The platinum solution is treated with formic acid, best at pH 6, and the precipitated platinum weighed. A 2 per cent solution of hypophosphorous (phosphinic) acid may also be used as the reducing agent.
In this determination any excess of nitric and/or hydrochloric acid present must be removed. Evaporate the solution of platinum, containing no other platinum metals (ruthenium, rhodium, palladium, osmium, and iridium) or gold, to a syrup on the steam bath so as to remove as much hydrochloric acid as possible. If nitric acid was present, dissolve the residue in 5 mL of water, heat on the water bath for a few minutes, add 5 mL of concentrated hydrochloric acid, and again evaporate to a syrupy consistency. Dissolve the residue in water, and dilute so that the solution does not contain more than 0.5 g of Pt in 100 mL. For each 100 mL of solution, add 3g of anhydrous sodium acetate and 1 mL of formic acid. Heat on the boiling water bath for several hours. Filter through a quantitative filter paper. Add a little more sodium acetate and formic acid to the filtrate and digest in order to ensure complete precipitation. Wash the precipitate with water until free from chloride, dry and ignite the filter paper in contact with the precipitate to constant weight. Weigh as metallic Pt.