一元弱酸准确滴定的浓度下限

摘要

According to the titration rules (the pH range of titration jump should be greater than or equal to 0.60 pH units and the titration error should be less than or equal to 0.20%),we calculated concentration limits for a monoprotic acid.The results showed that if the concentration is too low (c ≤1.5×10-4mol?L-1),either the strong acid or the weak acid cannot be titrated accurately.The titration criterions for the weak acids with different dissociation constants are also different.Therefore,the titration criterion should be modified as follows:to obtain an accurate titration result,both 1.0×10-3mol·L-1c≤0.10 mol?L-1and cKa≥1.1×10-8are required.%酸碱滴定分析时,滴定突跃范围≥0.60pH单位被教科书定义为指示剂法进行准确滴定(滴定误差Et≤0.20%)的临界数据.对某一强度(Ka)的一元弱酸而言,在控制滴定突跃范围等于0.60pH单位情形下,溶液浓度越大,滴定误差越小.存在某一临界浓度c,其对应的滴定误差恰好等于0.20%.对不同强度的弱酸,本文计算出了其临界浓度c,即滴定浓度下限.计算结果表明,对于较强弱酸,其分析浓度必须大于临界浓度才可准确滴定,而仅仅满足教科书要求的判据cKa≥1.1×10-8是达不到准确滴定的要求和目标.根据不同强度弱酸对应浓度下限结果,建议滴定判别式修正为:待测酸溶液浓度介于0.10~0.0010mol·L-1之间,并且满足cKa≥1.1×10-8.