- pH is determined by the negative log of the hydrogen ion (H+) concentration in a solution, while pOH is determined by the negative log of the hydroxide ion (OH-) concentration. - The pH of water is 7 because at 25°C, the H+ concentration is 1.0 x 10-7 and the log of that value is 7. However, pH varies with temperature as the self-ionization constant of water (Kw) changes with temperature. - pOH can be calculated from pH using the relationship that pH + pOH must equal 14 for any aqueous solution.