This titrant is used for hardness analysis in water samples.

This titrant is used for hardness analysis in water samples.

• A. Calcium chloride
• B. EDTA
• C. Sodium thiosulfate
• D. Acetic acid

Answer: (B)

Hardness comes from polyvalent metal ions, mainly calcium and magnesium, in water. To measure the amount of these ions, we use a titrant that binds them very strongly so they can be counted away from the solution. Ethylenediaminetetraacetic acid, or EDTA, is the standard choice because it forms stable, 1:1 complexes with Ca2+ and Mg2+. At a higher pH (around 10) EDTA is fully activated to bind these ions, and as titration proceeds, the metal ions are chelated away one by one. An indicator such as Eriochrome Black T helps signal the endpoint: while calcium and magnesium are present, the solution remains red; once all are complexed by EDTA, the color shifts to blue, indicating that the hardness-causing ions have been exhausted. This reliable stoichiometry is why EDTA is used for hardness analysis. Calcium chloride would add calcium rather than measure it, sodium thiosulfate is used for oxidant titrations (like chlorine), and acetic acid functions mainly as a buffer, not a titrant for hardness.

Hardness comes from polyvalent metal ions, mainly calcium and magnesium, in water. To measure the amount of these ions, we use a titrant that binds them very strongly so they can be counted away from the solution. Ethylenediaminetetraacetic acid, or EDTA, is the standard choice because it forms stable, 1:1 complexes with Ca2+ and Mg2+. At a higher pH (around 10) EDTA is fully activated to bind these ions, and as titration proceeds, the metal ions are chelated away one by one.

An indicator such as Eriochrome Black T helps signal the endpoint: while calcium and magnesium are present, the solution remains red; once all are complexed by EDTA, the color shifts to blue, indicating that the hardness-causing ions have been exhausted. This reliable stoichiometry is why EDTA is used for hardness analysis.

Calcium chloride would add calcium rather than measure it, sodium thiosulfate is used for oxidant titrations (like chlorine), and acetic acid functions mainly as a buffer, not a titrant for hardness.