Hydrogen ion concentration is more conveniently expressed as pH, which is the logarithm of the reciprocal of the hydrogen ion concentration in gram moles per liter. Thus, in a neutral solution the hydrogen ion (H+) and the hydroxyl ion (OH−) concentrations are equal, and each is equal to 10−7. A pH of 7 is neutral.

An acid produces hydronium ions in solution by donating a proton (H+) to water. In contrast, a base (B) is a substance that produces hydroxide ion (OH-) by accepting a proton from water. Hydroxyl is a compound containing an oxygen atom bound covalently with a hydrogen atom.

pH = – log [H3O+]. The hydronium ion concentration can be found from the pH by the reverse of the mathematical operation employed to find the pH. [H3O+] = 10-pH or [H3O+] = antilog (- pH) Example: What is the hydronium ion concentration in a solution that has a pH of 8.34?

Key Concepts

1. The hydrogen ion concentration in a solution, [H+], in mol L-1, can be calculated if the pH of the solution is known.
2. pH is defined as the negative logarithm (to base 10) of the hydrogen ion concentration in mol L-1 pH = -log10[H+]
3. [H+] in mol L-1 can be calculated using the equation (formula): [H+] = 10-pH

If a solution has pH=8 , then its pOH is 14−8=6 and the corresponding concentration of hydroxyl OH− ions is 10−6 mol per litre.

The pH scale starts from 1 being the most acidic to 14 being the most basic. The neutral pH is 7. Thus, for a pH of 7, the H+ and the OH- concentrations are equal. Since the concentrations are equal, the ratio is equal to 1.

Electrolysis of acidified water Water is a poor conductor of electricity, but it does contain some hydrogen ions , H +, and hydroxide ions, OH -. These ions are formed when a small proportion of water molecules naturally dissociate .