pH is a term in chemistry that is used as a unit of measurement for the acidity or basicity of solutions. The pH scale ranges from 0 to 14, with 0 as the most acidic and 14 as the most basic. A pH of 7 means the solution is neutral. Examples of acidic solutions include vinegar, lemon juice and gastric juices. Examples of basic solutions include milk of magnesia, ammonia and lye.

pH is logarithmic which means for instance, a pH value of 6 means that it is 10 times more acidic than a solution with a pH value of 7, while a solution with a pH value of 5 is 100 times more acidic than a pH value of 7.

Litmus is a compound used to determine the acidity of solutions. By applying to paper this compound that is derived from lichens, a blue litmus paper is made. A litmus paper is then dipped into a solution and when it turns red, this means that it is acidic. Interestingly, there occurs in nature a natural litmus test which is the flowering plant hydrangea. The blossoms can be either pink or blue hued, depending on the alkalinity or acidity of the soil.

pH was first used at the Carlberg laboratory in 1909 by Danish chemist Soren Peder Lauritz Sorensen. There is no final concensus on what pH actually stands for, but according to the Carlsberg laboratory, it stands for ‘power of hydrogen’.