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Anion Exchange Separation of Nickel and Zinc

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CHEM 1220

ANALYTICAL CHEMISTRY LABORATORY

LAB REPORT

Experiment 8: Anion exchange separation of nickel and zinc

Objective

To separate nickel and zinc on an anion-exchange column by using anion-exchange.

To determine the amount of zinc in the sample by titrating with EDTA using Eriochrome black T as indicator.

Introduction

Ion exchange is a process involving an interchange among ions of like sign between a solution and an essentially insoluble solid in contact with the solution. Many substances, both natural and synthetic, act as ion exchangers. The solution to be injected is usually called a sample, and the individually separated components are called analytes. It is often used in protein purification, water analysis, and quality control.

The ion exchanger used in this experiment is resins. Synthetic ion exchange resins are high molecular weight polymeric materials containing large numbers of an ionic functional group per molecule. The resins may exchange cations or anions and consequently they are called cation exchangers or anion exchangers. The resins containing sulfonic acids groups dissociate in acid as well as in base media. The iongenic groups are fixed to the matrix of the resin whereas the counter ions are exchanged with other ions. Hence cation exchange resins can be defined as a polyvalent anions which has positively charged counter ions. In an analogous manner the anion exchange resins can be defined as polyvalent cations which have negatively charged counter ions. Cation exchange resins can be either a strong acid type containing sulfonic acid groups or a weak acid type containing carboxylic acid groups. Anion exchange resins contain basic functional groups, generally amines, attached to the polymer molecule. Strong base exchangers are quaternary amines; weak base types contain secondary or tertiary amines.

A cation-exchanger process is illustrated by the equilibrium

xRSO3H + Mx+ (RSO3)x Mx+ + xH+

solid solution solid solution

where Mx+ represents a cation and R represents a part of a resin molecule containing one sulfonic acid group. The analogous process involving a typical anion exchange resin can be written.

xRN(CH3)3+OH + Ax- [RN(CH3)3+]xAx + xOH

solid solution solid solution

where Ax is an anion.

In this experiment, zinc and nickel ions will be separated on an anion-exchange column. The zinc ion will be titrated with standard EDTA while the nickel will not be determined. In 9M hydrochloric acid , zinc (II) and nickel(II) will form coordination complex with chloride .

Zn2+ + 4 Cl- ZnCl42-

Ni2+ + Cl- NiCl+

The ZnCl42- is retained on the anion-exchange resin while the positively charged NiCl+ is not.

ZnCl42- + 2 R+Cl- (R+)2ZnCl42- + 2 Cl-

After nickel was completely eluted from the column, elution with deionized water would covert the zinc complex into a positively charged species.

ZnCl42- + 6 H2O Zn(H2O)62+ + 4 Cl-

The zinc containing eluate is then buffered to a pH of 10 and titrated with EDTA using Eriochrome black T for end point detection.

Zn(H2O)62+ + EDTA4- [Zn(EDTA)]2- + 6 H2O

The structure of EDTA

Zn-EDTA complex where M represents Zn2+

Eriochrome Black T is a complexometric indicator that is part of the complexometric titrations such as water hardness determination process. It is an azo dye.It is also known as ET-00. In its protonated form, Eriochrome Black T is blue. When it forms a complex with calcium, magnesium, or other metal ions, it would turn red. When this indicator is added to the Zn-containing eluate, the solution turns purple due to the mixing of blue indicator and red ZnIn- complex.

The structure formula of Eriochrome Black T

The Eriochrome Black T contains three ionizable protons, so it is represented by H3In. A small amount of indicator was added to the solution. After all the ZnIn- reacts with EDTA4-, the EDTA displaces the indicator from the magnesium, causing a change of colour from purple to blue .

Zn2+ + 2 H3In ZnIn- + HIn2- + 5 H+

(colorless) (red) (blue)

ZnIn- + EDTA4- [Zn(EDTA)]2- + In3-

(red) (colorless) (colorless) (blue)

A typical ion-exchange The Diagram of the Ion-exchange column

column set-up

Procedure and observations:

A. Elution of nickel

PROCEDURES OBSERVATION

1. 120ml 9M HCl was prepared by diluting conc. HCl (12M) in a 150ml beaker with wearing gloves in fumehood.

2. A column was prepared by inserting a plug of glass wool into the bottom of the glass column. A slurry of Dowex 1-X8 resin in 9M HCL was added to the column until the depth of the resin is 8-10cm. 9M HCl was kept 2cm above the resin. The Dowex 1-X8 resin is orange in colour.

3. The column was washed with two 5ml portions of 9M HCl using a flow rate of 2-3ml per minute. Very pale yellow eluate was collected.

4. 1cm of HCl was left above the resin level. 2ml of the unknown sample was added on to the column by pipette. The unknown sample was

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