The pH of a solution is defined as the negative logarithm of its hydrogen ion (H+) concentration. For example, If the concentration of H+ ion is 10^(-7), Therefore, as the Hydrogen Ion concentration increases, pH value decreases.he pH of a solution is defined as the negative logarithm of its hydrogen ion (H+) concentration. For example, If the concentration of H+ ion is 10^(-7), Therefore, as the Hydrogen Ion concentration increases, pH value decreases.