Potential hydrogen, or pH, is the measure of the acidity or alkalinity of an aqueous solution. A solution is acidic if its pH is less than 7. If its pH is greater than 7, the solution is basic or alkaline.
To measure pH, certain indicators are used because their color changes with pH. Some common pH indicators include thymol blue, litmus, methyl red and phenolphthalein. Litmus is the most commonly used pH indicator because it changes color around pH 7. It is a blue dye extracted from certain lichens, and it is used to make litmus paper, which is commonly used to test the pH of liquids. When blue litmus paper is immersed in an acidic solution, it turns red, and when red litmus paper is immersed in an alkaline solution, it turns blue. A universal indicator, which consists of a mixture of indicators, produces more accurate measurements. It displays a variety of colors over the pH scale, which ranges in number from 1 to 14.
On the pH scale, blood has a pH level of about 7.34, and gastric acid has a pH level of 1. Battery acid, lemon juice, vinegar, soda and tomato juice are all acidic as each has a pH that's less than 7. Bleaches, ammonia solution, baking soda and sea water are bases as each has a pH greater than 7. Water has a pH of about 7 at 25 degrees centigrade, and it is considered to be neutral. However, at 50 degrees centigrade, the pH of water decreases to about 6.55.