铜合金中锡的测定,加过量EDTA,二甲酚橙指示,Pb~(2+)滴去过量的EDTA为第一终点;加入NH_4F释放与锡络合的EDTA,Pb~(2+)滴定为第二终点。通常用硫脲-抗坏血酸混合溶液联合掩蔽主量元素铜,使用中发现第一终点很不稳定,频繁返黄,褪色现象严重。究其原因,部分Cu~(2+)-EDTA络合物,在常温下,抗坏血酸要将其中的Cu~(2+)还原为Cu~+,再让硫脲络合,速度缓慢。所以用Pb~(2+)滴定过量的EDTA时出现一系列虚假终点,频繁返黄,致使锡的测定结果大大偏高。如果改用硫脲-亚硫酸钠或者硫代硫酸钠混合溶液掩蔽铜就不会出现这种现象。试液不发生浑浊,且前后两个终点均明显稳定。注意使用硫脲-硫代硫酸钠时,有一 The determination of tin in copper alloy, plus EDTA, xylenol orange indicates that EDTA ~ (2+) drops excess EDTA as the first endpoint; the addition of NH_4F release and tin complexed EDTA, Pb ~ (2+) titration For the second end. Usually thiourea - ascorbic acid mixed solution masking the main element of copper, the use of the first end point is found to be very unstable, frequent yellowing, fading serious. The reason is that part of Cu2 +-EDTA complex, at room temperature, ascorbic acid should reduce Cu2 + to Cu2 +, then let thiourea complex slowly. So with Pb ~ (2+) titration excess EDTA when a series of false end point, frequent back to yellow, resulting in the determination of tin is much higher. If the switch to thiourea - sodium sulfite or sodium thiosulfate mixed solution masking copper will not have this phenomenon. Test fluid does not occur turbidity, and before and after the two endpoints were significantly stable. Note that when using thiourea - sodium thiosulfate, there is one