1. Hydrogen ion concentration or pH value is the logarithm of the reciprocal of the gram ionic hydrogen equivalents per liter: pH = log [1/(H+) per liter]. Water has a concentration of H+ ion of 10 - 7 moles per liter or a pH value of 7. An indication of the relative acidity or alkalinity. Values below 7 are increasing acid; above 7 increasing alkaline.