Complexometric Determination of Nickel using EDTA

EDTA, or ethylenediaminetetraacetic acid is able to form stable complexes with nickel ions due to its hexadentate structure with a one-to-one ratio:

$$ EDTA^{4-}_{(aq)} + Ni^{2+}_{(aq)} \rightarrow [EDTA - Ni]^{2-}_{(aq)} $$

This reaction is observed by adding murexide indicator which is yellow when bound to $\mathrm{Ni^{2+}}$ ions and violet when free in solution.
The theoretical percentage mass of nickel in hydrated nickel(II) sulphate was calculated to be 22.3%.

Experimental Procedure

Apparatus

Chemicals

Chemical Hazards

Hydrated Nickel(II) Sulphate: Harmful by ingestion and inhalation. Eyes and skin irritant. Wear gloves.

EDTA: Toxic if ingested in large quantities.

Aqueous Ammonia: toxic by inhalation. Eyes and skin irritant. Wear gloves and goggles, handle in fume cupboard.

Ammonium Chloride: Eye irritant. Murexide: Harmful by inhalation, eye irritant.

Nickel(EDTA) Complex: Low level toxicity.

Procedure

Accurately weigh out approximately \( 6g \) of nickel(II) sulphate. Add \(25 cm^3 \) deionised water to \( 250 cm^3 \) beaker and transfer the nickel salt to the water. Reweigh bottle and remaining salt. Stir until solid has dissolved. Transfer solution to \( 250 cm^3 \) flask. Rinse beaker with deionised water and add the rinsing to flask, make up to the graduation mark then mix contents of flask by inverting it several times. Fill burette with \( 0.1 mol L^{-1} \) EDTA. Rinse pipette with nickel salt solution and pipette \( 20.0 cm^3 \) of it into a conical flask. Dilute the solution to \( 100 cm^3 \) with deionised water. Add \( 0.05g \) of murexide indicator to the nickel salt solution together with \( 10 cm^3 \) of ammonium chloride solution. Titrate the mixture with EDTA solution, add \( 10 cm^3 \) of \( 0.88M \) ammonia solution to make the it alkaline after the addition of \( 10 cm^3 \) EDTA. Continue the titration to the end-point shown by the appearance of a blue-violet colour. Repeat for concordant results. Calculate percentage mass of nickel in the sample using results from titration.

Results

Mass Measurements for NiSO4 · 6H2O

Titrations


*Average taken from rough, #1 and #3 results
**Average taken from #1, #2 and #3 results

Conclusion

Uncertainties

Weighing by difference:

Volumetric Flask: 0.06% Pipette: 0.3%
EDTA concentration: 0.5%
Burette: Exp 1: 0.55% Exp 2: 0.58%
Absolute Uncertainty: Exp 1: 0.335% Exp 2: 0.315%

Evaluation

Frequent transfer of solutions lead to possible volume inaccuracy and exposed chemicals to contamination often. The end-point of the titration was initially difficult to detect so indicator and ammonium chloride were decided to be added before any EDTA.
Results show that the values from each experiment are not within each other’s uncertainty values. The theoretical value is also slightly off from the results’ uncertainty values. This indicate that techniques within the procedure are inaccurate.