Determination of phosphate and silicate in lead-acid electrolyte using molybdenum blue

Abstract Simple and reliable methods are described for the routine determination of phosphate and silicate content of the lead-acid cell electrolyte, based on the well-known molybdenum blue complex. The molybdate complexes are formed in the same acid electrolyte and a solution of iron(II) and fluoride ions is used as reductant for the silicomolybdic acid, while the phosphomolybdic acid is reduced by an ascorbic acid-potassium antimonyltartrate solution. Absorbance is respectively read at 828 (Si) and 882 nm (P). Solutions are quite stable and permit determinations down to 5 μg liter −1 with errors less than ±4%. The addition of citrate/tartrate removes phosphate interference of silicate determination, at least at the ratios found in the lead-acid cell electrolyte. Silicate, however, does not interfere with phosphate determination. The effect caused by several other ions has been determined.