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R9 Laboratory SOP 570
TOTAL HARDNESS, TITRIMETRIC METHOD
Summary
This method is applicable to drinking, surface, and saline waters and domestic and industrial wastes. This SOP is based on EPA Method 130.2.
The method is suitable for all concentration ranges of hardness; however, in order to avoid large titration volumes, the procedures use a sample aliquot containing not more than 25 mg CaCO3. The quantitation limit (QL) is 5 mg/L CaCO3 hardness.
High concentrations of heavy metals interferes with these procedures therefore hardness determination for samples known to contain heavy metals should be done by calculation using calcium and magnesium determined from inductively coupled plasma (ICP) analysis.
Calcium and magnesium ions are the primary sources of hardness in aqueous samples. Samples are titrated with a disodium ethylenediamine tetraacetate solution (EDTA solution). The EDTA chelates the calcium and magnesium ions in the sample. The end point of the reaction is detected by means of Calmagite indicator, which has a red color in the presence of calcium and magnesium and a blue color when the cations are chelated by EDTA. Based upon the volume of EDTA solution used, the total hardness in the sample is calculated and reported in units of mg/L CaCO3.