The relative acidity of any solution expressed in a pH range of numbers. The pH value is the negative common logarithm of the hydrogen-ion concentration in a solution, expressed in moles per liter of solution. A neutral solution, that is, one that is neither acidic nor alkaline, such as pure water, has a concentration of 10 moles per liter; its pH is thus 7. Acidic solutions have pH values ranging with decreasing acidity from 0 to nearly 7; alkaline or basic solutions have a pH ranging with increasing alkalinity from just beyond 7 to 14. In seawater, the alkalinity is a measure of the excess of hydroxyl ions over hydrogen ions, generally expressed as milliequivalents per liter.