SOP for ESTIMATION OF PHOSPHORUS BY STANNOUS CHLORIDE METHOD

                PRINCIPLE:

Molybdophosphoric Acid is formed and reduced by stannous chloride to intensely

 coloured molybdenum blue.

                 MINIMUM DETECTION LIMIT:  3 µg P/l

                 REAGENTS:

1.    Phenolphthalein indicator aqueous solution.

2.    Strong acid solution: Slowly add 300 ml conc. H₂SO₄ to about 600 ml distilled 

water. When cool, add 4.0 ml conc. HNO₃ and dilute to 1000 ml.

3.    Ammonium molybdate reagent: Dissolve 25 g ammonium molybdate in 175 ml

 distilled water. Cautiously add 280 ml conc. H₂SO₄ to 400 ml distilled water. 

Cool, add molybdate solution and dilute to 1000 ml.    

4.    Stannous chloride reagent: Dissolve 2.5 g fresh SnCl₂. 2H₂O in 100 ml glycerol.

 Heat in a water bath and stir with a glass rod to hasten dissolution. This reagent is

 stable and requires neither preservatives nor special storage.  

5.    Standard phosphate solution: Dissolve 219.5 mg anhydrous KH2PO4 and 

dilute with distilled water to 1000 ml.  (1ml.= 50 mg PO₄) 

 PROCEDURE:

1.    Sample pretreatment: To 100 ml sample containing not more than 200 mg P and

 free from colour and turbidity. Add 0.05 ml (1 drop) phenolphthalein indicator. 

If sample turns pink add strong acid solution drop wise to discharge the colour. 

If more than 0.25 ml (5 drops) is required, take a smaller sample and dilute to 

100 ml with distilled water after first discharging the pink colour with acid. 

2.    Colour development: Add with through mixing after each addition, 

4.0 ml molybdate reagent and 0.5 ml stannous chloride. Rate of colour removal

 development and intensity of colour depend on temperature of the final solution.

 Each 1^o C increases producing about 1 % increase in colour. Hence hold samples,

 standards and reagents within 2 ^o C of one another and in the 

temperature range between 20 ^oC and 30 ^oC.

3.    Colour measurement: After 10 min., but before 12 min. using 

it specified interval for all determinations.

Measure colour photo metrically at 690 nm and compare with calibration curve.

  

           CALCULATION:

                                      A                    B

     mg/L of P  =   ------------------- x -----------

                             ml of sample            C

  

Where,

            A = mg P determined from plotted curve,

            B = final volume of diluted sample (ml)

            C = Volume of diluted sample used for colour development (ml)

            REFERENCE: APHA – 4500 P D. Stannous chloride method