Autoionization (Dissociation) of Water

Author: John Hutchinson

Since we have the ability to measure pH for acid solutions, we can measure pH for pure water as well. It might seem that this would make no sense, as we would expect [H₃O⁺] to equal zero exactly in pure water. Surprisingly, this is incorrect: a measurement on pure water at 25°C yields pH=7, so that [H₃O⁺]=1.010^-7 M. There can be only one possible source for these ions: water molecules. The process

H2O(l) + H2O(l) H3O+(aq) + OH-(aq)

[6]

is referred to as the autoionization or dissociation of water. Note that, in this reaction, some water molecules behave as acid, donating protons, while other acid molecules behave as base, accepting protons.

Since at equilibrium [H₃O⁺]=1.010^-7 M, it must also be true that [OH^-]=1.010^-7 M. We can write the equilibrium constant for equation 6, following our previous convention of omitting the pure water from the expression, and we find that, at 25°C,

Kw = [H3O+][OH-] = 1.010-14 M

[7]

(In this case, the subscript "w" refers to "water"). Equation 6 occurs in pure water but must also occur when ions are dissolved in aqueous solutions. This includes the presence of acids ionized in solution. For example, we consider a solution of 0.1M acetic acid. Measurements show that, in this solution [H₃O⁺]=1.310^-3M and [OH^-]=7.710^-12 M. We note two things from this observation: first, the value of [OH^-] is considerably less than in pure water; second, the autoionization equilibrium constant (the dissociation constant) remains the same at 1.010^-14. From these notes, we can conclude that the autoionization equilibrium of water occurs in acid solution, but the extent of autoionization is suppressed by the presence of the acid in solution.

We consider a final note on the dissociation of water. The pH of pure water is 7 at 25°C. Adding any acid to pure water, no matter how weak the acid, must increase [H₃O⁺], thus producing a pH below 7. As such, we can conclude that, for all acid solutions, pH is less than 7, or on the other hand, any solution with pH less than 7 is acidic.