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Citation: Xu Lin, Wang Haishui. The Constraints for Estimating the pH of a Weak Monoprotic Acid Solution[J]. Chemistry, ;2017, 80(11): 1077-1079. shu

The Constraints for Estimating the pH of a Weak Monoprotic Acid Solution

Xu Lin ,
Wang Haishui ^,

School of Chemistry & Chemical Engineering, South China University of Technology, Guangzhou 510640

Corresponding author: Wang Haishui, wanghsh@scut.edu.cn
Received Date: 11 March 2017
Accepted Date: 12 September 2017

It is a routine work to determine the pH of a weak monoprotic acid in analytical chemistry. Based on the concentration (c) and dissociation constant (K_a) of a weak monoprotic acid, its pH value can be estimated by a quadratic equation. In this paper, we evaluate the working conditions of the equation for pH determination and reach the following conclusions:(1) In general, the pH of all monoprotic acid solutions can be obtained approximately if their concentrations are higher than 6×10^-6 mol·L^-1. If cK_a ≥ 10K_w, then \begin{document} $[{{\rm{H}}^ + }] = \frac{1}{2}(-{K_{\rm{a}}} + \sqrt {K_{\rm{a}}^2 + 4c{K_{\rm{a}}}})$ \end{document}; (2) if K_a ≥ 19c, then [H⁺]=c; (3) if c ≥ 105K_a, then \begin{document} $[{{\rm{H}}^ + }] = \sqrt {{K_{\rm{w}}} + c{K_{\rm{a}}}} $ \end{document}. Moreover, the physical meanings of the above equations were also given in this paper.
- Weak monoprotic acid,
- pH,
- Approximation calculation,
- Constraint,
- Physical meaning
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