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Citation: Liu Qing, Wang Haishui. The Universal Approximation Calculation: Range of Abrupt Change in Titration of a Monoprotic Weak Acid[J]. Chemistry, ;2020, 83(5): 476-479. shu

The Universal Approximation Calculation: Range of Abrupt Change in Titration of a Monoprotic Weak Acid

Liu Qing ¹ ,
Wang Haishui ^2,,

1.
Analytical and Testing Center, South China University of Technology, Guangzhou, 510640
2.
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640

Corresponding author: Wang Haishui, wanghsh@scut.edu.cn
Received Date: 5 November 2019
Accepted Date: 26 January 2020

The range of abrupt change in the titration curve plays a key role in selecting an indicator, and also is the important factor to determine whether or not the titration proceeds accurately. Before the stoichiometric point of the chemical reaction, the approximate formula \begin{document}$\mathrm{pH}=\mathrm{p} K_{\mathrm{a}}+\lg \frac{c_{\mathrm{B}^{-}}}{c_{\mathrm{HB}}} $\end{document} can be normally used to obtain the solution pH. However, if[OH^-] is close to c_HB or[H⁺] approaches c_HB, the above approximation should be a serious problem. In this paper, pH approximation formulas for monoprotic weak acid with different concentrations are derived, which satisfy the error requirement (within ±0.02 pH). Before the stoichiometric point (a=0.999), if the solution is acidic (pK_a ≤ 4.0 case), \begin{document}$\left[\mathrm{H}^{+}\right]=\frac{10^{-3} c K_{\mathrm{a}}}{c+2 K_{\mathrm{a}}} $\end{document}; if the solution is alkaline (pK_a>4.0 case), \begin{document}$\left[\mathrm{H}^{+}\right]=\frac{\left.c K_{\mathrm{a}}+\sqrt{c K_{\mathrm{a}}\left(c K_{\mathrm{a}}+8 \times 10^{-8}\right)}\right)}{2000 c} $\end{document}. After the stoichiometric point (a=1.001), \begin{document}$ \left[\mathrm{H}^{+}\right]=\frac{\left.-c K_{\mathrm{a}}+\sqrt{c K_{\mathrm{a}}\left(c K_{\mathrm{a}}+8 \times 10^{-8}\right)}\right)}{2000 c} $\end{document}. The three equations works more properly than the formula \begin{document}$ \mathrm{pH}=\mathrm{p} K_{\mathrm{a}}+\lg \frac{c_{\mathrm{B}}-}{c_{\mathrm{H} \mathrm{B}}}$\end{document}.
- Titration,
- Monoprotic weak acid,
- Stoichiometric point,
- Range of abrupt change
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