Determination of silicon dioxide in ferric oxide by polarography of silicomolybdenum blue heteropoly acid

Under acidic conditions, silicic acid radical reacts with ammonium molybdate to form a silicomolybdenum yellow heteropoly acid,which is reduced to a silicomolybdenum heteropoly blue by ascorbic acid.In NH_3·H_2O-NH_4Cl buffer(pH 9.6) the silicomolybdenum heteropoly blue can produce a sensitive polarographic wave with peak potential at-1.1 V(vs.SCE).The first derivative peak height is directly proportional to the silicon dioxide concentration in the range of 0.02-12 μg/25 mL and the detection limit was 0.012 μg/25 mL.The relative standard deviations are 0.46%0.62% and the recoveries are 97%-105%.The method has been used to the determination of silicon dioxide in the ferric oxide in the presence of EDTA as masking reagent with satisfactory results.