A measure of the degree of the acidity or the alkalinity of a solution as measured on a scale (“pH scale”) of 0 to 14. The midpoint of 7.0 on the pH scale represents neutrality–that is, a “neutral” solution is neither acid nor alkaline. Numbers below 7.0 indicate acidity; numbers above 7.0 indicate alkalinity. It is important to understand that pH is a measure of intensity, not of capacity. That is, pH indicates the intensity of alkalinity or acidity in the same way temperature tells how hot something is but not how much heat the substance carries.
More specifically, pH is the negative of the logarithm of the hydrogen ion concentration of a solution. The hydrogen ion concentration is the weight of hydrogen ions, in grams, per liter of solution. In neutral water, for example, the hydrogen ion concentration is 107- grams per liter; the pH is therefore 7.
Since it is hydrogen that is responsible for acidity and alkalinity, the abbreviation “pH” stands for “potential of hydrogen.” The neutral point of 7.0 actually indicates the presence of equal concentrations of free hydrogen and hydroxide ions.