Hardness T TesTabs® (6917A) contain Eriochrome Black T indicator which is blue in water with 0 ppm hardness. If calcium or magnesium ions are present, they combine with the indicator to form a red complex. The TesTabs® also contain EDTA (ethylenediaminetetraacetic acid). EDTA is a chelant which pulls the calcium and magnesium away from the red colored complex. The EDTA is added in known amounts until all of the calcium and magnesium have been chelated away from the complex and the indicator returns to the blue color. The amount of EDTA required to cause the color change is a direct indication of the amount of calcium and magnesium in the sample.