关于一元酸准确滴定判据的商榷

Reconsideration of the Criterion for Accurate Titrations of Monoprotic Acids

化学分析教科书中给出的弱酸能被准确滴定的条件是cKa≥1.0×10-8。本文根据这一判据的来源(滴定突跃≥0.3pH单位),对一元酸体系的滴定情况进行了严格数学处理。计算结果表明,只有同时满足弱酸的浓度c>1.0×10-3mol/L和cKa≥1.0×10-8条件下,弱酸才能被准确滴定。对于浓度极稀的酸溶液(c≤1.4×10-4mol/L),无论强酸还是弱酸都不能被准确滴定。对教材很少涉及的不同浓度酸碱互相滴定的情况也给出了有价值的结论。

As described in chemical analysis textbooks,the accurate titrations of monoprotic acids should fulfill the following criterion:cKa≥1.0 × 10-8,where c is the concentration of the acid,Kathe dissociation constant.According to the titration rules(the pH range of titration jump should be greater than or equal to 0.3 pH units),we calculated the pHs of some acid-base titration systems.The results showed that if the concentration is very low(c≤1.4 × 10-4mol /L),either the strong acid or the weak acid cannot be titrated accurately.Therefore,the titration criterion should be modified as follows:to obtain an accurate titration result,both c > 1.0 × 10-3mol /L and cKa≥1.0 × 10-8is required.