One of the common methods of removing hardness is by means of soap. When a soap, which is made with potash and soda as a base, is added to the water containing calcium and magnesium salts the soap is decomposed, and the fatty acid is set free in an insoluble state. It is upon this reaction that the old method of determining hardness, known as Clark's method, was based. This process has been varied in many ways by chemists in different countries, but the original principle has not been departed from.

The hardness is usually estimated in the number of milligrams of calcium carbonate in a standard volume of water - usually 100,000 parts. In England and America it is usually determined by the number of grains of calcium carbonate in an imperial gallon in England and a United States gallon in America, which means, in England, the number of grains of calcium carbonate in 70,000 parts, and, in America, in 58,381 parts.

The methods of determining hardness of water will be found in all works on water analysis.