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EDTA Titration with an Auxiliary Complexing Agent

EDTA Titration with an Auxiliary Complexing Agent
B.Sc (Hons, USJ) (Polymer Science and Technology, Chemistry, Physics)

Generally, EDTA titrations are occurred to find the metal ions concentration in a solution. EDTA will be complex with metal ions with a 1:1 stoichiometric ratio. But in some cases, we add auxiliary complexing agents to the solution. The auxiliary complexing agent is a complexing agent which is also complexing with the metal ion, less strong than EDTA.

Why do we add an auxiliary complexing agent?

EDTA titrations are taken place at around pH = 10. At this pH level, the concentration of Hydroxyl ions (OH-) in the medium is very high. Some metal ions tend to form insoluble products with OH- ions. Therefore, the free metal ion concentration is reduced.

When we are titrating the above solution, only free metal ions will form a complex with EDTA. Some amount of metal ions have already been used and precipitated. So, we get a less concentration of the metal ion than the actual concentration. Therefore, we should prevent metal ions to precipitate with OH- ions.

To prevent this, we add a new auxiliary complexing agent. Auxiliary complexing agents will complex with the metal ions more strongly than OH- ions and less strongly than EDTA.

Metal ligand equilibria

When an auxiliary complexing agent is added to the solution, it will be complex with metal ions. If we consider that the metal ions form a complex with two ligands, the formation of the complex can be represented as a two-step process.

EDTA Titration with an Auxiliary Complexing Agent eq 01

Formation constants for the above formation steps can be written as,

EDTA Titration with an Auxiliary Complexing Agent eq 02

Overall formation of the complex,

EDTA Titration with an Auxiliary Complexing Agent eq 03

Where M is the metal ion and L is the ligand.

β is the cumulative formation constant or overall formation constant.

If the formation is a single-step process, β = K. if the formation is a multiple-step process, β can be found by multiplying the formation constants of each step.

After adding the auxiliary complexing agent, metal ions will form a complex with it. Therefore, there is only a fraction of free metal ions in the solution. If the formation is a two-step process, metal ions are in the form of M, ML, and ML2.

EDTA Titration with an Auxiliary Complexing Agent eq 04

Where αM is the fraction of free metal ions in the solution.

This solution is titrated with EDTA. When adding EDTA, metal ions will be de-complexed with an auxiliary complexing agent and complexed with EDTA. The following equilibrium is written by considering the metal ion as a divalent ion.

EDTA Titration with an Auxiliary Complexing Agent eq 05

At pH = 10, we write the conditional formation constant of the [EDTA – metal ion] complex as follows.

EDTA Titration with an Auxiliary Complexing Agent eq 06

Where, αY4- is the fraction of EDTA and [EDTA] is the total concentration of EDTA. Since there is only a fraction of free metal ions in the solution, we need to modify the above conditional formation constant.

EDTA Titration with an Auxiliary Complexing Agent eq 07

By substitution of the above values to the formation constant expression, we can obtain a new conditional formation constant.

EDTA Titration with an Auxiliary Complexing Agent eq 08

Question 01

Find the fraction of free Zn2+ ion concentration of a 0.1 M solution of Ammonia (NH3). Zn2+ ions and Ammonia form a 1:4 complex. At 30℃, the cumulative formation constants for [Zn - Ammonia] complexes are as follows.

Complexβ
[Zn(NH3)]2+1.51 × 102
[Zn(NH3)2]2+2.69 × 104
[Zn(NH3)3]2+5.49 × 106
[Zn(NH3)4]2+5.01 × 108

Solution

EDTA Titration with an Auxiliary Complexing Agent eq 09

Question 02

Consider the titration of 50.0 mL of 0.001 M Zn2+ solution with 0.001M EDTA at pH 10.00. The solution has been added 0.10 M NH3 as an auxiliary complexing agent. If the equivalence point is at 50.0 mL, find pZn2+ at the equivalent point. At 25 ℃, αY4- = 0.3, αZn2+ = 1.79 × 10-5, the Formation constant of the [Zn- EDTA] complex is 3.16 × 1016.

Solution

At the equilibrium point, we consider that all the Zn2+ ions have complexed with EDTA. Therefore, the total mols of Zn2+ ions are equal to the total mols of the complex formed. The free Zn2+ ions come from the dissociation of the complex.

EDTA Titration with an Auxiliary Complexing Agent eq 10

At the equilibrium, the formed complex is dissociated into free Zn2+ ions and free EDTA. If we take the dissociated Concentration of the complex as x, the concentration of each component at the equilibrium can be represented as follows.

 Zn2+Y4-ZnY2-
Initial concentration--5 × 10-4
Concentration at the equilibriumxx5 × 10-4 -x
EDTA Titration with an Auxiliary Complexing Agent eq 11

When compared to 5 × 10-4, x is negligible.

Therefore, (5 × 10-4 - x) is approximately equal to 5 × 10-4.

EDTA Titration with an Auxiliary Complexing Agent eq 12

The above x value shows the total Zn2+ ions in the solution. But we need the concentration of free Zn2+ ions at the equilibrium point to calculate pZn2+ at the equilibrium point.

EDTA Titration with an Auxiliary Complexing Agent eq 13

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References and Attributes

Figures:

The cover image was designed using an image by André Künzelmann, licensed under CC BY-SA 3.0, via Wikimedia Commons


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