Science Project: Voltammetry Practise, Method.

This Blog entry is a write up of my first experiments involving Voltammetry. the aims and methods are described here, whilst the results, graphs and discussion shall be uploaded in the next entry.

Introduction: As a great many of the experiments likely to occur during the course of this project may include Voltammetry, specifically cyclic voltammetry, a simple experiment was designed to familiarise myself with the method and process. The experiment itself concerns a voltammteric analysis of Potassium Ferricyanide (K3Fe(CN)6). The experiment itself was conducted in 2 stages on seperate days. On the first day a HEPES/NaCl buffer solution was prepared and scanned using voltammetry. On the second day this buffer was used in the preparation of the K3Fe(CN)6 solution. 2 seperate dilutions of this solution were scanned and the data collected for analysis. Due to the nature of the exerirment, and the well known charcateristics of Ferricyanide the following predictions were made.

1) An average redox potential of 220mV against a standard Ag/AgCl reference electrode.

2) Increased seperation fo the oxidation/reduction peaks on the voltammogram as the scan rate is increased.

3) The current detected at a specific scan rate to be proportional to the concentration of the Ferricyanide solution i.e. a higher current at the higher concentration.

Method: Stage one involved the preperation of 100mL of 0.02M HEPES/0.1M NaCl buffer solution.

As a general note, each piece of equipment was thoroughly washed with deionised water and dried before use.

A small quantity od=f Deionised water (DI water) was placed in a beaker for easy access and use. A mass of 0.4758g (4d.p) of HEPES was weighed out. This mass was placed in a seperate beaker and dissolved in roughly 10mL of DI water. A mass of 0.5835g (4d.p) of NaCl was then weighed out and again dissolved in roughly 10mL of DI water. The salt solution was then stirred using a magnetic stirrer and DI water addede until the salts had completely dissolved (~40mL) and a clear colourless solution formed.

Using a pH meter, calibrated using pH 4 and 7 stock solutions, the pH of the salt solution was measured as 5.37. Small volumes of 0.1M NaOH solution were added until a pH of 6.99 was recorded. The buffer solution was then transferred into a 100mL volumetric flask and made up to measure using DI water. The pH was again recorded aa 6.94. The final concentrations of the buffer solution are calculated as 0.1000M NaCl/0.0199M HEPES (4d.p). A series of Voltammograms were run for this buffer at 3 seperate scan rates. Graphs 1-3 of the next blog entry show the results of the scans. The buffer was then labeled and stored in a chilled room overnight.

On the second day of the experiment a mass of 0.0162g (4d.p) of K3Fe(CN)6 was weighed out. This was dissolved in 2mL of Buffer solution and a clear yellow solution formed. This solution was then transferred into a 10mL volumetric flask andmade up to volume using buffer solution. The final concentration of ferricyanide was calculated at 4.9201mM (4d.p).

An initial voltammogram of the buffer solution alone was run to calibrate and test the equipment. Following this the working electrode was cleaned and prepared. A 9:1 diltution of the ferricyanide was prepared with a volume of 1 mL, thus containing 0.1mL ferricyanide solution and 0.9 mL of buffer solution, and placed in the voltammogram. This yields a concentration of 0.4920mM (4d.p). A series of scans with the following conditions and perameters were conducted.

Scan Rate. Step Potential. E(initial) E(max) E(min)

1)20mV/s 2mV/s 0.4V 0.5V -0.1V

2)20mV/s 2mV/s 0.4V 0.6V -0.2V
3)10mV/s 1mV/s 0.4V 0.6V -0.2V
4)50mV/s 5mV/s 0.4V 0.6V -0.2V

Multiple scans (10) were run for each set of conditions, and the results collected. An overlay graph showing each set of conditions is shown in the next entry in graph 4.

The voltammogram was emptied and cleaned and the working electrode washed. Finally an 0.00492mM solution of ferricyanide was prepared and a set of scans corresponding to condition 2-4 were conducted. See graph 5 for a similar overlay showing the results.

Notes:

1) HEPES=4-(2 hydroxyethyl)-1-piperazineethanesulfonic acid.

Structure shown on the right.

2) The voltammograms done use a 3 electrode set up, with a working electrode, reference electrode and counter electrode. The working electrode was submerged in the buffer/ferricyanide solutions and the reference electrode in the buffer solution. These are seperated and connected via a ceramic plug. The entire apparatus was placed in a Faraday cage which was connected to an earth wire, to reduce noise in the signal.