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定性分析中,以(NH_4)_2S来沉淀阳离子第三组时,还常常加入NH_4OH。加NH_4OH的作用,被解释为防止(NH_4)_2S水解,使S~(-2)浓度增高,避免第三组硫化物沉淀不完全。这种讲法是从平衡移动的概念出发的,未用计算作量上的考虑。究竟(NH_4)_2S的水解度有多大,加NH_4OH对其水解度有多大的影响,沉淀第三组时,有无必要加入过量NH_40H?关于这些问题计算讨论如下: 一、(NH_4)_2S的水解度及溶液中S~(-2)浓度的计算:
Qualitative analysis, to (NH4) _2S to precipitate the third group of cations, often also added NH4OH. The effect of adding NH 4 OH was explained as preventing the hydrolysis of (NH 4) 2 S and increasing the concentration of S 2 to avoid the incomplete precipitation of the third group of sulfides. This argument is based on the concept of balanced movement, not calculated for consideration. What is the degree of hydrolysis of (NH_4) _2S, how much NH_4OH is affected by its degree of hydrolysis, and whether it is necessary to add excess NH_40H when the third group is precipitated? The discussion of these problems is as follows: 1. Hydrolysis of (NH_4) _2S Degree and solution of S ~ (-2) concentration calculation: