为什么在使用莫尔法测定氯离子时要控制pH值？
参考答案：
①莫尔法测定CL-必须在pH值为6.5—10.5的溶液中进行，最好pH值控制在6.5-7.2范围内
②若酸度过大，CrO42-与H+形成HCrO4-、H2Cr04，不能形成红色的Ag2CrO*4沉淀，不能指示终点；
③若碱度太强，Ag+会水解成AgOH，甚至形成A2O沉淀。如果水样中含有NH4+，在强碱性下转变为NH3，它能和Ag+形成[Ag（NH3）]+、[Ag（NH3）2]+等络离子，也使测定产生较大的正误差。
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