The pH scale

Water auto-ionizes to form a small amount of H⁺ and OH^-.  In pure water the H⁺ concentration equals the OH^- concentration.  In water the concentrations of H⁺ and OH^- are in a delicate balance, if we increase H⁺ concentration the OH^- will decrease.  If we increase OH^- concentration the H⁺ will decrease.  Look at the chart below.

In pure water, the H⁺ concentration is 1x 10^-7.  As we add an acid the H⁺ concentration will increase and the pH will decrease. The pH is defined as the negative log of the H⁺ concentration. As we add base the H⁺ concentration will decrease and the pH will increase.

In an acidic solution the H⁺ concentration is greater than the OH^- concentration. In an basic solution the OH^- concentration is greater than the H⁺ concentration.

Please watch animation 10.5 on indicators.