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目前,用于分光光度法测定微量钍的显色剂不少,常用的有钍试剂、偶氮胂Ⅰ、偶氮胂羧、偶氮胂Ⅲ和偶氮氯膦Ⅲ。其中偶氮胂Ⅲ为最常用的显色剂,该显色剂只有在中等酸度下测钍才具有较高的灵敏度。其它显色剂在低酸介质中测钍时灵敏度并不理想。偶氮氯膦-mA是由上海师大于1979年首先合成并应用于稀土元素分析的新型显色剂,它为深紫红色结晶粉末,易溶于水,在酸性介质中与钍形成蓝紫色络合物,其结构式如下:
Currently, many spectrophotometric determination of trace amounts of thorium chromogenic agents, commonly used thorium reagents, arsenazo Ⅰ, arsenazo carboxylate, arsenazo Ⅲ and chlorophosphonazo Ⅲ. Among them, arsenazo III is the most commonly used developer, and the developer has high sensitivity only under moderate acidity. Sensitivity of other developers when measuring thorium in low acid media is not ideal. Chlorophosphonazo-mA is a new color former synthesized by Shanghai Normal University in 1979 and applied to the analysis of rare earth elements. It is a dark purple crystalline powder that is easily soluble in water and forms purple-blue with thorium in acid medium Complexes, the structural formula is as follows: