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 act as a masking agent

To form complex with Ca2+ and Mg2+

To be used as a metal ion indicator

To maintain the pH of the solution around 10

Answer 1

Correct answer is d) To maintain the pH of solution around 10.

Explanation: EDTA is insoluble in water at low pH. It is because, the most prominant species is H₄Y.

For metals ion to form complexes with EDTA, it must be present in anionic form so that metals do not have to abstract protons from them. When pH is increased, the protons start dissociating. At pH 10, Y^4- is predominant and hence, the complexometric reaction proceeds swiftly.