Patent Description:
Attempts have been made for many years to reduce phosphate pollution in the environment, in particular from wastewaters.

According to the German Waste Water Ordinance, water treatment plants having a designed capacity for servicing more than <NUM>,<NUM> inhabitants must adhere to a discharge concentration of phosphate of not more than <NUM>/l. This value is <NUM>/l for a designed capacity for more than <NUM>,<NUM> inhabitants.

An exact determination of phosphate content is therefore necessary.

The molybdenum blue method and the molybdenum yellow method are among the currently most commonly used methods for measuring orthophosphate in a water system, in particular in industrial water. The molybdenum blue method is very sensitive and is suitable for measuring orthophosphate in low concentrations. However, in industrial water system, the orthophosphate concentration may sometimes reach as high as tens of ppm. When the orthophosphate concentration in water is high, the molybdenum blue method is easily saturated so that the water sample must be diluted before the measurement is performed. In contrast to the molybdenum blue method, the molybdenum yellow method has a wide range of measurement and is therefore more suitable for industrial water samples.

The documents<NPL>, <NPL>, and <NPL> disclose the molybdenum method in water sample applications.

The first step of the molybdenum yellow method is to acidify the water sample and to add a molybdate, where the orthophosphate and the molybdate react in the acidic aqueous solution to generate a solution that contains the phosphomolybdic acid (a heteropoly acid). A color development agent, such as vanadate, is then added to the aforesaid phosphomolybdic acid containing solution to develop a yellow color. The absorbance of the water sample is subsequently measured photometrically using a spectrometer, thereby optically determining the concentration of the orthophosphate contained in water.

<CIT> describes a method for automatic phosphate determination using the molybdenum yellow method which uses as a reagent a solution which contains <NUM> of ammonium heptamolybdate, <NUM> of ammonium monovanadate, <NUM> of concentrated sulfuric acid, and <NUM> of sodium chloride per liter. The molybdate and vanadate components form a yellow-colored complex together with phosphate. It is presumed that the following reaction takes place:.

(NH<NUM>)<NUM>Mo<NUM>O<NUM> + NH<NUM>VO<NUM> + PO<NUM><NUM>- → (NH<NUM>)<NUM>PO<NUM> x NH<NUM>VO<NUM> x <NUM> MoO<NUM>.

The equilibrium lies on the right-hand side of the equation in an acid medium (pH < <NUM>). In this method, a wastewater sample is measured photometrically, the reagent is added, and after a delay time of from <NUM> to <NUM> minutes, a second measurement is carried out. The phosphate concentration can be determined from the yellow coloration with the aid of a standard solution. The intensity of the yellow color is here proportional to the phosphate concentration when the measurement solution is strongly acidic.

Variants of the above method are also known.

For example, <CIT> describes a variant where the composition for measuring the concentration of orthophosphate in a water system is provided in solid form, in particular as a dried powder, in order to facilitate transportation, packaging and use. The composition for measuring the concentration of orthophosphate in a water system comprises:.

<CIT> describes that the color development agent for developing the phosphomolybdic acid is a color development agent used in the molybdenum yellow method that develop a color for the phosphomolybdic acid formed from the reaction between the molybdate and the orthophosphate in the water system under the acidic conditions, for example, vanadates, for example, ammonium vanadate, sodium vanadate and potassium vanadate.

Another variant of molybdenum yellow method is described, for example, in <CIT> which describes a process for determining the amount of organic phosphonate present in an aqueous solution, comprising the steps of:.

<CIT> describes a variant of the molybdenum yellow method which focuses on reliably determining small phosphate concentrations. <CIT> thereby describes a method for determining phosphate in a water sample comprising:.

<CIT> describes that the coloring reagent, i.e., a reagent based on molybdate and vanadate, is divided by firstly adding an acid before a first photometric measurement is carried out and then adding the coloring component to the same water sample solution and carrying out the second photometric measurement.

Each of <CIT>, <CIT>, <CIT> and <CIT> disclose examples of the molybdenum yellow method.

A disadvantage of the molybdenum yellow method is that inaccuracies have been observed, in particular, when processing a wastewater sample and/or an industrial water sample having a low phosphate content at warm temperatures.

An object of the present invention is to provide an improvement of the molybdenum yellow method to overcome the above inaccuracies.

The present invention, defined in appended independent claim <NUM>, provides a method for determining a phosphate concentration in a water sample in an analyzer with a photometric reaction chamber which can be heated and cooled. The method comprises: providing the water sample; determining a first phosphate concentration of the water sample via a molybdenum yellow method at a first temperature equal to or above <NUM>; and if the first phosphate concentration falls within a specified range below or equal to <NUM> ppm, determining a corrected phosphate concentration of the same water sample via the molybdenum yellow method at a second temperature, below the first temperature.

In an embodiment of the present invention, if the specified range of the first phosphate concentration is < <NUM> ppm, the second temperature for the corrected phosphate concentration is preferably <NUM>-<NUM>, if the specified range of the first phosphate concentration is from <NUM>-<NUM> ppm, the second temperature for the corrected phosphate concentration is preferably <NUM>-<NUM>, and if the specified range of the first phosphate concentration is from <NUM>-<NUM> ppm, the second temperature for the corrected phosphate concentration is preferably <NUM>-<NUM>. According to the invention, as defined in appended claim <NUM>, if the specified range of the first phosphate concentration falls below or equal to <NUM> ppm, the second temperature is below a first temperature of equal to or above <NUM>.

In an embodiment of the present invention, the first temperature is preferably from <NUM>-<NUM>, and most preferably at approximately <NUM>. The first temperature can, for example, be <NUM>, <NUM>, <NUM>, <NUM>, °C <NUM>, <NUM>, <NUM>, °C, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> <NUM>, <NUM>, <NUM>, <NUM> or <NUM>.

In an example which does not fall within the scope 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. The determining of the first phosphate concentration is preferably performed with one of the two water samples, for example, with the first water sample, while the determining of the second phosphate concentration is performed with the other of the two water samples, for example, with the second water sample.

In an embodiment of the present invention, in accordance with appended claim <NUM>, the determining of the first phosphate concentration and the determining of the second phosphate concentration is performed by adjusting the temperature of the water sample from the first temperature to the second temperature. A skilled person knows that the molybdenum yellow method will take a certain amount of time, for example, up to five minutes. Instead than running two samples, which would then up to <NUM> minutes, excluding any preparation time, the skilled person can instead use a single sample and perform the analysis with an analysis system where the single sample can be heated and/or cooled in situ. Analysis systems with heating and/or cooling systems have previously been described, for example, in <CIT>, <CIT> or in <CIT>.

An aspect of the present invention therefore includes an analyzer having a photometric reaction chamber, for example with a flexible temperature range of <NUM>-<NUM>, which can be heated and/or cooled, for example between any temperature from <NUM>-<NUM> within a matter of seconds. This practically immediate temperature adjustment allows a photometric measurement to be conducted first at the first temperature, and almost immediately thereafter, at the second temperature, with the same sample. A skilled person could, merely as an example, perform a first analysis at <NUM>, and a second analysis at <NUM>. Any difference in the analysis result could thereby be the result of an interference. The use of two temperatures could therefore also be used by a skilled person to check and/or validate the analysis result obtained at another temperature. As stated above, any combination of temperatures between <NUM> and <NUM> is possible (provided that the starting temperature is not equal to the ending temperature), for example (in embodiments in accordance with the invention, the starting temperature must be equal to or above <NUM>; the starting temperatures which are not equal to or above <NUM>, and listed in the table below, are related to examples which do not fall within the scope of the invention defined by the appended claims):.

In an embodiment of the present invention, the water sample is preferably a wastewater sample or an industrial water sample. A skilled person knows that such samples are usually photometrically analyzed at approximately <NUM>.

The corrected phosphate concentration preferably mitigates an effect of an interference of the phosphate concentration at this temperature.

The interference is preferably a silicate interference. The term "silicate" is thereby understood to be any member of a family of anions consisting of silicon and oxygen, usually with the general formula <MAT> where <NUM> ≤ x < <NUM>. The family includes orthosilicate <MAT> (x = <NUM>), metasilicate <MAT> (x = <NUM>), and pyrosilicate <MAT> (x = <NUM>, n = <NUM>). The name is also used for any salt of such anions, such as sodium metasilicate; or any ester containing the corresponding chemical group, such as tetramethyl orthosilicate. The term silicate is used to mean silicate minerals, ionic solids with silicate anions, as well as rock types that consist predominantly of such minerals. In that context, the term also includes the non-ionic compound silicon dioxide SiO<NUM> (silica, quartz), which would correspond to x = <NUM> in the general formula. The term also includes minerals where aluminum or other tetravalent atoms replace some of the silicon atoms, as in the aluminosilicates. The interference of the present invention is of course not limited to silicate, other examples of interference are also known to the skilled person, for example, Zr(IV) and Ti(IV), Ta(V) and Nb(V), W(VI).

A variant of the method for determining a phosphate concentration in a water sample, which does not fall within the scope of the present invention, includes providing the water sample, determining a first phosphate concentration of the water sample via a molybdenum yellow method or via a variant of the molybdenum yellow method at a temperature of from <NUM>-<NUM>, preferably from <NUM>-<NUM>, very preferably at approximately <NUM>, determining a silicate concentration of the water sample, and correcting the first phosphate concentration by applying a phosphate correction value so as to eliminate an interference effect of the silicate concentration.

A skilled person would know many methods of determining the silicate concentration of a water sample. One such method is described in ASTM D859-<NUM> (<NUM>)e1. Another method is the Spectroquant® Silicate Test <NUM> of Merck KGaA which starts by pipetting <NUM> of sample solution into a plastic test vessel, after which <NUM>µL of a reagent (Si-<NUM>) is added. The solution is then mixed and then left to stand for <NUM> minutes. After the standing time, <NUM>µL a second reagent (Si-<NUM>) is added and the solution is mixed, then <NUM> of a third reagent (Si-<NUM>) is added. The solution is mixed once again, left to react for <NUM> minutes, and then measured in the photometer against a reagent blank prepared with Ultrapure water in an analogous manner. Another method is the "Silicomolybdate/Heteropoly Blue Method" of Hach Lange GmbH. The Silicomolybdate Method involves the reaction of molybdate ion with silica and phosphate under acid conditions to form a yellow color. Citric acid is added to destroy the phosphomolybdic acid complex (the yellow color formed due to phosphate), but not the silicomolybdic acid complex. For large amounts of silica, the remaining yellow color is intense enough to be read directly. For low concentrations, an amino-naphthol sulfonic acid reducing agent is used to convert the faint yellow color to a dark heteropoly blue species. The color formed is directly proportional to the amount of silica present in the original sample; a colorimetric measurement of this intensity provides an accurate means of determining the silica concentration. Some forms of silica (usually polymeric) will not react with ammonium molybdate and must be digested with sodium bicarbonate to be converted to a reactive form. Silicic acid reacts with water and hydrates as follows:.

H<NUM>SiO<NUM> + <NUM><NUM>O → H<NUM>SiO<NUM>.

This hydrated silicic acid reacts with molybdate in the presence of acids to form silicomolybdic acid.

H<NUM>SiO<NUM> + <NUM>(NH<NUM>)<NUM>MoO<NUM> + <NUM><NUM>SO<NUM> → H<NUM>[Si(Mo<NUM>O<NUM>)<NUM>] + <NUM>(NH<NUM>)<NUM>SO<NUM> + <NUM><NUM>O.

This silicomolybdic acid is then reduced to a blue color (heteropoly species) by an amino naphthol sulfonic acid for low concentrations. The above examples are not intended to be limiting and a skilled person would have knowledge of additional methods.

In an example which does not fall within the scope of the present invention, the phosphate correction value is preferably,.

The above correction values are merely provided as an example. A skilled person can calculate narrower or broader ranges based on the water sample usually used. In an example which does not fall within the scope of the present invention, the phosphate correction value can also be provided by a look-up table and/or an assumption modeling which represents the interference effect of the silicate concentration at a specific temperature. An example of an assumption modeling is shown, for example, in <FIG>. A skilled person would then deduct this interference effect from the first phosphate concentration based on the silicate concentration of the water sample determined, for example, in ppm silicate, and temperature at which the first phosphate concentration was determined. The corrected first phosphate concentration will then be obtained absent the interference effect of the silicate concentration.

A variant of the method for determining a phosphate concentration in a water sample in an example which does not fall within the scope of the present invention is to eliminate the interference effects of silicate by always determining the phosphate concentration at a temperature where no, or at least minimal, such interference effects occur. This variant preferably provides the water sample, establishes a temperature of the water sample of preferably <NUM>-<NUM>, and then determines the phosphate concentration of the water sample via the molybdenum yellow method or a variant thereof. The temperature of the water sample can, for example, even be lower than <NUM>, for example, from <NUM>-<NUM>. Preference is however given to a temperature of <NUM>, <NUM>, <NUM>, <NUM> and <NUM>, for example, <NUM>.

The present invention, defined by the appended claims, is described in greater detail below on the basis of embodiments and of the drawings in which:.

The present invention, defined by the appended claims, is explained in greater detail below based on the following examples which are provided to show how the present invention can be practically applied in the real world. The scope of the invention is defined and limited by the appended claims.

In order to investigate the impact that silicate (here SiO<NUM>) has on an orthophosphate measurement, a sample run was performed under standard conditions using the Phosphax sc Phosphat-Analysator from Hach Lange GmbH, <NUM>-<NUM>/l PO<NUM>-P (<NUM>µl reagent to <NUM> sample at <NUM> and less than <NUM> Minutes reaction time) with a silicate range of <NUM>-<NUM>/l SiO<NUM>. It was thereby determined that the interference of silicate can result in up to <NUM> ppm phosphate equivalents as PO<NUM>-P with <NUM> ppm silicate. Table <NUM> sets forth the data of samples run between <NUM>-<NUM> ppm SiO<NUM>.

A first sample run was repeated under optimized conditions using the Phosphax sc Phosphat-Analysator from Hach Lange GmbH, <NUM>-<NUM>/l PO<NUM>-P (<NUM>µl reagent to <NUM> sample at <NUM> and less than <NUM> Minutes reaction time) with a silicate range of <NUM>-<NUM>/l SiO<NUM>. The only difference between Examples <NUM> and <NUM> was therefore that more reagent (<NUM>µl instead of <NUM>µl) was used. It was thereby determined that the interference of silicate can result in up to <NUM> ppm phosphate equivalents as PO<NUM>-P with <NUM> ppm silicate. Table <NUM> sets forth the data of samples run between <NUM>-<NUM> ppm SiO<NUM>.

The interference impact could therefore be reduced by up to <NUM> % by manipulating the sample-to-reagent ratio.

Example <NUM> was repeated at a temperature of <NUM> instead of <NUM>. It was thereby determined that the interference of silicate can be significantly reduced. Table <NUM> sets forth the data of samples run between <NUM>-<NUM> ppm SiO<NUM>. A comparison of the results of Examples <NUM> and <NUM> are set forth in <FIG> thereby clearly shows that the interference of silicate is significant at higher temperatures (i.e., <NUM>) compared to lower temperatures (i.e., <NUM>).

Examples <NUM> and <NUM> shows that the interference of silicate can be corrected based on a combination of temperature and sample-to-reagent ratio to approximately <NUM>-<NUM> %.

The above Examples demonstrate that varying only the ratio of sample-to-reagent will not in and of itself significantly minimize or entirely eliminate silicate interference. The above Examples also show, however, that changing the reaction temperature, for example, in the range of <NUM>-<NUM>, can significantly minimize or entirely eliminate silicate interference. While the present invention does not propose any conclusive underlying theory for this technical effect, it is believed that reaction kinetics may be influenced based on the Arrhenius equation in combination with the chosen concentration level. A practical application of this observation thereby involves dividing the measuring range into segmented reaction temperature areas or zones to thereby minimize or entirely eliminate silicate interference. For example, <NUM>/l of silicate showed an impact of <NUM>/l phosphate under standard conditions at <NUM> (see Example <NUM> above). A reduction of the temperature to <NUM> reduced this interference entirely (see Example <NUM> above). An analysis with high resolution down to <NUM>/l is therefore possible.

Claim 1:
A method for determining a phosphate concentration in a water sample in an analyzer with a photometric reaction chamber which can be heated and cooled, the method comprising:
providing the water sample;
determining a first phosphate concentration of the water sample via a molybdenum yellow method at a first temperature equal to or above <NUM>; and
if the first phosphate concentration falls within a specified range below or equal to <NUM> ppm, determining a corrected phosphate concentration of the same water sample via the molybdenum yellow method at a second temperature, below the first temperature.