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.
The following apparatus was collected:
50cmᶟ burette ●Glass stirring rod
20cmᶟ pipette ●Hydrated nickel(II) sulphate (NiSO₄.6H₂O)
100cmᶟ standard flask ●Standardised 0.10moll⁻¹ EDTA solution
250cmᶟ conical flask ●1 moll⁻¹ ammonium chloride
Weighing bottle ●Murexide indicator
Balance (accurate to 0.01g) ●0.88 aqueous ammonia
100cmᶟ beakers ●Deionised water
25cmᶟ measuring cylinder
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....
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