The present invention relates to a method for determining phosphate in a water sample.

[0002] For many years, efforts have been made to reduce phosphate pollution in the environment, especially from wastewater.

[0003] According to the German Waste Water Ordinance, water treatment plants with a design capacity for serving more than 10,000 inhabitants must comply with a phosphate discharge concentration of no more than 2 mg / l. For design capacities of more than 100,000 inhabitants, this value is 1 mg / l.

[0004] Therefore, accurate determination of phosphate content is required.

[0005] The molybdenum blue method and the molybdenum yellow method are currently the most commonly used methods for measuring orthophosphate in water systems, especially industrial water. The molybdenum blue method is very sensitive and suitable for measuring low concentrations of orthophosphate. However, in industrial water systems, the orthophosphate concentration may sometimes be as high as tens of ppm. When the orthophosphate concentration in the water is high, the molybdenum blue method is easily saturated, making it necessary to dilute the water sample before taking the measurement. Compared with the molybdenum blue method, the molybdenum yellow method has a wide measurement range and is therefore more suitable for industrial water samples.

[0006] The first step of the molybdenum yellow method is to acidify the water sample and add molybdate, wherein orthophosphate and molybdate react in an acidic aqueous solution to produce a solution containing phosphomolybdic acid (heteropolyacid). A color developer such as vanadate is then added to the aforementioned solution containing phosphomolybdic acid to develop a yellow color. The absorbance of the water sample is then measured photometrically using a spectrometer, thereby optically determining the concentration of orthophosphate contained in the water.

[0007] DE 27 28 706 describes a method for automatic phosphate determination using the molybdenum yellow method, which uses a solution containing 25 g of ammonium heptamolybdate, 1 g of ammonium monovanadate, 56 ml of concentrated sulfuric acid and 1 g of sodium chloride per liter as reagent. The molybdate component and the vanadate component form a yellow complex with the phosphate. The following reaction is assumed to occur:

[0008] (NH4)6Mo7O 24 + NH4VO3 + PO4 3- → (NH4)3PO4 x NH4VO3 x 16 MoO3

[0009] In an acidic medium (pH < 2), the equilibrium is on the right side of the reaction. In this method, a wastewater sample is measured photometrically, the reagent is added, and after a delay time of 4 to 5 minutes, a second measurement is carried out. The phosphate concentration can be determined by a yellow color development with the aid of a standard solution. When the measuring solution is strongly acidic, the intensity of the yellow color is proportional to the phosphate concentration.

[0010] Variations of the above-described method are also known.

[0011] For example, WO 2016 / 133882 A1 describes a variation in which a composition for measuring the orthophosphate concentration in a water system is provided in a solid form, particularly in a dry powder form, to facilitate transportation, packaging and use. The composition for measuring the orthophosphate concentration in a water system comprises:

[0012] (a) an acid in solid form;

[0013] (b) water-soluble molybdates; and

[0014] (c) A color developing agent for developing color of phosphomolybdic acid.

[0015] WO 2016 / 133882 A1 describes that the color developer used to develop phosphomolybdic acid is a color developer used in the molybdenum yellow method, which develops the phosphomolybdic acid formed by the reaction of molybdate with orthophosphate in an aqueous system under acidic conditions. The color developer is, for example, a vanadate, for example, ammonium vanadate, sodium vanadate and potassium vanadate.

[0016] For example, another variation of the molybdenum yellow method is described in US 4,544,639, which describes a method for determining the amount of organic phosphonate present in an aqueous solution, the method comprising the following steps:

[0017] (a) adding 1000 to 20,000 ppm of a strong oxidant and 8 ppb to 5000 ppm of silver ions to an aqueous solution;

[0018] (b) reacting organic phosphonates present in the solution to form orthophosphates;

[0019] (c) precipitating any excess silver in the solution by adding from 1000 to 50,000 ppm of a reducing agent or at least an equimolar amount of a halide based on the silver ions added in step (a), and removing any precipitate formed;

[0020] (d) adding 5 to 30 volume percent of an acid aqueous solution containing 500 to 2000 ppm of ions selected from the group consisting of molybdate ions and vanadate ions to the solution, and if a halide is used in step (c), adding 1000 to 50,000 ppm of a reducing agent;

[0021] (e) measuring the absorbance of the solution at 625 to 880 nanometers and determining the orthophosphate concentration from a calibration curve of orthophosphate concentration versus absorbance; and

[0022] (f) subtracting any orthophosphate present in the original aqueous solution from the orthophosphate concentration determined in step (e) to obtain a total organic phosphonate concentration.

[0023] US 2020 / 0340925 describes a variant of the molybdenum yellow method, which focuses on determining low phosphate concentrations. US 2020 / 0340925 thus describes a method for determining phosphate in a water sample, the method comprising:

[0024] (a) Providing water samples;

[0025] (b) adding an acid so that the concentration of the acid in the water sample is at least 0.5% by weight;

[0026] (c) performing a first photometric measurement on the water sample solution;

[0027] (d) adding a coloring component to the photometric water sample solution;

[0028] (e) performing a second photometric measurement; and

[0029] (f) calculating the phosphate concentration from the difference between the first photometric measurement and the second photometric measurement.

[0030] US 2020 / 0340925 describes partitioning a coloring reagent, namely a molybdate- and vanadate-based reagent, by first adding an acid before performing a first photometric measurement, then adding the coloring component to the same aqueous sample solution and performing a second photometric measurement.

[0031] DE 27 28 706 A, WO 2016 / 133882 A1, US 4,544,639 and US 2020 / 0340925 are each incorporated herein by reference. Other variations will be apparent to the skilled person, and the above examples of the above molybdenum yellow process are not intended to be limiting.

[0032] A disadvantage of the molybdenum yellow method is that inaccuracies occur, particularly at warm temperatures, when processing wastewater samples and / or industrial water samples with a low phosphate content.

[0033] It is an object of the present invention to provide an improvement to the molybdenum yellow method to overcome the above-mentioned inaccuracies.

[0034] In one embodiment, the present invention provides a method for determining the phosphate concentration in a water sample. The method comprises: providing a water sample, determining a first phosphate concentration of the water sample at a first temperature by a molybdenum yellow method or by a variation of the molybdenum yellow method, and if the first phosphate concentration is within a specified range, determining a corrected phosphate concentration of the water sample at a second temperature by a molybdenum yellow method or by a variation of the molybdenum yellow method.

[0035] In one embodiment of the present invention, if the specified range of the first phosphate concentration is <5ppm, the second temperature of the corrected phosphate concentration is preferably 30-35°C, if the specified range of the first phosphate concentration is 5-10 ppm, the second temperature of the corrected phosphate concentration is preferably 35-40°C, and if the specified range of the first phosphate concentration is 10-15 ppm, the second temperature of the corrected phosphate concentration is preferably 40-44°C.

[0036] In one embodiment of the present invention, the first temperature is preferably 40-60° C., more preferably 43-47° C., and most preferably about 45° C. The first temperature can be, for example, 40° C., 41° C., 42° C., 43° C., 44° C., 45° C., 46° C., 47° C., 48° C., 49° C., 50° C., 51° C., 52° C., 53° C., 54° C., 55° C., 56° C., 57° C., 58° C., 59° C. or 60° C.

[0037] In one embodiment of the present invention, the water sample is preferably divided into two water samples, for example, a first water sample and a second water sample. Preferably, one of the two water samples, for example, the first water sample, is used to measure the first phosphate concentration, while the other of the two water samples, for example, the second water sample, is used to measure the second phosphate concentration.

[0038] In one embodiment of the invention, the determination of the first phosphate concentration and the determination of the second phosphate concentration are performed by adjusting the temperature of the water sample from the first temperature to the second temperature. It is known to the technician that the molybdenum yellow method will consume a certain amount of time, for example, up to five minutes. Instead of running two samples (which will consume up to 10 minutes, excluding any preparation time), the technician can use a single sample and analyze it using an analysis system, in which the single sample can be heated and / or cooled in situ. An analysis system with a heating and / or cooling system has been previously described, for example, in US 4,004,708, US 4,619,530 or CN 104316479 A, ​​each of which is incorporated herein by reference. Therefore, one aspect of the present invention includes an analyzer with a flexible photometric reaction chamber, the photometric reaction chamber having a flexible temperature range of 25-60°C, which can be heated and / or cooled between any temperature of 26-60°C in about a few seconds. This almost instant temperature adjustment enables the same sample to be first temperature measured at a first temperature and then almost immediately temperature measured at a second temperature. As just one example, a technician may perform a first analysis at 45°C and a second analysis at 30°C. Therefore, any differences in the analytical results may be the result of interference. Therefore, the use of two temperatures can also be used by the technician to check and / or verify analytical results obtained at other temperatures. As described above, any combination of temperatures between 25 and 60°C is feasible (provided that the starting temperature is not equal to the ending temperature), for example:

[0039]

[0040] In one embodiment of the present invention, the water sample is preferably a wastewater sample or an industrial water sample. As known to the skilled person, such samples are usually analyzed photometrically at approximately 45°C.

[0041] The corrected phosphate concentration preferably mitigates the effects of interference with the phosphate concentration at this temperature.

[0042] The interference is preferably a silicate interference. The term "silicate" is therefore understood to mean any member of the family of anions consisting of silicon and oxygen, which generally have the general formula , where 0≤x<2. This family includes orthosilicate (x=0), metasilicate (x=1) and pyrosilicate (x=0.5, n=2). The name is also used for any salt of such an anion, such as sodium metasilicate; or any ester containing the corresponding chemical group, such as tetramethyl orthosilicate. The term silicate is used to denote silicate minerals, ionic solids with silicate anions, and rock types consisting mainly of such minerals. In this case, the term also includes the non-ionic compound silicon dioxide SiO2 (silica, quartz), which corresponds to the case where x=2 in the general formula. The term also includes minerals in which aluminum or other tetravalent atoms replace a portion of the silicon atoms, as in aluminosilicates. The interferences in the present invention are of course not limited to silicates, and other examples of interferences are also known to the skilled person, for example, Zr(IV) and Ti(IV), Ta(V) and Nb(V), W(VI).

[0043] A variation of the method of the present invention for determining the phosphate concentration in a water sample comprises providing the water sample, determining a first phosphate concentration of the water sample at 40-50°C, preferably 43-47°C, and very preferably approximately 45°C by a molybdenum yellow method or by a variation of the molybdenum yellow method, determining the silicate concentration of the water sample, and correcting the first phosphate concentration by adopting a phosphate correction value to eliminate the interfering influence of the silicate concentration.

[0044] The skilled person will be aware of a number of methods for determining the silicate concentration of a water sample. One such method is disclosed in ASTM D859-16 (2021) e1. Another method is the Spectroquant ®Silicate Test 101813, which begins with pipetting 20 mL of the sample solution into a plastic test container, after which 200 µL of a reagent (Si-1) is added. The solution is then mixed and then left to stand for 5 minutes. After the standing time, 200 µL of the second reagent (Si-2) is added and the solution mixed, after which 1.00 mL of the third reagent (Si-3) is added. The solution is mixed again, allowed to react for 5 minutes, then measured in the photometer, and a reagent blank prepared with ultrapure water is measured in a similar manner. Another method is the "Silicomolybdate / Heteropoly Blue Method" of Hach Lange GmbH. The silicomolybdate method involves the reaction of molybdate ions with silicon dioxide and phosphate under acidic conditions to form a yellow color. Citric acid is added to destroy the phosphomolybdic acid complex (yellow color is formed due to the phosphate), but not the silicomolybdic acid complex. For large amounts of silicon dioxide, the remaining yellow color is strong enough to be read directly. For low concentrations, the use of amino-naphtholsulfonic acid reducing agent converts the pale yellow color to a dark heteropoly blue material. The color formed is proportional to the amount of silica present in the original sample; colorimetric measurement of this intensity provides an accurate means of determining silica concentration. Some forms of silica (usually polymeric) will not react with ammonium molybdate and must be digested with sodium bicarbonate to convert to a reactive form. Silicic acid reacts with water and hydrates as shown below:

[0045] H2SiO3 + 3H2O → H8SiO6

[0046] The hydrated silicic acid reacts with molybdate in the presence of an acid to form silicomolybdic acid.

[0047] H8SiO6 + 12(NH4)2MoO4 + 12H2SO4 → H8[Si(Mo2O7)6] + 12(NH4)2SO4 + 12H2O

[0048] For low concentrations, the silicomolybdic acid is then reduced to a blue color (heteropoly species) with aminonaphtholsulfonic acid. The above examples are not intended to be limiting, and the skilled artisan will be aware of other methods.

[0049] In one embodiment of the present invention, the phosphate correction value is preferably:

[0050] a) from 0 to 25 ppm silicate: (ppm silicate measured) x 0.10 / (25 ppm silicate),

[0051] b) from >25 to 50 ppm silicate: except a), (ppm silicate measured above 25 ppm) x 0.07 / (25 ppm silicate), and

[0052] c) From >50 to 100 ppm silicate: except a) and b), (ppm silicate determined above 50 ppm) x 0.20 / (50 ppm silicate).

[0053] The above correction values ​​are provided as an example only. A skilled person can calculate a narrower or wider range based on commonly used water samples. In one embodiment of the present invention, the phosphate correction value can also be provided by a look-up table and / or a hypothetical model, which represents the interference effect of silicate concentration at a specific temperature. For example, an example of a hypothetical model is Figure 4 The technician will then infer this interfering influence in the first phosphate concentration based on the measured silicate concentration of the water sample (e.g., in ppm silicate) and the temperature at which the first phosphate concentration is measured. A corrected first phosphate concentration will thus be obtained without the interfering influence of the silicate concentration.

[0054] A variation of the method of the present invention for determining the phosphate concentration in a water sample is to eliminate the interfering influence of silicate by always determining the phosphate concentration at a temperature where such interfering influence does not occur or at least such interfering influence occurs at a minimum. This variation preferably provides a water sample, determines the temperature of the water sample, preferably 30-35°C, and then determines the phosphate concentration of the water sample by the molybdenum yellow method or its variation. The temperature of the water sample can, for example, be even lower than 30°C, for example, 10-30°C. However, temperatures of 30°C, 31°C, 32°C, 33°C and 35°C are preferred, for example, 30°C.

[0055] The present invention is described in more detail below based on embodiments and drawings, wherein:

[0056] Figure 1 Optimization of silicate interference at temperatures of 30°C and 45°C is shown.

[0057] Figure 2 An algorithm for predicting the results of increasing / decreasing phosphate concentration slope is shown.

[0058] Figure 3 A typical operation of a wastewater plant in the <5 mg / l PO4-P range over time is shown.

[0059] Figure 4 A hypothetical model between reaction temperature and silicate interference is shown.

[0060] The present invention is explained in more detail hereinafter based on the following examples, which are provided in order to show how the present invention can be practically applied in the real world.These examples are not intended to be limiting. Example

[0061] Example 1

[0062] In order to investigate the influence of silicate (here SiO2) on the orthophosphate measurement, sample runs were performed under standard conditions using a Phosphax sc Phosphat-Analysator from Hach Lange GmbH with a silicate range of 0.05-15 mg / l PO4-P (at 45 °C, 55 µl reagent to 1 ml sample and a reaction time of less than 5 minutes) and 0-100 mg / l SiO2. It was determined that at 100 ppm silicate, the silicate interference can lead to up to 0.37 ppm of phosphate (as PO4-P) equivalents. Table 1 lists the data for the sample runs between 0-100 ppm SiO2.

[0063] Table 1

[0064]

[0065] Example 2

[0066] The first sample run was repeated under optimized conditions using a Phosphax scPhosphat-Analysator from Hach Lange GmbH, 0.05-15 mg / l PO4-P (70 µl reagent per 1 ml sample at 45°C and a reaction time of less than 5 minutes) and a silicate range of 0-100 mg / l SiO2. Thus, the only difference between Examples 1 and 2 was the use of more reagent (70 µl instead of 55 µl). It was determined that at 100 ppm silicate, the silicate interference can lead to up to 0.22 ppm of phosphate (as PO4-P) equivalents. Table 2 lists the data for the sample runs between 0-100 ppm SiO2.

[0067] Table 2

[0068]

[0069] Therefore, the effects of interferences can be reduced by up to 40 % by controlling the sample-to-reagent ratio.

[0070] Example 3

[0071] Example 2 was repeated at a temperature of 30°C instead of 45°C. It was determined that the effect of silicate could be significantly reduced. Table 3 lists the data for sample runs between 0 and 100 ppm SiO2. A comparison of the results of Examples 2 and 3 is shown in Table 3. Figure 1 Given. Figure 1 It is thus clearly shown that the interference of silicate is significant at a higher temperature (ie 45°C) compared to a lower temperature (ie 30°C).

[0072] Table 3

[0073]

[0074] Examples 2 and 3 show that silicate interference can be corrected to approximately 93-100% based on a combination of temperature and sample to reagent ratio.

[0075] The above examples demonstrate that simply changing the sample-reagent ratio itself does not significantly minimize or completely eliminate silicate interference. However, the above examples also show that changing the reaction temperature, for example, changing the reaction temperature within the range of 30-60°C, can significantly minimize or completely eliminate silicate interference. Although the present invention does not propose any conclusive underlying theory for the technical effect, it is believed that the reaction kinetics may be affected based on the Arrhenius equation combined with the selected concentration level. The practical application of this observation thus involves dividing the measurement range into segmented reaction temperature regions or reaction temperature bands, thereby minimizing or completely eliminating silicate interference. For example, 50 mg / l of silicate shows the influence of 0.17 mg / l of phosphate at 45°C under standard conditions (see Example 1 above). Reducing the temperature to 30°C completely eliminates this interference (see Example 3 above). Therefore, high-resolution analysis as low as 0.05 mg / l is feasible.

[0076] It is also possible to run hypothetical models for various phosphate concentrations. For example, Figure 4 Shown is a hypothetical model for the relationship between reaction temperature and silicate interference based on a reagent ratio of 70 µl to 1 ml sample. Figure 4 It is thus shown that when the temperature is reduced, the effect of silicate interference is reduced. A skilled person can use such a hypothetical model to select a reaction temperature with expected silicate interference to minimize the effect of SiO2.

Claims

1. A method for determining the phosphate concentration in a water sample, the method comprising: providing the water sample; determining a first phosphate concentration of the water sample at a first temperature by a molybdenum yellow method or a variation of the molybdenum yellow method; as well as If the first phosphate concentration is within a specified range, a corrected phosphate concentration of the water sample is determined at a second temperature by the molybdenum yellow method or by a variation of the molybdenum yellow method.

2. The method according to claim 1, wherein: If the specified range of the first phosphate concentration is <5 ppm, the second temperature of the corrected phosphate concentration is 30-35°C, If the specified range of the first phosphate concentration is 5-10 ppm, the second temperature of the corrected phosphate concentration is 35-40° C., and If the specified range of the first phosphate concentration is 10-15 ppm, the second temperature for the corrected phosphate concentration is 40-44°C.

3. The method according to claim 1 or 2, wherein the first temperature is 40-60°C, preferably 43-47°C, most preferably about 45°C.

4. The method according to at least one of claims 1 to 3, wherein: dividing the water sample into a first water sample and a second water sample, and The first phosphate concentration determination is performed using the first water sample, and the second phosphate concentration determination is performed using the second water sample.

5. The method according to at least one of claims 1 to 3, wherein the determination of the first phosphate concentration and the determination of the second phosphate concentration are performed by adjusting the temperature of the water sample from the first temperature to the second temperature.

6. The method according to at least one of claims 1 to 5, wherein the water sample is a wastewater sample or an industrial water sample.

7. The method according to at least one of claims 1 to 6, wherein the correcting phosphate concentration mitigates the effect of interferences in phosphate concentration.

8. The method of claim 7, wherein the interference is a silicate interference.

9. A method for determining the phosphate concentration in a water sample, the method comprising: providing the water sample; determining a first phosphate concentration of the water sample by a molybdenum yellow method or by a variation of the molybdenum yellow method at a temperature of 40-50° C., preferably 43-47° C., preferably about 45° C.; determining the silicate concentration of the water sample in ppm silicate; and The first phosphate concentration is corrected by adopting a phosphate correction value to eliminate the interfering effect of the silicate concentration.

10. The method according to claim 9, characterized in that The phosphate correction value is: a) from 0 to 25 ppm silicate: (ppm silicate measured) x 0.10 / (25 ppm silicate), b) from >25 to 50 ppm silicate: except a), (ppm silicate measured above 25 ppm) x 0.07 / (25 ppm silicate), and c) From >50 to 100 ppm silicate: except a) and b), (ppm silicate determined above 50 ppm) x 0.20 / (50 ppm silicate).

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