Environmental Engineering LaboratoryBy: Ngoc Thuan Le

Phosphate (P-PO4-3) - ascorbic method

Principle: Ammonium molybdate and potassium antimonyl tartrate react in acid medium

with orthophosphate to form a heteropoly acid-phosphomolybdic acid that is reduced to

intensely colored molybdenum blue by ascorbic acid.

Apparatus

Volumetric tubes, incubator, Spectrophotometer.

Procedure

1. Place 40ml of sample to volumetric tubes.

2. Add 4ml of solution of 5V (NH4)6Mo7O24 (ammonium molybdate) and 1V C6H8O7 7.2%

(ascorbic acid).

3. Bring the volume to 50ml, mixing after addition.

4. Keep at 20-40oC in incubator for 15 minutes

5. Measure color photometrically at 880nm and compare with calibration curve, using

distilled water blank.

6. Calibration curve: P-PO4 solution 0.005mg/ml (5mg/L)

Concentration range: 0, 1, 2, 4, 6, 8, 10, 15ml

Calculation

Reagents

- 6g of Ammonium molybdate ((NH4)6Mo7O24.4H2O, 1235.86) + 0.24 gr of

antimonyl Potassium tartrate 3-hydrate (C8H4K2O12Sb2.3H2O, 667.87) and 300ml

of H2O. Then, adding solution of 120ml H2SO4 (2+1) (mean 2V H2SO4+1V H2O)

+ 5gr ammonium sulfamate (NH4OSO2NH2, 114.12), bring the solution to 500ml

with water.

- Ascorbic 7.2% (w/v): dissolve 7.2 g ascorbic powder in 100ml dW.

- Working solution: 5V ammonium molybdate solution: 1V ascorbic. This mixture

is stable for 4h.

- Standard solution, dissolve 0.439gr potassium phosphate monobasic (KH2PO4,

136.09) in water to 1000ml (~ 0.1gr P-PO4 /L, 100mg/L, dilute 20 times before

using to have sol’s 5mg/L).

- Solution for confirmation: dilute 25ml of standard solution to 500ml