Question

EDTA (ethylenediaminetetraacetic acid) is the most widely used metal chelator in analytical chemistry, which forms strong 1:1 complexes with most metal ions. Water hardness can be determined by titrating Ca2+, Mg2+ and other metal ions with EDTA. During the titration, ammonium-ammonium chloride buffer is often used. Which of the following descriptions about the function of ammonium-ammonium chloride is/are correct?

-To react with EDTA

-To be used as a metal ion indicator

-To minimize the affect of other metal ions

-To maintain the pH of the solution around 10

Answer 1

EDTA is of the form H₄Y. It forms a complex with cations of group 2(Ca²⁺ and Mg²⁺) when it is in the form HY^3- .

The titration is done at a pH around 10 with ammonia-ammonium chloride buffer (NH₃/NH₄⁺) as it keeps the EDTA in the form HY^3- and hence prevents EDTA to form a complex with other cations(like Fe³⁺) that may be present as impurities in water.

Hence, reacts with EDTA to form anion that complexes with group 2 cations and hence prevents the effect of other cations.