Hydronium ions (`H_3O^+` ) are formed when an acid dissolves in the water. The reaction will result in the release of hydrogen ions (`H^+` ) which will combine with water molecule to generate the hydronium ions. In water, hydrogen ion does not exist, due to high charge density, it bonds with water and gives us hydronium ion. In water, at a given pH, the concentration of hydronium and hydroxide (`OH^-` ) ions is always related. This relationship is given by the ionic product of water as:
`k_w = [H_3O^+][OH^-] = 10^(-14)`
at 25 degree Celsius.
The concentration of hydronium ions is given by pH measurement. The pH is related to hydronium concentration as:
`pH = -log [H_3O^+]`
pH can be measured by a number of methods, including pH strips and pH electrodes (most commonly used).
Hope this helps.