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.
pH Impact on Water Quality
The pH of precipitation, and water bodies, vary widely across the United States. Natural and human processes determine the pH of water. The National Atmospheric Deposition Program has developed maps showing pH patterns, such as the one below showing the spatial pattern of the pH of precipitation at field sites for 2013.