What is the primary substance used in the titration for hardness testing?

The primary substance used in titration for hardness testing is EDTA (Ethylenediaminetetraacetic acid). This compound is a chelating agent that binds to metal ions, particularly calcium and magnesium, which are the primary contributors to water hardness.

In a titration process, EDTA is added to a water sample containing these metal ions until all the calcium and magnesium are complexed or bound to the EDTA. The endpoint of the titration can be detected using an indicator, which changes color when all the hardness ions have reacted with the EDTA. This allows for the determination of the concentration of hardness in the water sample.

The effectiveness of EDTA in this context comes from its ability to form stable complexes with divalent ions like Ca²⁺ and Mg²⁺, making it an ideal choice for accurately measuring water hardness. Other substances listed, such as phosphate, silicate, and carbonate, do not serve the same function in hardness testing and are therefore not suitable for this specific analytical process.