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Measurement of Phosphate of Water and Waste Water samples.

 

 Phosphate analysis of Water and Waste Water samples.

PURPOSE:

                    To describe the Laboratory standard operating procedure for measurement of Phosphate of Water and Waste Water samples.

                     

 SCOPE:

                    Applicable to the Laboratory for analysis of Water and Waste Water samples, where measurement of Phosphate is required.

 

 RESPONSIBILITY:

Lab chemist

:

Analysis of sample for measurement of Phosphate.

Technical Manager

:

Review of activity

Quality Manager

:

Implementation and compliance of SOP.

                     PROCEDURE:

1.     Principle:

·       Molybdophosphoric acid is formed and reduced by stannous chloride to intensely colored molybdenum blue. 



2.     Instrument and Equipment:

·       UV VIS spectrophotometer

3.     Reagents:

·       Phenolphthalein indicator:

·       Sulfuric acid Solution: Carefully add 300ml conc. H2SO4 to approximately 600ml distilled water and dilute it to 1 L with distilled water.

·       Ammonium persulfate, (NH4)2S2O8 or potassium persulfate K2S2O8,solid

·       Standard Phosphate Solution: Dissolve 219.5 mg anhydrous KH2PO4 in distilled water & dilute it to 1000ml. This solution contains 50.0 mg PO4 3- P

·       Ammonium Molybdate Reagent: Dissolve 25gm (NH4)6Mo7O24.4H2O in 175ml distilled water. Cautiously add 280ml conc H2SO4 to 400ml distilled water. Cool, add molybdate solution, and dilute it to 1000ml.

·       Stannous Chloride: Dissolve 2.5gm fresh SnCl2.2H2O in 100ml glycerol. Heat in a water bath and stir with glass rod to hasten dissolution.

4.     Test Method:

Pretreatment of sample:

·       Take 50ml of thoroughly mixed sample into 150 ml beaker.

·       Add 0.5ml Phenolphthalein indicator; if pink colour develops, add H2SO4 solution drop wise to just discharge the colour. Then add 1ml excess H2SO4

·       Add either 0.4 g (NH4)2S2O8 or 0.5 g K2S2O8.

·       Boil gently on a preheated hot plate for 30-40 minutes or until the final volume of 10 ml is reached.

·       Cool and dilute to 30ml with distilled water.

·       Add 0.05 ml Phenolphthalein indicator, and neutralize to a faint pink color.

·       Make up to 100 ml with distilled water.

Preparation of calibration curve:

·       Take 10ml of standard phosphate solution & dilute it to 100ml with distilled water. This solution contains 5 mg PO4 3- P.

·       Prepare 100ml distilled water blank & a series of 100ml phosphate standard containing 10,20,30,40,50 mg .

·       Add 4.0ml ammonium molybdate reagent   & mix thoroughly.

·       Add 0.5ml stannous chloride reagent & mix thoroughly.

·       Measure the color photometrically at 690nm after 10min but before 12 min.

Color development:

·       Place a 100-mL sample, or a suitable portion diluted to 100 ml.

·       Add a drop of Phenolphthalein indicator. If sample turns pink, add a drop of strong acid solution to discharge its color.

·       Follow the same procedure described for calibration

 

 

 

5. Calculation:

·    P (mg/L)    =       mg obtained from graph

                                                    -----------------------------------------------

                                                         Volume of sample ml


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