Determination of the formula of complex ion

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Chemistry Experiment 31

  1. Title:

Determination of the formula of complex ion

  1. Objective:

To determine the formula of complex formed between nickel (II) and EDTA

  1. Theory:

Nickel (II) ion (Ni2+) forms complex with water, murexide and EDTA

[Ni(H2O)6]2+ + murexide  [Ni(murexide)6]4- + H2O

[Ni(H2O)6]2+ + EDTA  [Nickel-EDTA complex] + 6H2O

EDTA (HO2CCH2)2NCH2CH2N(CH2CO2H)2), being a polydentate ligand, is of higher strength than that of murexide and thus form complex more readily than murexide. Also, murexide, by experiment, form complex more readily than water. Therefore, by the principle of competitive complexing, in a mixture of aqueous nickel ions with EDTA and murexide added, complex are formed between nickel ions and EDTA instead of with murexide. Similarily complex are formed between nickel ions and murexide instead of with water.

Below are the colours of the different species in this experiment.

In order to determine the formula of the Nickel-EDTA complex [Ni(EDTA)y] where y is a positve interger, the mole ratio of the reaction between Ni and EDTA has to be determined. This can be done by titrating nickel(II) solution against EDTA with murexide as indicator. Initially the mixture in the conical flask should appears yellowish green due to the domination of the green nickel-water complex over a trace amount of yellow Nickel-murexide complex. When EDTA are gradually added, the solution should start to turn blue due to the increasing concentration of Nickel-EDTA complex. When all the Nickel-murexide complex are finally replaced by Nickel-EDTA complex, a pale blue colour should appear, any further addition of EDTA would cause the solution turns dark blue and then violet, which is due to the formation of the original murexide, and this indicates the end-point of the titration.

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By carrying out several titrations, the average value of the volume of EDTA used for reaching the end-point can be calculated, together with the known molarity of EDTA and nickel (II) ions (both 0.1M), the no. of moles of EDTA used and that of nickel (II) ions can be found, by considering the mole ratio between them, the formula of Nickel-EDTA complex can be obtained.

  1. Results:

Average volume of 0.1M EDTA used = (25.3+24.6+24.8) / 3 = 24.9 dm3

No. of moles of EDTA used = (24.9/1000) x 0.1 = 2.49x10-3 ...

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