Molybdates [Mo(VI)] are quantitatively reduced in 2M hydrochloric acid solution at 60-80 °C by the silver reductor to Mo(V). The reduced molybdenum solution is sufficiently stable over short periods of time in air to be titrated with standard cerium(IV) sulphate solution using ferroin or N-phenylanthranilic acid as indicator. Nitric acid must be completely absent; the presence of a little phosphoric(V) acid during the reduction of the molybdenum (VI) is not harmful and, indeed, appears to increase the rapidity of the subsequent oxidation with cerium(IV) sulphate. Elements such as iron, copper, and vanadium interfere; nitrate interferes, since its reduction is catalysed by the presence of molybdates.
Weigh out accurately about 2.5 g ammonium heptamolybdate
(NH4)6Mo7O24,4H2O,
dissolve in water and make up to 250 mL in a graduated flask. Pipette 50 mL of
this solution into a small beaker, add an equal volume of 4M hydrochloric acid,
then 3 mL of 85 per cent phosphoric(V) acid, and heat the solution to 60-80°C.
Pour hot 2M hydrochloric acid through a silver reductor, and then pass
the-molybdate solution through the hot reductor at the rate of about 10 mL min -1.
Collect the reduced solution in a 500 mL beaker or 500 mL conical flask, and
wash the reductor with six 25 mL portions of 2M hydrochloric acid; the first
two washings should be made with the hot acid (rate: l0mL min-1) and
the last four washings with the cold acid (rate: 20-25 mL min-1).
Cool the solution, add one drop of ferroin or 0.5 mL N-phenylanthranilic acid,
and titrate with standard 0.1M cerium(IV) sulphate. The precipitate of cerium
(IV) phosphate, which is initially formed, dissolves on shaking. Add the last
0.5 mL of the reagent dropwise and with vigorous stirring or shaking.