University Chemistry ›› 2023, Vol. 38 ›› Issue (2): 260-265.doi: 10.3866/PKU.DXHX202204021

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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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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