The sample was dissolved with dilute acid; in slightly acidic solution, silicic acid reacted with ammonium molybdate to generate the silicon-molybdenum heteropoly acid, it was reduced to silicon molybdenum blue with ammonium ferrous sulfate in the presence of oxalic acid, at last its absorbance was measured. Silicon that reacted with molybdate to form heteropoly acid must be orthosilicate state, so the polymerization of silicic acid in the sample dissolution process must be avoided. Silicic acid easily polymerizes in the solution, the extent of polymerization has relation to solution acidity, temperature during dissolution, time and final solution CKS silicate ion concentration%试样用稀酸溶解,在微酸性溶液中,硅酸与钼酸铵生成硅钼杂多酸,在草酸存在下,用硫酸亚铁铵还原成硅钼蓝,测其吸光度.与钼酸盐形成杂多酸的硅必须以正硅酸状态存在,所以在试样溶解过程中必须避免硅酸的聚合.在溶液中硅酸较易于聚合,其聚合程度与溶液的酸度、溶解时加热的程度和时间以及最终溶液中正硅酸离子的浓度有关
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