When working with acidic or basic solutions, another way of expressing concentration is often used and that is pH. There is more to understanding pH than knowing its definition, so let's set the stage for pH by first looking at the self-ionization of water. After a definition of pH, we will then look at how the hydronium/hydroxide ion balance determines the acidity or basicity of a solution. The concentrations of hydronium and hydroxide ions in an aqueous solution are related to one another by the ionization constant for water, K_w. That constant can be used to calculate hydronium and hydroxide ion concentrations, and these in turn can be used to calculate pH and pOH values. Each of these topics can be found in the pages of this section.