Precipitation of lead as lead molybdate from homogeneous solution

Abstract

Lead molybdate can be precipitated in a granular form from a homogeneous solution by taking advantage of the long induction period for the formation of the precipitate in strong acid solution. The rate of precipitation is probably controlled by the rate at which the polymolybdate species depolymerize to yield simple molybdate ions. Although the bulk (90%) of the precipitate forms at pH ca. 1.0, quantitative recovery requires final adjustment of the pH of the digested solution, either homogeneously (urea hydrolysis) or heterogeneously (dropwise addition of ammonia), to pH 3.0. Contamination by molybdic oxide occurs but both methods yield reliable results if the excess of molybdate is limited to 5% and the precipitate is ignited to 725°. Contamination by copper and cadmium was also studied.