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Chapter 12: Q14P (page 283)

State the purpose of an auxiliary complexing agent and give an example of its use.

Short Answer

Expert verified

The bond between metal cation and the auxiliary complexing agent must be strong enough so that precipitation of hydroxide can be prevented as well as without interference with EDTA titration.

NH3 retains Zn2+ in solution at high pH , but easily displaced by EDTA

Step by step solution

01

Introduction

If an auxiliary complexing agent is induced while EDTA titration, formation of hydroxide became restricted. This agent is added if any of the titration solution is a metal cation.

02

Purpose

The bond between metal cation and the auxiliary complexing agent must be strong enough so that precipitation of hydroxide can be prevented as well as without interference with EDTA titration.

03

Example

NH3 retains Zn2+ in solution at high pH , but easily displaced by EDTA

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Most popular questions from this chapter

Calculate pCu2+ at each of the following points in the titration of 50.00 mL of 0.001 00 M Cu2+ with 0.00100 M EDTA at pH 11.00 in a solution with [NH3] fixed at 1.00 M:

(a) 0 mL(b) 1.00 mL (c) 45.00 mL (d) 50.00 mL (e) 55.00 mL

Spreadsheet equation for formation of the complexes ML and ML2.Consider the titration of metal M (initial concentration = CM, initial volume = VM) with ligand L (concentration = CL, volume added = VL), which can form 1:1 and 2 : 1 complexes:

M+L⇋MLβ1=[ML][M][L]M+2L⇋ML2β2=[ML2][M][L]2

Let αM be the fraction of metal in the form M, αML be the fraction in the form ML, and αML2 be the fraction in the form ML2. Following the derivation in Section 12-5, you could show that these fractions are given by

αM=11+β1[L]+β2[L]2αML=(β1[L])1+β1[L]+β2[L]2αML2=β2[L]21+β1[L]+β2[L]2

The concentrations of ML and ML2 are

[ML]=αMLCMVMVM+VL[ML]=αML2CMVMVM+VL

becauseCMVMVM+VL is the total concentration of all metal in the solution. The mass balance for ligand is

[L]+[ML]+2[ML2]=CLVLVM+VL

By substituting expressions for [ML] and [ML2] into the mass balance, show that the master equation for a titration of metal by ligand is

ϕ=CLVLCMVM=αML+2αML2+LCM1-LCL

Calculate pCu2+ at each of the following points in the titration of 50.00 mL of 0.001 00 M Cu2+ with 0.00100 M EDTA at pH 11.00 in a solution with [NH3] fixed at 1.00 M:

(a) 0 mL(b) 1.00 mL (c) 45.00 mL (d) 50.00 mL (e) 55.00 mL

Spreadsheet equation for auxiliary complexing agent. Consider the titration of metal M (initial concentration = CM, initial volume = VM) with EDTA (concentration = CEDTA, volume added = VEDTA) in the presence of an auxiliary complexing ligand (such as ammonia). Follow the derivation in Section 12-4 to show that the master equation for the titration is

ϕ=CETDAVETDACMVM=1+Kf"[M]free-[M]free+Kf"[M]freeCMKf"[M]free+[M]free+Kf"[M]free2CETDA

where role="math"> is the conditional formation constant in the presence of auxiliary complexing agent at the fixed pH of the titration (Equation 12-18) and [M]free is the total concentration of metal not bound to EDTA. [M]free is the same as [M] in Equation 12-15. The result is equivalent to Equation 12-11, with [M] replaced by [M]free and Kf replaced by Kf".

List four methods for detecting the end point of an EDTA Titration
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