1.1 This International Standard specifies a method of preparing water of known calcium hardness for use in testing surface active agents and products containing them. 1.2 Systematic investigations have shown that, in many tests on surface active agents with hard water, there is no essential difference between calcium hardness and magnesium hardness, so that these tests can usually be carried out with an aqueous solution of calcium chloride of known hardness. If, in certain cases, it is necessary to use other ions that give rise to the hardness of water, this fact shall be mentioned in the test report.