Abstract It is difficult to give a clear and accurate criterion for ignoring the second dissociation process during the hydrogen ion concentration estimation for a weak diprotic acid, because the analytical concentration c, the dissociation constants of Ka1 and Ka2 will influenced this criterion in a very sophisticated manner. In order to solve this tough but important problem, an imaginary acid A, acid B and a special diprotic acid were designed and their properties were investigated. The results of this paper are as follows: 1) The larger of $\frac{[H^{+}]_{AV}}{2K_{a2}}$,the better of the approximate result, where the approximate value of $[H^{+}]_{AV}$ is given by $[H^{+}]_{AV}$=$\frac{-K_{a1}+\sqrt{K^{2}_{a1}+4cK_{a1}}}{2}$. 2) The smaller of $\frac{K_{a1}}{K_{a2}}$ is, the more stringent criterion needs. 3) The criterion for the special diprotic acid $\lgroup{\frac{K_{a1}}{K_{a2}}=1}\rgroup$ is $\frac{2K_{a2}}{[H^{+}]_{AV}}$≤0.01,this rule can apply to all diprotic acids. 4) The criterion of $\frac{2K_{a2}}{[H^{+}]_{AV}}$≤0.01 is powerful and robust, it can be widely used to make a right decision whether we ignore the second dissociation process or not.
CAI Jun-Xian, LIU Jian-Yu, WANG Hai-Shui. The Constraints for Ignoring the Second Dissociation Process and Estimating the Hydrogen Ion Concentration for A Weak Diprotic Acid Solution[J]. Chinese Journal of Chemical Education, 2020, 41(4): 21-25.
