Indicator tablets for determination of water hardness by titration with EDTA.

Specifications
Minimum weight: ≥ 0.20 g
Maximum weight: ≤ 0.23 g
Disintegration time (at 25°C): ≤ 150 s
Application test: conforms

The tablets are dissolved in 100 ml of the water sample to be tested, which has been buffered at pH 10 with 1 ml of Ammonia/ Ammonium chloride buffer solution, and then titrated with EDTA solution (0,01 M) till the sample turns from red to blue.
Total hardness in mg/l = 10×ml of titrant used.

Control samples of  calcium carbonate should be used to calibrate the method.