Self Ionization of Water

Ionization Equilibrium

The bonds between water molecules rapidly break and re-form. Sometimes the bonds break in an uneven way to give solvated H3O+ and solvated HO-.

We typically abbreviate the cation part as H+(aq) and the anion part as HO-(aq) with the realization that the solvated proton and solvated hydroxide anion are complexes and stabilized by many water molecules.

Relative to water molecules, the concentration of H+(aq) and of HO-(aq) are very low, but because they are in equilibrium with liquid water, the concentrations are constant.

pH and Acids

Because of the self ionization, water is a Bronsted acid. That is, it produces H+(aq) ions in solution. We can calculate the molar concentration of solvated protons from the expression for Kw. In neutral water the concentration of H+(aq) and of HO-(aq) must be equal.

We can define a quantity related to acid concentration, pH. The pH is -1 time the log of the hydrogen ion concentration. The pH of neutral water is 7.

We can determine the pH of any solution by converting the concentration to a power of 10. The pH is then -1 time the exponent. Remember that we've used an abbreviated log table to convert whole numbers to powers of 10.

pOH and Bases

Water is also a Bronsted base because it produces HO-(aq) ions in solution.

We define pOH as (-1) times the log of the hydroxide ion concentration. The pH and the pOH of any aqueous solution are related through the Kw.

Professor Patricia Shapley, University of Illinois, 2011