The potential of hydrogen. A "neutral" pH is expressed as 7.0 (water), with greater being alkaline and lesser being acidic. Expressed logarithmically like the Richter's Scale, 6.9 pH is twice as acidic as 7.0. 9.0 is ten times as alkaline as 8.0, etc., all based on the presumed amount of hydrogen ion (acidity) present. This is a chemical literality, not to be confused with the vitalist and cyto-hologrammic implications of Acid and Alkaline metabolism or foods. A complex protein has a literal pH close to neutral. Run it through your body and it gets broken down into an incredible array of amino acids, ending up as nitrogenous acid waste products. The more rapid the metabolism, the more acids are produced...the ashes of life are acids. The literal pH of the life media, such as blood, lymph and cytoplasm...and most food, is alkaline. This acid/alkaline is a concept only applicable "in vivo"; pH defines acid/alkaline "in vitro".