Determination of Phosphate

The neutralized sample is reacted with ammonium molybdate and stannous chloride.the blue color obtained is matched against that produced with a series of standard phosphate solution, visually or with spectrophotometer at 690mu.

Reagents:

1.Phenolphthalein indicator solution

2.Approximately 4N sulfuric acid

3.Ammonium Molybdate Solution

4.Standard Phosphate solution

Procedure:

To a 50 ml.sample containing not more than 0.15mg.of phosphate(as Po₄)add one drop of Phenolphthalein indicator. if pink color is obtained , add dilute H₂SO₄ drop wise to discharge the color.add,with through mixing after each addition 2.0ml. ammonium molybdate solution and 0.25ml(5drops)of stannous chloride solution.the rate of color development and intensity of color depend upon the temperature of the final solution, each 1°C increase producing about 1 percent increase in color.Hence samples and reagents should be within 2°C of one another and at a temperature between 20 to 30°C.After 10 minutes but before 15 minutes, measure the color photometrical or visually in Nessler tubes against standards prepared simultaneously. A blank shall always be run on the reagents and stilled water

Calculation:

Phosphate as Po₄ mg ./ litre = 1000 X W / V

Where,

W=Weight in mg. of phosphate (as Po4)in sample from standard graph (or solution)

V=Volume in ml.of the sample taken.