一、引言 常见阳离子的系统分析,一般教科书多采用经典的硫化氢系统分组法进行分离。对于第四组阳离子C_a~(2+)、S_r~(2+)、B_a~(2+)的分离,是利用其碳酸盐难溶于水的特性,采用(NH_4)_2CO_3作组成试剂进行分离。由于(NH_4)_2CO_3是弱酸弱碱盐,在溶液中水解趋势很大: NH_4~++CO_3~(2-)=NH_3+HCO_3~- K_t=Ka(NH_4~+)/Ka_2(H_2CO_3)=10 为了有足够的碳酸根供沉淀需要,应加入氨 I. INTRODUCTION Systematic analysis of common cations, the general use of the classic textbook of hydrogen sulfide system grouping method to separate. For the fourth group of cations C_a ~ (2 +), S_r ~ (2 +) and B_a ~ (2+) were separated by using the characteristic of poor solubility of carbonate in water and using (NH_4) _2CO_3 as reagent Separated. Since (NH 4) 2 CO 3 is a weakly acidic weak base salt, the tendency of hydrolysis in solution is very large: NH 4+ ++ CO 3 2- = NH 3 + HCO 3+ - K_t = Ka (NH 4+) / Ka 2 (H 2 CO 3) = 10 In order to have enough carbonate for precipitation, ammonia should be added