ANALYTICAL cHEMISTRY - II

MSCM201

Experiment 7

Aim of the Experiment

To determine the phosphate concentration in water by SnCl₂.2H₂O.

Principle

Phosphorous exists in water as orthophosphate (PO₄³⁻, HPO₄²⁻, H₂PO₄⁻), phosphate, and organic phosphate. Orthophosphates are assimilated by bacteria during their growth process. Organic phosphate is present in molecules such as DNA, RNA, and nucleotides. Orthophosphates and molybdate ions condense in an acidic solution to give molybdophosphate acidic solution, which, upon selective reduction by stannous chloride, produces a blue color due to the formation of molybdenum blue in uncertain composition. The absorbance is measured at 690 nm.

Reactions:

Ammonium molybdate + Phosphate ---> Ammonium Phospho Molybdate + SnCl2 ---> Blue

yellow complex

(NH4)2 MoO4 PO4 3- (NH4)3[PO4(MoO3)12]

Materials required

Pipette, Nessler’s tubes, Standard volumetric flasks, Phosphate solution (100 ppm), Glycerol, Stannous chloride solution, Ammonium molybdate solution, etc.

Preparation of reagents

50 ppm phosphate stock solution:

Weigh 71.65 mg of potassium dihydrogen phosphate and dissolve it in conductivity water and make up the volume to 100 mL in a 100 mL volumetric flask.

KH₂PO₄ ≡ PO₄³⁻

136.086 g/mol ≡ 94.9714 g/mol

71.65 mg ≡ 50 mg

Ammonium molybdate solution:

Dissolve 2.5 gm ammonium molybdate in 17.5 mL conductivity water. Then add 28 mL concentrated H₂SO₄ solution diluted with 40 mL water and make up the volume to 100 mL using conductivity water.

Stannous chloride solution:

Take 2.5 gm SnCl₂.2H₂O in 100 mL of glycerol and 100 mL water (It is not soluble in 100 mL glycerol, so the addition of water is required).

Procedure

Pipette out 0.2 mL, 0.4 mL, 0.6 mL, 0.8 mL, 1 mL, and an unknown amount of 50 ppm phosphate solution in separate Nessler’s tubes and add 2 mL ammonium molybdate to each Nessler’s tube.
Now add 0.5 mL SnCl₂.2H₂O solution (solution changes to blue colour).
Add conductivity water to make the total volume to 50 mL in Nessler’s tube.
Prepare blank in the same way except for the addition of 50 ppm phosphate solution.
Measure the absorbance at 690 nm after 10 mins using Colorimeter.
Prepare a calibration chart using the absorbance values of the standard solutions on the Y-axis and the concentration of the standard solutions on the X-axis. Use the absorbance value of the unknown phosphate solution to find the concentration from the calibration chart.