Total hardness is defined as the sum of all alkaline earth ions; these are mainly calcium and magnesium. In solution, these are "paired" with chlorides, sulfates, carbonates and other anions. 

The best-known and most accurate determination method for total hardness is complexometric titration with an aqueous solution of the disodium salt of ethylenediaminetetraacetic acid (EDTA, trade name: Titriplex III) at a known concentration. EDTA forms soluble, stable chelate complexes with the hardness components Ca2+ and Mg2+. To the water sample to be analyzed, 25% ammonia solution, a pH 11 buffer (ammonia-ammonium acetate) and the indicator Eriochrome Black-T are added. 

The indicator forms a red colored complex with the Ca2+ and Mg2+. If these ions are bound by the EDTA at the end of the titration, the Eriochromschwarz-T is present freely and is colored green. The total hardness is calculated from the consumed ml of EDTA solution. This allows a highly accurate ACTUAL value analysis.