ANALYTICAL cHEMISTRY - II

Experiment 2

Principle

• Scope and application: Determination of Ni²⁺ and Fe^3+/2+ in environmental analysis and from alloys. In an acidic medium, Fe³⁺ forms Fe–oxinate complex and can be extracted selectively in the organic solvent because Ni²⁺ will not form a complex in low pH with oxine. Therefore, the remaining aqueous layer containing Ni²⁺ can be extracted as Ni–DMG complex in an alkaline medium.

• Interferences: For, Fe³⁺ at pH > 2.5 (Ni, Co, Ce, Al) may interfere and thus, these interferences can be avoided by maintaining the pH between 2 to 2.5. For, Ni²⁺, Co²⁺ and Cd²⁺ can act as interfering ions.

Materials required

Ferrous ammonium sulphate (FAS), 1% Oxine (8-hydroxyquinoline) solution, Liquor ammonia solution, Dimethyl glyoxime, Nickel sulphate hexahydrate, Citric acid, Beaker, Pipette, Nessler's tube, Colorimeter, etc.

Procedure

1. Weigh accurately 86.3348 mg of FAS. Add into it a few drops of concentrated H₂SO₄. Add dropwise dilute KMnO₄ solution till the slight permanent pink color persists and then dilute it up to the mark (100 mL) using de-ionized water.

2. Similarly, prepare the solutions for Ni²⁺ by taking 44.7855 mg of NiSO₄.6H₂O and dilute it up to the mark.

3. Pipette out 1.0, 1.2, 1.4, 1.6, and 1.8 mL of Fe³⁺ solution and into the same beakers, pipette out 1.0, 1.2, 1.4, 1.6, and 1.8 mL of Ni²⁺ solution.

4. Adjust the pH (between pH 2 – 2.2) and transfer the solution quantitatively into the separating funnel. Add 10 mL of 1% oxine solution in CHCl₃ into it and shake the contents of the separating funnel for 1 min. Allow the phases to separate and transfer the organic layer into the Nessler’s tube. Use anhydrous Na₂SO₄ to remove the water.

5. Measure the absorbance of the solutions at 470 nm. Prepare a calibration plot using the absorbance values.

6. In the aqueous phase, add 1 gm of citric acid, adjust the pH to 8 – 8.5 using dilute NH₃ solution and transfer this solution back into the separating funnel.

7. Add 20 mL of aqueous DMG solution and extract this Ni–DMG complex into an organic phase using 10 mL of CHCl₃. Separate the organic layer into the Nessler's tube, add anhydrous Na₂SO₄ to remove the water. Measure the absorbance of the solutions at 560 nm. Prepare a calibration plot using the absorbance values.

8. Repeat the same procedure for the given unknown mixture of Fe²⁺ and Ni²⁺ salts. Record the corresponding absorbance values. Using the absorbance values, find the concentration of Fe²⁺ and Ni²⁺ in the given mixture using the calibration plots.

Result

The concentration of Fe²⁺ and Ni²⁺ in the given mixture is __________ and __________ ppm.

Reference Material

1. G H Jeffery, J Bassett, J Mendham, and R C Denney, Vogel's Textbook of Quantitative Chemical Analysis, 5th Edition

2. S. Suzanne Nielsen, Food analysis, 4th Edition