Experiment 8
Electrochemical Methods in Inorganic
Chemistry
Electrochemistry - 1
EXPERIMENT 8: Electrochemical Methods in Inorganic
Chemistry.
Aims
to measure current-potential curves for an electrochemical reaction
to determine the formal potential, E, of the reaction
to assess the electrochemical reversibility of the reaction
to prepare a compound (K2S2O8) by electrochemical means
to assess the chemical reactions of K2S2O8
Introduction
Oxidation and reduction processes are very important in inorganic chemistry. One way in
which such processes can be characterised is by cyclic voltammetry. A potential is applied
across a solution of the species to be studied and the current measured. The voltage is
scanned from a suitable starting point to a predetermined end point and then back again. (See
Appendix 5). In this experiment the following redox couple is investigated:
Its formal potential, E, is determined and its electrochemical reversibility assessed from (i) the
ratio ipa / ipc (ii) the peak separation Epa / Epc.
Oxidation or reduction by an electrolytic process sometimes provides a method of preparation
of compounds in which a central atom has either a rather high or a rather low oxidation state.
Apart from peroxosulphates, oxidizing agents such as chlorates, perchlorates and silver (II)
oxide are also readily obtained electrochemically. Reduction to the metallic state (including
the formation of amalgams) and the preparation of compounds of transition metals in low
oxidation states (e.g. Ti(III), V(III), Cr(II)) are the most frequent applications of cathodic
reduction.
Experimental
Before starting you should watch the video which accompanies this experiment.
a) Cyclic Voltammetry of [Fe(CN)6]4- Prepare a solution of concentrated ammonia (10 cm 3) made up to 250 cm 3 with distilled
water. This is Solution A.
This is the supporting electrolyte solution, needed to carry the current in the cell.
- Prepare a solution of K3[Fe(CN)6] (3.29 g) in distilled water (50 cm3). The molarity of this
solution is 0.2 M. This is Solution B.
- Prepare the following solutions:
Solution (i)
Solution (ii)
Solution (iii)
Solution (iv)
Solution (v)
Solution (vi)
50 cm3 of Solution A
49 cm3 of A and 1 cm3 of B
48 cm3 of A and 2 cm3 of B
46 cm3 of A and 4 cm3 of B
42 cm3 of A and 8 cm3 of B
40 cm3 of A and 10 cm3 of B
Electrochemistry - 2
- In each case, measure the appropriate quantity of B into a 50 cm 3 graduated flask using a
pipette and make up to volume with solution A.
- Set up the cell comprising two platinum wire electrodes and a calomel reference electrode.
The platinum electrodes should be preconditioned by the following procedure:(i)
(ii)
(iii)
(iv)
(v)
Wash in distilled water
Leave for 5 minutes in 50% nitric acid
Use the second power supply (Weir 4000) in the laboratory to provide 4V and
apply to the electrodes dipped in dilute sodium hydroxide, reversing the
polarity several times.
Apply 4V to the electrodes dipped in dilute sulphuric acid, reversing
the polarity several times.
Wash and keep the electrodes in distilled water until use.
- Obtain voltammograms from solutions (i) to (vi). (Instructions for operation may be found
near the potentiostat) . Use a scan rate of 20 mV/s.
- Obtain voltammograms from solution (vi) at several scan rates (200, 100, 50, 20, 10, 5 mV/s)
- Plot each of the spectra individually and stack plot the series at fixed concentration and
varying scan rate.
b) Electrochemical Preparation of Potassium Peroxodisulphate, K2S2O8.
- Prepare a saturated solution of potassium sulphate in sulphuric acid as follows. To a
solution of 60 cm 3 of concentrated sulphuric acid in 150 cm 3 of water add approx. 15 g
potassium sulphate. Heat to dissolve then cool to 0C, allow to stand for 10 minutes and pour
off the saturated solution into the tube.
- Insert the electrode assembly and immerse the tube in an ice bath.
- Connect up the electrodes to the power supply as shown in the diagram with the equipment.
Ensure that the top black button is at INT and press the red button at the bottom to switch on.
- Set a constant current (0.4 amps) and press the middle black button.
- Accurately record the time elapsed and after about 45 minutes, switch off.
- Collect the crystals which have formed at the anode and wash successively with ethanol and
diethyl ether.
- Record the product yield and calculate the yield based on the total charge passed.
c) Chemical Reactions of the Peroxodisulphate Ion.
- Prepare a saturated solution of potassium peroxodisulphate in water. (If you don’t have
enough of your product then use some from the bottle on the chemicals shelf).
- Place a small amount in four test tubes (2-3 cm3).
- Add a small quantity of the following reagents, one reagent per tube:
(a) Acidified potassium iodide solution. Warm gently. Could this be made the basis of
a volumetric determination of peroxodisulphates?
(b) Silver nitrate solution.
(c) Manganous sulphate in dilute sulphuric acid to which had been added one drop of
silver nitrate solution.
(d) Sodium molybdate or ammonium vanadate solution.
- Record your observations.
Data Analysis.
1.
2.
3.
Draw up a table which reports values of Epc, Epa, Epc - Epa, Eo, ipc and ipa for each of
your cyclic voltammograms.
From your data, plot ipa (or ipc) as a function of concentration.
From your data, plot ipa (or ipc) as a function of the square root of the scan rate (v1/2).
Electrochemistry - 3
Questions.
1.
2.
3.
4.
5.
Determine and report Eo for the process.
Determine Epa - Epc and comment on the value. (Is the couple reversible?)
What factors affect the electrochemical reversibility of electron transfer reactions?
Explain your observations of the reactions of the peroxodisuphate anion.
Draw and discuss the structures of the peroxodisulphate and peroxosulphate anions.
References
1.
2.
3.
4.
5.
6.
P.T. Kissinger and W.E. Heineman, J. Chem. Educ., 1983, 60, 702.
J.J. Van Benschoten, J.Y. Lewis, W.R. Heineman, D.A. Roston and P.T. Kissinger,
J. Chem. Educ., 1983, 60, 772.
A.J. Bard and L.R. Faulkner, "Electrochemical Methods", Wiley, New York, 1980,
Chap. 6.
R.N. Adams, "Electrochemistry at Solid Electrodes", Dekker, New York, 1969,
Chap. 5.
D.H. Evans, K.M. O'Connell, R.A. Peterson and M.J. Kelly, J. Chem. Educ., 1983,
60, 290.
J.T. Maloy, J. Chem. Educ., 1983, 60, 285.
Electrochemistry - 4
OPERATING INSTRUCTIONS FOR THE PC-DRIVEN EG&G Model 263A POTENTIOSTAT
A. Initialise PC.
1.
Make sure the PC is switched on. It will boot into windows.
2.
Using the mouse, position the cursor on the Echem icon in the Echem programme
group and double click (using the left hand button). This starts the electrochemistry
control software.
3.
Press any key to remove the EG&G banner screen and bring up the main programme
screen.
B. Connect Cell.
!!WARNING!! If the potentiostat power is on when changing samples etc always make
sure the red cell indicator light above the “multicore” lead is out before touching any of
the leads as it is possible to get an electric shock if it is live.
4.
Connect the cell to the potentiostat as follows:
The “multicore” lead from the front of the potentiostat has four colour coded
connectors on the end. Connect the red lead to the auxilliary electrode (Pt), the green
lead to the working electrode (Pt) and the white lead to the reference electrode
(standard Calomel Electrode (SCE)). Ignore the black lead.
5.
Fill the cell with the appropriate solution.
6.
Switch on power to potentiostat (button at bottom left of front panel) if not already on.
C. Obtaining the Cyclic Voltammograms.
6.
Using the mouse click on [Setup] on the menu bar.
(A new menu bar appears).
7.
Click on [Get setup].
(A dialogue box appears).
8.
Type in cvlab.dat and press ENTER.
(This brings up a parameter screen).
Most of the parameters are preset, but you will need to input the appropriate scan rate
and check that the initial potential and vertex 1 potential are correctly set (600mV and
-120 mV respectively).
9.
Click on [Run] on the menu bar.
(The potentiostat should now run - the cyclic voltammogram will appear on the
screen).
10.
When the run is complete it is necessary to give the data a title and save it.
Click on [Graph], then [Labels] and lastly [Edit Title]. Enter a title (which should
include solution number and scan rate, the date and your name) and press ENTER.
Then Click on [Main], then [File], and on the pull down menu [Save Data]. Enter a
filename which should be in the form c:\temp\*.dat, where * = your initials and a
number identifying the run. Press ENTER.
Finally, click on [Print/Plot Data] on the pull down menu and then on [Graph] in the
dialogue box which appears and then on [Printer] in the “Output to ?” dialogue box. .
Return to step 6, and repeat for each of your solutions.
Changing Sample.
11.
Before changing your sample make sure the red light above the multicore cable is not
on.
12.
Remove the electrodes carefully.
13.
Empty out the previous solution, rinse the cell and electrodes with a little of the next
solution and fill the cell.
14.
Replace the electrodes in the solution and run as above.
Further Data Manipulation.
Do two stacked plots of the cyclic voltammograms to show how they vary with
a) concentration and b) scan rate.
Electrochemistry - 5