When water passes through or over deposits such as limestone, the levels of Ca2+, Mg2+, and HCO3 ions present in the water can greatly increase and cause the water to be classified as hard water. This term results from the fact that calcium and magnesium ions in water combine with soap molecules, making it “hard” to get suds. In Test 13, Calcium and Water Hardness, an Ion-Selective Electrode was used to determine calcium hardness, in mg/L as CaCO3. In this test, total hardness will be determined. Total hardness is defined as the sum of calcium and magnesium hardness, in mg/L as CaCO3. In addition to total hardness, the test described here will allow you to determine the concentration of Mg2+, in mg/L.

High levels of hard-water ions such as Ca2+ and Mg2+ can cause scaly deposits in plumbing, appliances, and boilers. These two ions also combine chemically with soap molecules, resulting in decreased cleansing action. The American Water Works Association indicates that ideal quality water should not contain more than 80 mg/L of total hardness as CaCO3. High levels of total hardness are not considered a health concern. On the contrary, calcium is an important component of cell walls of aquatic plants, and of the bones or shells of aquatic organisms. Magnesium is an essential nutrient for plants, and is a component of chlorophyll.


  • Determine the total water hardness in a stream or lake.