大学化学 >> 2023, Vol. 38 >> Issue (2): 260-265.doi: 10.3866/PKU.DXHX202204021

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两性物质NaHA溶液pH近似式的物理意义及使用条件

柳青1, 王海水2,*(), 陈超3,*()   

  1. 1 华南理工大学分析测试中心, 广州 510640
    2 华南理工大学化学与化工学院, 广州 510640
    3 华南农业大学生命科学学院, 广州 510642
  • 收稿日期:2022-04-08 录用日期:2022-04-27 发布日期:2022-05-19
  • 通讯作者: 王海水,陈超 E-mail:wanghsh@scut.edu.cn;cchen@scau.edu.cn
  • 基金资助:
    中央高校基本科研业务费专项资金(2019MS031);国家自然科学基金项目(21773072)

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

Qing Liu1, Haishui Wang2,*(), Chao Chen3,*()   

  1. 1 Analytical and Testing Center, South China University of Technology, Guangzhou 510640, China
    2 School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
    3 College of Life Sciences, South China Agricultural University, Guangzhou 510642, China
  • Received:2022-04-08 Accepted:2022-04-27 Published:2022-05-19
  • Contact: Haishui Wang, Chao Chen E-mail:wanghsh@scut.edu.cn;cchen@scau.edu.cn

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摘要：

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pH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NaHA溶液呈酸性(Ka1Ka2 > Kw)或呈碱性(Ka1Ka2 < Kw)的属性，详述了NaHA溶液各pH近似式的物理意义，修正了各近似式的使用条件。cKa2 ≥ 20Kw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pH近似式；c ≥ 20Ka1 (相当于cKb2 ≥ 20Kw)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pH近似式；最简式pH = (pKa1 + pKa2)/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pH (相对误差±5%)有重要参考价值。

关键词: 两性物质, NaHA, pH近似式, 物理意义, 使用条件

Abstract:

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 (Ka1Ka2 > Kw) or alkaline (Ka1Ka2 < Kw). When cKa2 ≥ 20Kw, by assuming that the production of OH− from water and from HA− is negligible, the approximate formula of an acidic NaHA solution is obtained. When c ≥ 20Ka1 (namely cKb2 ≥ 20Kw), the approximate formula of an alkaline NaHA solution is obtained by ignoring the production of H+ from water and HA−. The formula pH = (pKa1 + pKa2)/2 is suitable for both acidic and alkaline NaHA 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.

Key words: Amphiprotic substance, NaHA, pH approximate formula, Physical meanings, Applicable conditions