How to determine Inorganic Phosphate

Phosphate Ortho, Inorganic and Total

Various forms of phosphate are used in the treatment of boilers and cooling systems. Orthophos phate ion, the simplest form has the formula PO4 =. It may also exist in water as H2 PO4 _or HPO 4=, depending on the pH. Complex forms of this ion, or polyphosphates, are also used for both scale control and corrosion protection in operating systems. Examples of complex inorganic phosphates are pyrophosphate, hexametaphosphate, and other polyphosphates.

Modern water treatment practices also make use of a variety of phosphate ions having different organic functional groups attached. A broad class of these compounds known as phosphonates is particularly useful as scale control agents. In these applications, phosphonates function through retarding or preventing crystal growth and through dispersing scale-forming particles in solution. Examples of typical phosphonates include he di-,tri-, and tetraphosphonic acids. Because of the large number of titratable protons for each of these acids, the ionic forms present in aqueous media strongly depend on the system pH.

Phosphates are often present in surface waters from biological processes or systems run-off from fertilized land. In cooling systems, polyphosphates are used to stop the precipitation of calcium carbonate, thus preventing scale formation. In boiler systems, phosphate is used for internal conditioning. Water distribution systems often include phosphate treatment to reduce corrosion  and to control tuberculation.

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Phosphate in the Ortho (PO4=) form reacts with molybdic acid to produce a lightly colored phosphomolybdate ion. When reacted with reducing agents, the intensely colored molybdenum blue complex is formed. All other forms of phosphate do not react. To determine them, they must first be converted to the orthophosphate (OP) ion.

The complex inorganic phosphates revert very slowly at neutral pH and normal temperatures to the ortho form. High temperature and strong acid will complete the reversion in about 30 minutes. The results of testing this reverted solution for phosphate are a combination of ortho and complex inorganic forms. It is referred to as Total Inorganic Phosphate (TIP) for this reason. A small amount of organically bound phosphate may slowly react under these conditions and be included in TIP results, but this amount is usually insignificant and the concentration of complex inorganic phosphate is considered to be the difference between the (TIP) and (OP) results.

The reversion conditions for organo-phosphate ions are more severe. In addition to heat and acid, oxidizing agents must be used to destroy the organic functional groups attached to the phosphate “core” of the ion. Under these conditions the complex forms will of course also revert to the orthophosphate ion. For this reason the test is referred to as Total Phosphate (TP)

To determine the amounts of complex inorganic or organic phosphate present the differences between the test results are used. Complex inorganic phosphate is approximately the difference between TIP and OP. Likewise, complex organic phosphate is approximately the difference between TP and TIP.

In the laboratory, all three-phosphate tests can be determined on a small amount of sample by using an automated colorimeter.

Orthophosphate, Photometric Method (0-10ppm as PO4 ) Theory of Test

This test for orthophosphate is based on the reaction of the orthophosphate ion with molybdic acid to produce phosphomolybdic acid which is reduced to the intensely colored molybdenum blue complex by stannous chloride. Other inorganic anions, especially arsenate and silicate, also form molybdic acid complexes. However, the reductant for this test will not reduce these complexes to the molybdenum blue species.

The intensity of the blue color is proportional to the original orthophosphate concentration, and the absorbance of the reduced molybdenum blue is measured at 640 nm in a photometer. It is  necessary to prepare a standard calibration graph by treating standard samples of known concentrations of orthophosphate by the test procedure and plotting the measured absorbance vs. concentration. Orthophosphate concentrations of water samples are found by reading the correct concentration from the graph at the absorbance measured for that sample.

Apparatus Required

Filter photometer complete with assorted glassware.

Chemicals Required

Molybdate Reagent

Phosphate Standard

Stannous Chloride Reagent

Procedure for Test

This procedure uses a wavelength of 640 nm and a cell with a light path of approximately 5 mm. Prepare a calibration curve for the photometer using successive dilutions of phosphate solution to cover the range of the test. Treatment of the standards should be exactly the same as that described below for samples.

Prepare the sample for analysis by filtering it using filter paper for fine precipitates (Eaton- Dikeman 610 paper or the equivalent). Pipet 100ml of the filtered sample into a flask and add 4 ml of molybdate reagent. Mix the sample and reagent making sure that the temperature stays at 70F. Use sample to set photometer reading at zero. Return sample to flask, add 0.5-ml stannous chloride reagent to sample in flask, and swirl. Allow color development for 10 minutes before measuring absorbance on photometer.

Calculation of Results

The orthophosphate in parts per million as PO4 is obtained by reference to the orthophosphate calibration curve.

Limitations of Test

This method measures orthophosphate. High concentrations of chromate (over 150 ppm)  or organic material may interfere.

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