EDTA titrations of metal ions are pH sensitive. Which statement is NOT true?
A. At low pH, the amount of EDTA in the form Y4− decreases.
B. As the pH is lowered, the end point becomes less distinct.
C. Metals with higher formation constants can be titrated at lower pH.
D. All metal ions require alkaline titration conditions.
E. At higher pH values, the metal hydroxide might precipitate.
The correct answer and explanation is:
The correct answer is D. All metal ions require alkaline titration conditions.
Explanation:
EDTA (ethylenediaminetetraacetic acid) titrations are highly sensitive to pH because the protonation state of both the EDTA and the metal ions influences the complexation reaction. The statements provided refer to various aspects of this sensitivity.
- A. At low pH, the amount of EDTA in the form Y4− decreases.
This is true. EDTA is a weak acid and can lose protons, forming various protonated species (H4EDTA, H3EDTA−, etc.). At low pH, EDTA exists in its protonated forms, reducing the amount of the fully deprotonated form (Y4−), which is the form most capable of binding metal ions. - B. As the pH is lowered, the end point becomes less distinct.
This is also true. At low pH, the competition between free metal ions and protonated forms of EDTA increases, which makes the formation of the metal-EDTA complex less efficient. This leads to a less distinct titration endpoint, as the metal complex might not form as strongly or quickly. - C. Metals with higher formation constants can be titrated at lower pH.
This is true. Metals that form very stable complexes with EDTA (i.e., those with high formation constants) can be titrated at lower pH because their strong affinity for EDTA allows them to complex even in the presence of protons, preventing other reactions (like hydrolysis) from interfering. - D. All metal ions require alkaline titration conditions.
This is not true. While many metal ions, especially those that form stable complexes with EDTA, are typically titrated under alkaline conditions (because EDTA tends to be more deprotonated and effective at high pH), not all metal ions require alkaline conditions. For instance, metals like aluminum and iron can form stable complexes with EDTA even under acidic conditions. The need for alkaline conditions depends on the metal and its ability to form a stable complex with EDTA at various pH levels. - E. At higher pH values, the metal hydroxide might precipitate.
This is true. As the pH increases, some metal ions (such as iron and copper) can precipitate as their hydroxides. This is a concern during EDTA titrations, as the precipitation of metal hydroxides may interfere with the titration process.
Therefore, the incorrect statement is D, because not all metal ions require alkaline conditions for titration with EDTA.