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H3PO4 + 12(NH4)2MoO4 + 21HNO3 = (NH4)3H4[P(Mo2O7)6] + 21NH4NO3 + 10H2O Filter the solution through weighted glass filter (���F��� or ���4��� porosity) with pore size 5 ��m maximum. Wash the precipitate 6-8 times with small portions of the solution (2% HN03 + 5% NH4N03) and cool deionized water (CO2 free) until the washings are free of acid, as indicated by phenolphtalein test (5 drops of washing water + phenolphtalein + 1 drop of 0.1 N NaOH = pink color). Dry the filter and the precipitate in the oven at 110��C for 1-2 hours. Cool in a dessicator and weigh. Calculation: (A - B) x 0.0164 x 100 P%= C Where: A - weight of filter and precipitate (NH4)3P04.12Mo03, g. B - weight of filter, g. C - weight of sample, taken in aliquote of 20 ml, g. Photometric method has the advantage of economy in apparatus and is selective. It was found that better precision and greater sensitivity were obtainable by using the determination of phosphorus by the molybdivanadophosphoric acid method [4]. REAGENTS Phosphorus solution (0.4 mg/ml): Transfer 40 ml of 1 mg/ml standard solution into a 100- ml volumetric flask. Add 15 ml (1:5) HN03 , dilute to mark and mix. Ammonium molybdate solution (100 g/l): Dissolve 100 g of ammonium molybdate [(NH4)6Mo7O24 x 4H2O] in 600 ml of water at 50��C and dilute to 1 liter. Filter before using. Ammonium vanadate solution (2.5 g/I): Dissolve 2.5 g of NH4V03 in 500 ml of hot water. When dissolution is complete, add 20 ml of (1:1) HN03 , cool and filter if necessary and dilute to 1 liter. SUMMARY OF PHOTOMETRIC METHOD Yellow colored complex is formed when an excess of molybdate solution is added to an acidified mixture of vanadate and orthophosphate. Photometric measurement is made at approximately 470 nm. PROCEDURE Transfer 1.5 ml aliquote sample (from solution A) to a 150-ml flask and add 20 ml of (1:5) HN03 and cover. Boil gently for about 1-2 minutes. Add 2 ml of 1% KMn04 solution and heat just to boiling. Add 1ml of 3% H202 solution and swirl the flask until excess of KMn04 is destroyed and the solution clears. Add 10 ml 272

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