The only method which is at all widely used for the gravimetric determination of manganese is the precipitation as ammonium manganese phosphate, MnNH4PO4,H2O, in slightly ammoniacal solution containing excess of ammonium salts. The precipitate may be weighed in this form after drying at 100-105°C, or it may be ignited and subsequently weighed as manganese pyrophosphate, Mn2P2O7. The latter procedure is by far the better one. The method is, however, of limited application because of the interfering influence of numerous other elements. Titrimetric methods are generally preferred; the potentiometric determination of manganese.
The solution (200 mL) should be slightly acid, contain not more than 0.2 g of Mn(II) in 200 mL, and no other cations except those of the alkali metals (see Note). Almost neutralise the solution with dilute ammonia solution, add 20 g of ammonium chloride and a considerable excess of diammonium hydrogenphosphate (NH4)2HPO4 (say, 2g of the solid). If a precipitate forms at this point, dissolve it by the addition of a few drops of 1:3 hydrochloric acid. Heat the solution almost to boiling (90-95 °C), and add dilute ammonia solution (1:3) dropwise and with constant stirring until a precipitate {Mn3(PO4)2} begins to form; immediately suspend the addition of the alkali. Continue the heating and stirring until the precipitate becomes crystalline (MnNH4PO4,H2O). Then add another drop or two of ammonia solution, stir as before, etc., and so continue until no more precipitate is produced and its silky appearance remains unchanged. The precipitate must be maintained at 90-95 °C throughout; a large excess of ammonia solution must be avoided. Allow the solution to stand at room temperature (or, better, at 0 °C) for 2 hours. Filter through a weighed porcelain filtering crucible, and wash the precipitate with cold, 1 per cent ammonium nitrate solution until free from chloride. Dry at a gentle heat, and then heat at 700-800 °C (in an electric crucible furnace or within a larger nickel crucible) to constant weight. Weigh as Mn2P2O7. Alternatively, but less desirably, the precipitate in the porcelain filtering crucible may be dried at 100-105 °C to constant weight and weighed as MnNH4PO4,H2O; in this case, a sintered-glass filtering crucible may also be used.