两性物质NaHA溶液pH近似式的物理意义及使用条件

Physical Meanings and Applicable Conditions of pH Approximate Formulas of NaHA Solution

两性物质NaHA溶液的pH计算是化学教学的重要内容,当前教科书介绍了各类p H近似公式,但对各近似式的物理意义和使用条件缺少论证,存在物理意义不清晰、使用条件错误等弊端。按照Na HA溶液呈酸性(K_(a1)K_(a2)>K_(w))或呈碱性(K_(a1)K_(a2)<K_(w))的属性,详述了Na HA溶液各p H近似式的物理意义,修正了各近似式的使用条件。c K_(a2)≥20K_(w)时,忽略碱式解离产生的OH^(-)和水离解产生的OH^(-),可得到酸性两性物质溶液p H近似式;c≥20K_(a1) (相当于cK_(b2)≥20K_(w))时,忽略酸式解离产生的H^(+)和水离解产生的H^(+),可得到碱性两性物质溶液p H近似式;最简式pH=(pK_(a1)+pK_(a2))/2可适用于酸性和碱性两性物质溶液,但其使用条件存在明显差异。本文对正确理解近似式的物理意义和准确利用近似式获得溶液p H (相对误差±5%)有重要参考价值。

Approximate calculation of the pH of a NaHA solution plays an important role in analytical chemistry.However,the physical meanings and applicable conditions of the pH approximate formulas are not demonstrated in detail in current textbooks.In this paper,the applicable conditions and physical significance of various approximate formulas are described.The approximate formulas with incorrect physical meanings and inappropriate applicable conditions should be modified.NaHA solutions can be categorized as acidic (K_(a1)K_(a2)>K_(w)) or alkaline (K_(a1)K_(a2)<K_(w)).When cK_(a2)≥20K_(w),by assuming that the production of OH^(-) from water and from HA^(-)is negligible,the approximate formula of an acidic Na HA solution is obtained.When c≥20K_(a1) (namely cK_(b2)≥20K_(w)),the approximate formula of an alkaline Na HA solution is obtained by ignoring the production of H^(+)from water and HA^(-).The formula pH=(pK_(a1)+pK_(a2))/2 is suitable for both acidic and alkaline Na HA solutions,but their applicable conditions are quite different.This paper carefully discussed the pH calculation of NaHA solutions.The computation results demonstrated that some formulas should be disabled and the related wrong statements in textbooks be corrected.