Eriochromschwarz

Eriochromschwarz T, also written Eriochrom Schwarz T, is a water-soluble indicator dye used in complexometric titrations with EDTA (ethylenediaminetetraacetic acid). It is part of the Eriochrom family of indicators and is commonly employed to determine water hardness by quantifying calcium and magnesium ions.

In typical applications, the indicator forms a colored complex with Ca2+ and Mg2+ at alkaline pH. When EDTA is added, it sequesters the metal ions, releasing the indicator and causing a distinct color change at the titration endpoint. The endpoint is defined by the transition between the color of the metal–indicator complex and the color of the free indicator, with the exact hues depending on pH and the conditions of the assay. The method is widely used because it provides a simple, direct means to assess total hardness and the concentrations of calcium and magnesium in aqueous samples.

Procedure and conditions commonly involve buffering the solution to a basic pH (often around 9–11), adding a small amount of Eriochromschwarz T, and titrating with a standardized EDTA solution until the endpoint color change is observed. The results are reported as calcium and magnesium equivalents or as total hardness, depending on the scope of the analysis. Safety data sheets should be consulted for handling and disposal, as with many laboratory dyes.