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当钙镁比率接近1:100时,pH>12测钙,由于析出大量氢氧化镁沉淀吸附钙离子和指示剂,使低量钙难以测准。pH10时,镁不沉淀,钙镁与EGTA形成的络合物在稳定度上有较大差异,是直接滴定钙的最佳条件。但在此条件下,很难找到一种指示剂只对钙显色而不对镁显色。本文试验用Zn—EGTA一锌试剂—百里酚酞这一体系作简接指示剂效果较好。结果表明;在pH 9～9.5的饱和硼砂缓冲液中,以三乙醇胺-酒石酸钾钠联合掩蔽干扰元素,允许5毫克三氧化二铁、三氧化二铝,至少可在100毫克氧化镁共存下直接测定0.1毫克以上的氧化钙。以同样掩蔽条件,在pH 10
When the ratio of calcium and magnesium is close to 1: 100, the pH is more than or equal to 12, and calcium is precipitated because of precipitation of large amount of magnesium hydroxide. At pH10, magnesium does not precipitate, and the complexes formed by calcium and magnesium with EGTA have great differences in stability, which is the best condition for direct titration of calcium. However, under these conditions, it is difficult to find an indicator that only develops color with calcium without coloring the magnesium. In this paper, Zn-EGTA a zinc reagent - thymolphthalein system as a simple indicator is better. The results show that in a saturated borax buffer pH 9-9.5, triethanolamine-potassium sodium tartrate in combination with masking interfering elements allow 5 mg of ferric oxide, aluminum oxide, at least in the coexistence of 100 mg of magnesium oxide directly Determination of 0.1 mg or more of calcium oxide. With the same masking conditions at pH 10