Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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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 µm 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 wash- ing 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: P%= (A - B) x 0.0164 x 100 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 selec- tive. 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 mea- surement 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 145

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