Question

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Which statement is NOT true for auxiliary complexing agents?

Question options:

a) Auxiliary complexing agents prevent the precipitation of metal hydroxide at high pH.

b)Auxiliary complexing agents are chosen to bind metal strong enough to prevent metal hydroxide precipitation but weak enough to be displaced by EDTA.

c)Low concentrations auxiliary complexing agent changes the end point of the titration.

d) k'_f is the effective formation constant for fixed pH and fixed concentration auxiliary complexing agent.

e) Ammonia for example can be used as an auxiliary complexing agents.

f) All these statements are not true.

Answer 1

Answer: c)Low concentrations auxiliary complexing agent changes the end point of the titration. Explanation: a) Auxiliary complexing agents prevent the precipitation of metal hydroxide at high pH. True In general, as pH increases a titration of a metal ion with EDTA will have a higher Kf. Exception: if Mn+ reacts with OH to form an insoluble metal hydroxide. An auxiliary complexing agent is added to an EDTA titration to prevent hydroxide formation. This is added where one of the solutions of titration is a metal cation. This bond between the agent and the metal is important. It must be strong enough to prevent the hydroxide from precipitating but at the same time weak enough to release the cation when EDTA is added. To permit many metals to be titrated in alkaline solutions with EDTA, we use an auxiliary complexing agent.

b)Auxiliary complexing agents are chosen to bind metal strong enough to prevent metal hydroxide precipitation but weak enough to be displaced by EDTA. True A ligand or auxiliary complexing agents that binds strongly enough to the metal to prevent hydroxide precipitation, but weak enough to be displaced by EDTA (e.g., ammonia, tartrate, citrate, or triethanolamine) HO OH НО, CO2H N(CH2CH2OH)3 НО,С CO2H HOC CO2H Triethanolamine Tartaric acid Citric acid ) Low concentrations auxiliary complexing agent changes the end point of the titration. NOT true Because the concentration of NH3 in a buffer is essentially constant. Effective formation constant, K”f, that accounts for both pH and the auxiliary complexing agent's concentration.

d) k's is the effective formation constant for fixed pH and fixed concentration auxiliary complexing agent. True Zn2+ is usually titrated in ammonia buffer, which fixes the pH and complexes the metal ion to keep it in solution. Now consider a titration of Zn2+ by EDTA in the presence of NH3. We need a new conditional formation constant to account for the fact that only some of the EDTA is in the form 74- and only some of the zinc not bound to EDTA is in the form Zn2+. K) = Q 2,2-Qy_K; K”f is the effective formation constant at a fixed concentration of auxiliary complexing agent. For particular values of pH and (NH3), we can compute Kf". An assumption in this process is that EDTA is a much stronger complexing agent than ammonia, so essentially all EDTA binds Zn2+ until the metal ion is consumed.

e) Ammonia for example can be used as an auxiliary complexing agents. True Ammonia is a common auxiliary complex for transition metals like zinc K; = Q 2,3; QyK f