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Separation of Cadmium and Zinc on an Anion-Exchange Resin, and Their Determination Using Standardized EDTA Solution

Introduction

In a solution having high chloride , cadmium and zinc form complex anions as shown below:

+2 - -2 Zn + 4Cl ⇔ ZnCl4 Kst = 0.71 +2 - -2 Cd + 4Cl ⇔ CdCl4 Kst = 51.0

These complexes can be exchanged for other anions on anion-exchange resin, using a hydrochloric solution of appropriate strength. At a concentration of 0.01M HCl, zinc does not exist as the anion complex, but cadmium does. If the solution of zinc and cadmium contains chloride and hydroxide, zinc forms mixed -2 -3 complexes such as [ZnCl3OH] or [(ZnCl3)2OH] . Cadmium doesn’t form mixed complexes. Some of the zinc and cadmium form hydroxy complexes as shown below:

+2 - -2 15 Zn + 4OH ⇔ Zn(OH)4 Kst = 2 x 10 +2 - -2 12 Cd + 4OH ⇔ Cd(OH)4 Kst = 1 x 10

We can take advantage of the differences in zinc and cadmium to separate a mixture of zinc and cadmium.

Briefly, a sample containing zinc and cadmium is made about 1.5 M in chloride , converting all of the zinc and cadmium into the chloride complexes. This solution is put onto the top of a column of Dowex 1-X8 anion-exchange resin. The zinc and cadmium complexes stick to the anion-exchange resin. A suitable solution containing and sodium chloride is passed through the column. The zinc forms the hydroxide complex and the mixed chloride and hydroxide complexes, while cadmium forms hydroxide complexes only.

The cadmium chloride complex binds more tightly to the resin than does the various zinc complexes, and therefore zinc is eluted fairly rapidly from the column. To remove the cadmium, the cadmium chloride complex must be destroyed. After all zinc has been separated, dilute nitric acid is passed through the column. This destroys the cadmium chloride complex, releasing cadmium cation. Cations have no attraction for anion exchange resin, and therefore the cadmium elutes from the column immediately. The chemical reactions describing the chemistry of this separation are shown below.

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Sample Addition to Anion-Exchange Resin:

+ - -2 + -2 - 2R Cl + ZnCl4 ⇔ (R )2ZnCl4 + 2Cl + - -2 + -2 - 2R Cl + CdCl4 ⇔ (R )2CdCl4 + 2Cl

NOTE: anion exchange resin can be conveniently pictured as an ionic compound, composed of an organic cation (R+) and an anion (Cl- in this case, although other types of resins having different anions are available).

Elution of Zinc:

+ -2 + -2 - + - + -2 -2 - (R )2ZnCl4 + (R )2CdCl4 + 6OH ⇔ 2R OH + (R )2CdCl4 + Zn(OH)4 + 4Cl

NOTE: the zinc chloride complex is converted into the zinc hydroxide complex. The zinc hydroxide complex does not bind to the anion-exchange resin as tightly as the zinc chloride complex, and therefore the zinc hydroxide complex is eluted from the column. The cadmium complex remains intact and bound to the anion exchange resin.

Elution of cadmium:

+ -2 - + - +2 - (R )2CdCl4 + 2NO3 ⇔ 2R NO3 + Cd + 4Cl

NOTE: addition of nitric acid destroys the cadmium chloride complex. The nitrate ion binds to the anion exchange resin, while the cadmium cation is strongly repelled from the organic cation (R+) and therefore remains in solution.

Once the cadmium and zinc are separated, a quantitation method is needed to determine the amount of cadmium and zinc present. The classic method of determining zinc, cadmium, and other suitable cations, is with a standardized solution of ethylenediaminetetraacetic acid (EDTA).

EDTA has the structure shown below. Instead of repeatedly drawing this structure or writing out the chemical formula, the EDTA is represented as -1 -2 -3 “Y”. Each acid hydrogen on EDTA can be removed, producing H3Y , H2Y , HY , and -4 Y . The disodium dihydrate of EDTA, Na2H2Y•2H2O is commonly used to prepare standard EDTA solutions.

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This salt is readily available from many commercial sources, and often in such a high purity that solutions need not be standardized for routine work. carbonate can be used to standardize EDTA solutions, but high purity zinc solutions permit faster with a sharper endpoint. Therefore zinc is preferred for standardization.

A variety of indicators are used for EDTA titrations. In this titration, we will be using Eriochrome black T (EBT) indicator, having the structure shown below.

- (H2In )

Most metals form “wine red” colored complexes with Eriochrome black T, while the unbound indicator is blue (See Figure 1 below). Metal ions bind more strongly to EDTA than to EBT, so as a wine red solution of metal-EBT is titrated with EDTA, the color changes from wine red, to purple, to blue.

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Figure 1. Wine red and blue colors for Eriochrome black T indicator.

The zinc-EDTA complex has a stability constant of 3.26 x 1016. While other procedures for titrating zinc with EDTA are found in the literature, the most common procedure is to titrate zinc at pH 10. However, the product (Ksp) of zinc hydroxide is 7.1 x 10-18, and at pH 10 most of the zinc is in the insoluble zinc hydroxide form. (In most cases, zinc will start to precipitate as zinc hydroxide about pH 7.) In order for the zinc-EBT complex to form, the solution must be basic, so some sort of auxiliary complexing agent must be used to prevent the zinc from precipitating as the hydroxide. forms a tetra complex with zinc, 8 and the Kst for the zinc tetra amine complex is sufficiently high (4.2 x 10 ) that solutions containing 1.0 M ammonia will not precipitate zinc hydroxide at a pH of 10.

The titration reaction between the zinc tetra amine complex and HY-3 (the form of EDTA predominating at pH 10) is shown in the below.

+2 -3 -2 + Zn(NH3)4 + HY → ZnY + NH4 + 3NH3

The conditional stability constant for this reaction under the experimental 9 conditions (pH = 10, [NH3] = 0.5M) is 2.3 x 10 . The stability constant for the zinc-

4 EBT complex under these conditions is 5.6 x 106. The indicator forms a weaker complex with zinc ions than does the EDTA, therefore the free indicator is produced at the end point of the titration, causing the color to change at the appropriate point.

Solutions needed for this experiment:

Solutions prepared by the student Solutions provided by the instructor

0.01M disodium EDTA (standardized by 12M student) 6M Hydrochloric acid 50% Sodium hydroxide solution Dilute (<0.10M) sodium hydroxide 8.5M NH3-NH4Cl Buffer and ~ 1 M sodium hydroxide 0.5M Hydrochloric acid Eriochrome black T indicator 1M Nitric acid 0.12M HCl-10%NaCl solution Standard Zn solution 2M NaOH-2%NaCl solution Concentrated nitric acid

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Specialized equipment needed:

In this experiment, you will be using a column (see Figure 1 below). There are three separate pieces to the chromatography column; a long (~30 cm) glass tube, a valve and stopcock assembly, and a delivery tip. Assemble the pieces as shown in the figure, and test the column by filling it with water. If the column leaks, notify your instructor so that the column can be replaced.

Figure 1. Chromatography column showing glass tube, valve and stopcock assembly, and delivery tip.

6 Experimental Procedure

Preparation and standardization of 0.01 M EDTA solution.

1. Using the top loading balance, weigh between 3.6 and 3.7 grams of disodium EDTA dihydrate into a clean 1 L plastic bottle. EDTA will leach metal ions from soft glass containers, and should never be stored in glass containers. Add 1 L of deionized water.

EDTA dissolves SLOWLY. Shaking or stirring the solution vigorously speeds the dissolution process. Heating the solution to 50 – 60 oC also speeds the process. Nevertheless, even under these conditions EDTA dissolves SLOWLY. It is strongly recommended that you prepare the EDTA solution several hours or even the day before you plan on using it. Before use, check the solution to make sure that all solid EDTA has dissolved.

2. Using the analytical balance, weigh out ~0.5 grams of high purity zinc metal and transfer the metal to a 150 mL beaker. The actual mass of metal used is not important provided that the mass is precisely known.

3. Working in the hood, slowly add 10 mL of 6M hydrochloric acid to the zinc metal, and cover the beaker with a watch glass. Allow the zinc metal to dissolve completely, which may take some time (an hour or more). The initial reaction is vigorous, and zinc solution can be lost by splattering if the beaker is not covered quickly with a watch glass. When it appears that dissolution is complete, carefully check that all small zinc particles have been dissolved. Hydrogen gas is produced by this reaction. This gas is flammable and can form explosive mixtures with the air.

4. After the zinc has dissolved, analytically transfer the solution to a 500.00 mL volumetric flask and QS with DI water. Mix thoroughly, and store the solution in a plastic bottle. Label the solution with the molar concentration of zinc.

5. 25.00 mL of standard zinc solution into a 250-mL . Using dilute sodium hydroxide solution (~ 0.1M) raise the pH of the solution until a very slight turbidity persists. (Alternatively, until the pH of the solution is ~ 10). The exact amount of sodium hydroxide solution depends on the acid concentration of your zinc standard and the concentration of sodium hydroxide solution used, but should be ~25 mL. Addition of sodium hydroxide should be stopped at the first appearance of zinc hydroxide precipitate or when the pH is ~ 10, whichever occurs first.

7 pH Test Paper color chart:

6. Add 10 mL of 8.5M NH3-NH4Cl buffer. This buffer has been prepared for your use. CAUTION! This buffer is dangerous; it is caustic and ammonia can cause pulmonary paralysis (it can interfere with your ability to breath). Exercise caution in handling and dispensing this buffer!!

7. Add 50 mL of deionized water and two or three drops of Eriochrome black T indicator. Warm the solution slightly (30 – 40 oC) and titrate the zinc with the EDTA solution until the color changes from wine red, through purple, to a pure rich blue color. At the end point, the last traces of purple in the solution will have just disappeared. This color change is sharp, and should be observable within a fraction of a drop of EDTA. If the reaction seems to proceed slowly near the equivalence point, after each addition of EDTA wait a few seconds before adding the next drop.

Some students (not all of them male) have difficulty distinguishing between purple and blue. You may want to make a comparison sample, representing the endpoint of the titration. The comparison sample is made from 50 mL of DI water, 10 mL of 8.5M NH3-NH4Cl buffer, and 2 – 3 drops of EBT indicator solution. If necessary, ~2 mL of EDTA can be added. The solution should have a rich blue color and can be used for comparison with the end point of your zinc-EDTA titrations.

8 8. Perform the titration in triplicate. Normally, three titrations will agree within 0.04 mL. Do not use all of the EDTA solution in performing the standardization, since you must have enough EDTA to titrate the unknown cadmium and zinc solutions.

From the known mass of zinc, and the 1:1 stoichiometry between zinc and EDTA, you should be able to readily calculate the molarity of the EDTA solution. Calculate the average molarity and label your EDTA solution appropriately.

Column preparation – anion exchange resin.

1. Weigh out 15 – 20 g of Dowex anion exchange resin (chloride form) into a 150 mL beaker. Add ~10 mL of deionized water and stir for a few minutes to thoroughly wet the resin. Decant and discard the supernatant liquid. The volume of the wet resin should be ~25 mL.

2. Add 50 mL of 0.5M hydrochloric acid to the resin, stir, allow the resin to settle, and decant and discard the supernatant liquid. Repeat this washing process until the supernatant liquid is colorless.

3. Assemble your chromatography column. Into the bottom of the column tamp a small plug of glass wool. The glass wool should be tight enough and large enough to prevent any of the resin particles from passing through and clogging the stopcock or tip, but loose enough and small enough not to impede the flow of liquid through the column.

4. Using 0.5M hydrochloric acid as a wash liquid, transfer the wet resin slurry to the chromatography column. Rinse down the insides of the column and allow the resin to settle under gravity. A 20 cm column of well-packed resin should result. If the column is shorter than this, add more resin. Wash the completed column once with 0.5M hydrochloric acid. DO NOT ALLOW THE LEVEL OF LIQUID IN THE COLUMN TO FALL BELOW THE RESIN BED. Keep the liquid level ~ 1 cm above the resin bed at all times.

5. Wash the resin column with two 20-mL portions of 0.12M HCl – 10% NaCl solution, draining the liquid to about 0.5 cm above the top of the resin bed. (This solution has been prepared for your use.)

Sample Addition and Separation.

1. Pipette a 2.00 mL aliquot of the unknown cadmium-zinc solution onto the top of the column. This can best be achieved by holding the tip of the pipette about 1 cm above the resin bed and running the tip around the inner circumference of the chromatographic column while the sample is draining out of the pipette.

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2. Drain the sample to within 0.5 cm of the top of the resin bed. Use 5 mL of 0.12M HCl – 10% NaCl solution to wash down the inside walls of the column, draining the column to within 0.5 cm of the top of the resin bed. Repeat the washing with a 2 mL aliquot of 0.12M HCl – 10% NaCl solution. The resin may or may not change colors, depending on factors that aren’t important to this analysis.

3. Pass 150 mL of 2M NaOH – 2% NaCl eluting solution through the column at a rate of ~ 3 mL/minute. Collect the eluate in a clean 250.00 mL volumetric flask and QS with DI water. The 2M NaOH – 2% NaCl has been provided for you. This solution contains the zinc metal from your unknown sample and should be labeled appropriately.

4. Wash the resin column with ~ 50 mL of deionized water to remove most of the sodium hydroxide solution. Drain the wash water from the bottom of the column and discard.

5. Pass 150 mL of 1M nitric acid through the column, collecting the eluate in a separate, clean, 250.00 mL volumetric flask. QS with DI water. This solution contains the cadmium metal from your unknown sample and should be labeled appropriately.

6. When you have completed the separation, throw the used resin in the trashcan.

Analysis of cadmium and zinc.

1. Pipette a 50.00 mL aliquot of the zinc solution into a 250-mL Erlenmeyer flask. Use 6M hydrochloric acid to adjust the solution to ~pH 9. Titrate the zinc solution according to the instructions for the standardization of EDTA starting with step 6 (the addition of 8.5M NH3-NH4Cl buffer. Perform this analysis in triplicate.

2. Pipette a 50.00 mL aliquot of the cadmium solution into a 250-mL Erlenmeyer flask. Use sodium hydroxide solution to adjust the solution to ~pH 9. Titrate the cadmium solution according to the instructions for the standardization of EDTA starting with step 6 (the addition of 8.5M NH3- NH4Cl buffer). Perform this analysis in triplicate.

Calculations

Since EDTA forms a 1:1 complex with either metal under these experimental conditions, the millimoles of metal present can be found by calculating the number of millimoles of EDTA consumed during the titration using the equation:

10 millimoles EDTA = mL EDTA X molarity EDTA

Since the total volume of the eluate has not been titrated, the millimoles of metal titrated (calculated from the above equation) are the millimoles present in the aliquot used. A dilution factor must be used to calculate the total number of millimoles of metal present in the 250.00 mL volumetric flask:

Total millimoles metal = millimoles metal X (250.00 mL/50.00 mL)

Once the total number of millimoles of metal present in the sample is calculated, the mass in milligrams of metal present is readily determined. In your report, make sure you report the milligrams of cadmium and zinc separately (do not report the total of cadmium and zinc combined).

Laboratory report.

A sample report is shown at the end of the procedure.

WASTE DISPOSAL: All solutions containing cadmium or zinc must be disposed of in the aqueous metals container.

11 SAMPLE REPORT – ALL VALUES ARE FICTITIOUS AND HILARIOUS

Separation of Cadmium and Zinc Unknown #3.14159265358979323846 Lothar of the hill people

Cadmium Sample 1 Sample 2 Sample 3 Vol. eluate, mL 50.00 50.00 50.00 EDTA conc., M 0.009978 0.009978 0.009978 Vol. EDTA, mL 33.45 33.50 33.55 mmoles metal titrated 0.3337641 0.334263 0.3347619 mmoles metal total 1.6688205 1.671315 1.6738095 milligrams Cadmium 187.5921124 187.8725192 188.1529259 Mean mg Cadmium 187.9

Zinc Sample 1 Sample 2 Sample 3 Vol. eluate, mL 50.00 50.00 50.00 EDTA conc., M 0.009978 0.009978 0.009978 Vol. EDTA, mL 47.65 47.75 47.55 mmoles metal titrated 0.4754517 0.4764495 0.4744539 mmoles metal total 2.3772585 2.3822475 2.3722695 milligrams Zinc 155.4489333 155.775164 155.1227026 Mean mg Zinc 155.4

Sample calculations: show one complete sample calculation for cadmium and zinc.

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