The quantity of hydrogen or hydroxyl ions in a solution determines whether the solution is acid or alkaline. Using a logarithmic scale, pH measures, the relative alkalinity or acidity of a solution. The pH scale was first proposed by S.P.L Sorensen, a Dane, in 1909 to identify the concentration of hydrogen ions (H+) and hydroxide ions (OH -).
Of the several methods used to measure pH, two of the simplest are litmus paper and liquid acid-base indicators. Two types of litmus paper, red and blue, are coated with a dye that changes color in the presence of acids and bases. Red litmus paper will turn blue in the presence of a base. Blue litmus paper will turn red in the presence of an acid.
pH ranges from 0 to 14 (totally acid to totally alkaline). This range is shown in the pH chart below. Pure water has a pH very near 7 that is neutral.
Human blood has a normal level of 7.35 to 7.4. The body chemistry works to keep the blood in this range despite the intake of various foods with wide pH ranges.