The amount of hydronium ions (H3O+) and hydroxide ions (OH-) in a solution is inversely related. This relationship is determined by the pH of the solution. pH is a measure of the acidity or basicity of a solution and is defined as the negative logarithm of the hydrogen ion concentration ([H+]).

In pure water at 25 °C, the concentration of hydronium ions is equal to the concentration of hydroxide ions, and both are very low (approximately 1 x 10^-7 moles per liter). This means that pure water is neutral, with a pH of 7.

When the concentration of hydronium ions increases, the pH of the solution decreases, indicating an acidic solution. Conversely, when the hydroxide ion concentration increases, the pH increases, and the solution becomes basic or alkaline.

The relationship between hydronium ions and hydroxide ions can be expressed mathematically using the following equation:

[H3O+] × [OH-] = Kw

where Kw is the ion product constant for water, which is a constant value at a given temperature (approximately 1 x 10^-14 at 25 °C).

This equation shows that if the concentration of hydronium ions increases, the concentration of hydroxide ions must decrease to maintain the constant value of Kw. Similarly, if the hydroxide ion concentration increases, the hydronium ion concentration must decrease.

Therefore, in an acidic solution, the hydronium ion concentration is higher, while the hydroxide ion concentration is lower. In a basic solution, the hydroxide ion concentration is higher, while the hydronium ion concentration is lower. In a neutral solution, both hydronium and hydroxide ion concentrations are very low and equal.