ANALYTICAL cHEMISTRY - III

Experiment 2

Principle

• The experiment is based on the ion exchange chromatography, which is a separation/purification technique based on the reversible exchange of ions in the solution with the ions, that are electrostatically bound to the insoluble matrix or a stationary phase.

• Types of the ion exchange chromatography

1. Cation exchange chromatography (contains negatively charged stationary-phase particles; analyte is positively charged)

2. Anion exchange chromatography (contains positively charged stationary-phase particles; analyte is negatively charged)

• Several metal ions (Mg, Al, Zn, Co, Mn, etc.) can be absorbed from hydrochloric acid solution on anion exchange resin owing to the formation of negatively changed chloro- complexes. Each metal is absorbed over a well defined range of pH and this property can be used as the basis of the method of separation.

Apparatus required

Pipette, Conical flask, Column, etc.

Chemicals required

15 g anion exchange resin, 2 M HCl solution, Zn and Mg powder, Conductivity water, 80 mL 0.25 M HNO₃ solution, NaOH solution, EDTA solution, Buffer solution (pH 10), Solochrome Black indicator, etc.

Reagents preparation

1. Amount of Zn = 25 mg/mL, so that in 250 mL = 0.625 g of Zn powder = 625 mg

2. Amount of Mg = 15 mg/mL, so that in 250 mL = 0.375 g of Mg powder = 375 mg

Procedure

• Theory: Zinc is absorbed from 2 M acid, while magnesium does not and thus by passing a mixture of Zinc and Magnesium through the anion exchange column, we can efficiently separate the two. The zinc is eluted with dilute nitric acid.

• Prepare the anion exchange resin using about 15 g of anion exchange resin. The column should be made up in 2 M HCl solution.

• Prepare separate Zinc and Magnesium standard solutions. Zn - 2.5 mg/mL and Mg - 1.5 mg/mL

• Dissolving this in 2 M HCl solution and dilute it up to the mark.

• Pipette out 10 mL of Zn and 10 mL of Mg solution in small separating funnel and transfer it into / top of anion exchange resin. (After mixing the solutions)

• Allow the mixed solution to flow through the column at a rate of about 5 mL/min.

• Wash the funnel and column with 50 mL 2 M HCl solution. Do not allow the level of liquid to fall beyond a particular mark.

• Collect all the element in the conical flask. (this contains all Magnesium)

• Now, change the receiver. Elute the Zinc with 30 mL of conductivity water and then with 80 mL of 0.25 M HNO₃ solution.

• Determine the Magnesium and Zinc in the respective eluent by neutralization with NaOH followed by blank titration with standard EDTA solution using a buffer of pH 10 and Solo Chrome Black indicator.

Result

1. Amount of Mg²⁺ in,

Original = ____________

Eluted = ____________

Error = ____________

2. Amount of Zn²⁺ in,

Original = ____________

Eluted = ____________

Error = ____________

Questions

1. What is the working principle of anion exchange chromatography?

2. Discuss the factors affecting ion exchange chromatography.

3. Give few examples of cation and anion exchange resins.