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络合滴定钙的指示剂中,以钙黄绿素灵敏度较高。但此指示剂色变欠佳,为了增加终点变色的对比度,早期文献有加入百里酚酞作为衬色指示剂。百里酚酞在强碱性溶液中为纯蓝色,使钙络合滴定终点呈现红紫色。该混合指示剂实际对比度也不太理想,另外百里酚酞比例用量,对终点变化影响很大;量少,钙黄绿素残余萤光不易消除。量大,又使终点蓝色太深变化不明显。以后,一些文献在钙黄绿素一百里酚酞混合指示剂基础上加入酚酞,紫尿酸铵等做为三色混合指示剂,以消除残余萤光,增加终点的敏锐性。
Complexometric titration of calcium indicator to calcein sensitivity. However, this indicator is poorly pigmented and, in order to increase the contrast of the end discoloration, early literature incorporated thymolphthalein as a lining indicator. Thymolphthalein in strong alkaline solution is pure blue, calcium complexation titration end presents purple. The mixed indicator of the actual contrast is not ideal, while the amount of thymolphthalein dosage, the end point of a great impact on changes; less calcein residual fluorescence is not easy to eliminate. Large amount, but also so that the end of the blue is too dark not obvious changes. Later, some documents in the calcein thymolphthalein mixed indicator based on the addition of phenolphthalein, ammonium perrhenate as a three-color mixed indicator to eliminate residual fluorescence, increase the sharpness of the end point.