Standard Method # 2340. Hardness is due to the presence of multivalent metal ions which come from minerals dissolved in water. Hardness is based on the ability of these ions to react with soap to form a precipitate. Hardness is also a very important indicator of the ability of the water to deposit 'scale' or Ca/Mg salts over time in plumbing fixtures, therefore, the practice is to express hardness in terms of mg/L CaCO3, as calculated by either titration or ICP-AES analysis. In addition to the routine volumes and concentrations listed below, additional AccuSPEC Standards and Reagents are available in the "Standards and Reagents" Catalog.