Hardness is due to the presence of multivalent metal ions which come from minerals dissolved in the water. Hardness is based on the ability of these ions to react with soap to form a precipitate or soap scum.
In fresh water the primary ions are calcium and magnesium; however iron and manganese may also contribute. Carbonate hardness is equal to alkalinity but a non-carbonate fraction may include nitrates and chlorides.
Methodology: This is an electrochemical procedure. The technique for analysis uses potentiometric titration on the computer aided titrimeter (CAT) with a copper ion-specific electrode. A reference substance, EDTA, is used as a titrant. Hardness is expressed in mg/L of CaCO3 (even though all the hardness may not be due to CaCO3 ).
| Concentration mg/L CaCO3 | Description |
| 0 - 75 | soft |
| 75 - 150 | moderately hard |
| 150 - 300 | hard |
| 300 and up | very hard |
Environmental Impact: The most important impact of hardness on fish and other aquatic life appears to be the affect the presence of these ions has on the other more toxic metals such as lead, cadmium, chromium and zinc. Generally, the harder the water, the lower the toxicity of other metals to aquatic life. In hard water some of the metal ions form insoluble precipitates and drop out of solution and are not available to be taken in by the organism. Large amounts of hardness are undesirable mostly for economic or aesthetic reasons. If a stream or river is a drinking water source, hardness can present problems in the water treatment process. Hardness must also be removed before certain industries can use the water. For this reason, the hardness test is one of the most frequent analyses done by facilities that use water.
Criteria: There is no criteria for hardness.