What is the pKa of water?

The pKa value of water is 14.00. In the specific case of water autoprotonation, experiments and theoretical calculations have shown that the Kw of water at 25°C is 1.00×10−14, the same as its acid ionization constant, Ka. But the pKa value of water has been misleadingly given as 15.7 in many organic chemistry textbooks and some biochemistry textbooks, which is thermodynamically and chemically untenable.

Here, we trace this error back to Brønsted's early contributions to the Brønsted-Lowry acid-base theory in the 1920s. Organic chemists typically defend the use of these values by citing measurements of the equilibrium constant for the water + methanol acid-base reaction, which showed that methanol (pKa = 15.54) is stronger than water; from this, organic chemists conclude that the pKa of water must be 15.74, not 14.00. Here we discuss the issues that invalidate this conclusion, the most important of which is that it is based on the use of the standard state for pure liquids (molar fraction = 1), which is completely different from the standard state for acidity determined in dilute solutions (molarity = 1). Using the latter standard state, the equilibrium constants for the water/methoxide reaction range from 4 to 70, indicating that water is more acidic than methanol and justifying the use of the thermodynamically correct value of pKa(H2O) = 14.00.

References:

[1] TODD P. SILVERSTEIN* Stephen T H. pKa Values in the Undergraduate Curriculum: What Is the Real pKa of Water?[J]. Journal of Chemical Education, 2017, 94 6: 690-695. DOI:10.1021/acs.jchemed.6b00623.

[2] THOMAS L. NEILS Todd P S Stephanie Schaertel. The pKa of Water and the Fundamental Laws Describing Solution Equilibria: An Appeal for a Consistent Thermodynamic Pedagogy[J]. Helvetica Chimica Acta, 2024, 107 11. DOI:10.1002/hlca.202400103.

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