Hydronium (H3O+), Hydroxide (OH-), and pH
A water molecule (H2O) and a hydrogen ion (H+) combine to form hydronium ions (H3O+).
The pH scale is used to express the acidity or basicity of a solution in aqueous solutions and is based on the concentration of hydronium ions. The pH scale, which ranges from 0 to 14, measures the amount of hydronium ions present in a solution. A pH of 7 is regarded as neutral, meaning the amount of hydronium ions and hydroxide ions (OH-) in the solution are equal.
A pH value of less than 7 denotes an acidic solution, meaning that the ratio of hydronium to hydroxide ions is higher. On the other hand, a pH value greater than 7 denotes a basic solution, which means that the ratio of hydroxide to hydronium ions is higher. Essentially, measuring the concentration of hydronium ions is equivalent to measuring the concentration of protons/hydrogen ions (H+).
When water and many bases dissociate, negatively charged ions called hydroxide ions (OH-) are formed. In other words, together with hydronium ions (H3O+), they are a byproduct of the self-ionization of water. An increase in hydronium ion concentration causes a reduction in hydroxide ion concentration and vice versa in an aqueous solution because the concentration of hydroxide ions is inversely proportional to the concentration of hydronium ions. Since the pH of a solution is a measure of its hydronium ion concentration, it is also tangentially connected to the concentration of hydroxide ions. Lower pH values, or higher hydronium ion concentrations, result in lower concentrations of hydroxide ions, and vice versa.