EDTA (H₄Y) is widely used to determine metals in complexometric (a volumetric analysis where the formation of a coloured complex is used to indicate the end point of a titration) titrations as it forms stable complexes with most metal ions. EDTA is a tetracorboxylic acid and in alkaline conditions, it exists as Y⁴⁻ ions, which form 1:1 complexes with metal ions like nickel(II) ions:
Y⁴⁻ + Ni²⁺ NiY²⁻
A metal ion indicator (an organic dye which changes colour when it binds with metal ions) shows the end of an EDTA complexometric titration. However, for a metal ion indicator to be suitable in n EDTA titration, it must not bind as strongly with metal ions as EDTA does. Murexide is therefore suitable.
The aim of this experiment is to determine the percentage of nickel in a nickel(II) salt using EDTA.
Approximately 2.6g of hydrated nickel(II) sulphate was transferred to a weighing bottle and the contents weighed and about 25cmᶟ of deionised water was added to a 100cmᶟ beaker and the nickel transferred to the water. The bottle was weighed without any remaining salt and then the mixture was stirred until the solid was dissolved before the resulting solution was transferred to a 100cmᶟ standard flask.
The beaker was then rinsed several times with deionised water and the rinsings were added to the standard flask. The solution was filled up to the graduation mark with deionised water and the flask was stoppered and inverted several times to mix the contents.
The burette was rinsed with 0.01moll⁻¹ EDTA and filled with the same solution. The 20cmᶟ pipette was rinsed with a little of the nickel salt solution and then 20cmᶟ of it was pipetted into a conical flask. The solution was diluted to about 100cmᶟ with deionised water and murexide indicator (approximately 0.05g) was added to the diluted nickel salt solution along with appromimately 10cmᶟ of ammonium chloride solution.
The mixture was titrated with the EDTA solution and after and after the addition of about 15cmᶟ the solution was made alkaline by adding approximately 10cmᶟ of 0.88 aqueous ammonia (concentrated ammonia solution). The titration was then continued to the end point, shown by the first appearance of a blue-violet colour. Titrations were repeated until two concordant results were obtained.
Mass of 1 Mole of NiSO4.6H2O: 262.8g
Mass of 1 Mole of Ni: 58.7g
Theoretical Percentage by Mass:
Volumes of EDTA used in titration: 1st : 20.2cm3
Moles of EDTA used = 0.1 x 0.0205 = 0.00205 Moles
EDTA reacts with Nickel in a 1:1 ratio, so the number of moles of Nickel in 20cm3 of the salt solution = 0.00205Moles
Number of Moles in 100cm3 = 0.00205 x 5 = 0.01025 Moles
Number of Moles of Ni2+ in 2.6g of Nickel salt = 0.01025
Weight of Ni2+ in salt = 0.01025 x 58.7 = 0.601675g
% Mass of Nickel in salt = 0.601675 ÷ 2.60 = 23.14%