Source: Laboratory of Dr. Zhongqi He - United States Department of Agriculture

Acids and bases are substances capable of donating protons (H⁺) and hydroxide ions (OH^-), respectively. They are two extremes that describe chemicals. Mixing acids and bases can cancel out or neutralize their extreme effects. A substance that is neither acidic nor basic is neutral. The values of proton concentration ([H⁺]) for most solutions are inconveniently small and difficult to compare so that a more practical quantity, pH, has been introduced. pH was originally defined as the decimal logarithm of the reciprocal of the molar concentration of protons , but was updated to the decimal logarithm of the reciprocal of the hydrogen ion activity . The former definition is now occasionally expressed as p[H]. The difference between p[H] and pH is quite small. It has been stated that pH = p[H] + 0.04. It is common practice to use the term 'pH' for both types of measurements.

The pH scale typically ranges from 0 to 14. For a 1 M solution of a strong acid, pH=0 and for a 1 M solution of a strong base, pH=14. Thus, measured pH values will lie mostly in the range 0 to 14, though values outside that range are entirely possible. Pure water is neutral with pH=7. A pH less than 7 is acidic, and a pH greater than 7 is basic. As the pH scale is logarithmic, pH is a dimensionless quantity. Each whole pH value below 7 is 10x more acidic than the next integer. For example, a pH of 4 is 10x more acidic than a pH of 5 and 100x (10 x 10) more acidic than a pH of 6. The same holds true for pH values above 7, each of which is 10x more basic (or alkaline) than the next lower whole value. For example, a pH of 10 is 10x more basic than a pH of 9.