Instruments4Chem

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Electrochemistry Using a Potentiostat


Many interesting experiments can be carried out in an electrochemical cell containing 3 different electrodes. Here’s a brief description of the purpose of the 3 electrodes in a potentiostat -

(R)eference electrode : has a well known and stable equilibrium electrode potential and serves as a reference point against which the potential of the working electrode can be measured. It is commonly a silver (Ag) wire with a coating of silver chloride (AgCl) = Ag|AgCl.

(W)orking Electrode : is where electrochemical processes take place. It is usually made of a material that is not easily oxidised or reduced such as gold (Au), or glassy carbon. The potential at this electrode is controlled relative to the RE while the current at the WE is simultaneously measured.

(C)ounter Electrode : the potentiostat control amplifier injects current into the cell at this electrode (usually a platinum rod) to control VWE - VRE

Think of a potentiostat as a control system, setting the potential difference between a WE and an RE in an electrochemical cell, while also monitoring the current at the working electrode.

Two potentiostats that we have developed are described here.
The first instrument (opposite) is powered by a DC plugpack and uses through-hole components, with a PIC Microchip 16F74 as the control processor. A 128 kbyte static RAM chip (HM628128) serves the dual functions of excitation waveform buffer and current measurement buffer (32k words each).

In this instrument a MAX541, 16 bit serial DAC generates the electrochemical waveform and a MAX195, 16 bit serial ADC measures the current at the working electrode after current-to-voltage conversion.

Connection to the user's PC is via a USB cable.
The second potentiostat is smaller, being implemented on a miniature (50mm x 80mm) printed circuit board (PCB) that also connects to a personal computer (PC) via a USB cable.

This instrument draws its power from the USB bus so that no external supply is required.

An on-board 32 bit processor - a Parallax P8X32A - controls all instrument functions. Again there is an on-board buffer memory space – in this case provided by the HUB RAM of the P8X32A - one half stores the excitation waveform (potentials) and the other half the current measurements.
On the first instrument connections to the user’s electrochemical cell (working, reference and counter electrodes) are made via BNC connectors, while on the smaller instrument a 6 pin mini-DIN connector on the PCB brings out all necessary connections as well as providing two digital outputs to control stirring and gas purging operations. The latter instrument offers an extremely compact electrochemical analysis package, with the PCB sliding into a small diecast box.

Manipulation of the data in the two buffer spaces (voltage and current) in both instruments is via LabVIEWTM vi's. These allow the user to:

• Select and compute a waveform from a choice of standard electrochemical waveforms (custom waveforms are easily implemented, if required).
• Set parameters such as the current gain as well as the potentials/times to apply prior to, and after application of the excitation waveform.
• Run the electrochemical experiment (download the excitation waveform into the potentials buffer, start the acquisition, read back the current buffer and display the data).

These instruments support many standard electrochemical methods, including

- Cyclic voltammetry
- Anodic stripping voltammetry (linear scan and square wave)
- Pulsed amperometric detection
- Custom waveforms such as Fourier transform electrochemistry

Technical Description


The core of the instrument consists of a P8X32A-Q44 Propeller chip, a 32k serial EEPROM for program storage (24LC256), and a USB interface chip (FT245RL).

The VW-VR potential difference is set by a software DAC, consisting of a single Propeller pin connected to an RC filter that feeds an instrumentation amplifier (AD623), configured to provide a bipolar output VOS. This output signal drives the + input of an op-amp (1/4 TL074 - A) whose - input connects to the reference (R) input of the three electrode electrochemical cell. The output of this control op-amp connects to the cell’s counter electrode (C) via a solid-state relay that is also under Propeller control.
The working electrode (W) is connected to the – input of a current amplifier (1/4 TL074 - B) whose current gain is set by a feedback resistor R* that can be switched-in by a DG508 analog switch. Three Propeller I/O pins allow selection of 8 different current ranges. The + terminal of the current amplifier op-amp is grounded. This means that the working electrode sits at a virtual ground potential.

From the above discussion one sees that the control op-amp A holds the potential of Vref at VOS relative to a working electrode potential at ground. This means that if it is desired to have VW < VR then VOS should be positive, whereas if we require VW > VR then VOS should be negative.

The instrument can also be operated as a galvanostat, for example in potentiometric stripping experiments. In this case the working electrode is held at a known potential relative to the reference electrode during the deposition period, and the control op-amp is then disconnected from the counter electrode by opening the solid-state switch. The reference electrode is also connected to a voltage amplifier (1/4 TL074 - C) so that VW-VR can be monitored over time, as required in a potentiometric stripping application.

Two analog signals can be monitored by the instrument, one from the current amplifier B, and the other from the voltage amplifier C as described above. These signals (which are both nominally in the range ±5V first undergo level shifting into the range 0-3.3V using two op-amps (2 x 1/4 TL074) prior to analog-to-digital conversion in a two channel ADC (Linear Technology, LTC1865).

During an electrochemical experiment the Propeller chip sequentially extracts an ADC value value from its RAM buffer, sets the desired potential difference VW-VR, measures the current at the working electrode, stores the current reading into the RAM buffer and then repeats the process until the waveform generation is complete. The current buffer is then uploaded to the host PC via a fast USB interface.

Instrument Setup


Experiments are carried out in a 3 electrode electrochemical cell housing the reference, working and counter electrodes and the analyte solution. In addition, the analyte solution is stirred, and purged (using N2, to remove O2).

A convenient (but expensive) way to do all of this is to use a commercial cell stand, such as the one shown opposite. Here, the 3 electrodes are shown at 11,12 and 13; these sit at the appropriate depth inside cell 14 using o-rings.

In this setup the purging and stirring controls are located at right. The cell stand has a stirring motor inside housing 15 - this pivots downwards and swings out of the way so that access to the cell can be had to fill it with analyte solution.

A connector on the rear panel of the stand brings out the connections to the electrodes - this is where the R, C and W leads from the potentiostat are connected.

Screen Printed Electrodes


A low cost option for electrochemical experiments is to use screen printed electrodes. These have electrodes patterned onto a plastic substrate that are based on conductive carbon inks. The electrodes terminate at one end of the substrate in gold coated "fingers" that can be plugged into a connector so that external connections can be realized, for example to a mini-USB connector as shown here.

A small volume vial serves as the analyte reservoir, with the substrate mounted vertically in a cap that attaches to the vial.

Potentiostat External Connections


The Propeller potentiostat is housed in a small die cast box. There are two connections to the outside world. A USB connector is used for PC connectivity and a mini-DIN6 connector brings out the electrode connections to the reference, counter and working electrodes, as well as providing two logic levels to control stirring and purging operations under computer control if a BAS cell stand is being used.

Details of the mini-DIN6 pinout are shown below. A custom cable mates to the cell stand or to a screen printed electrode connector, depending on the user's experimental configuration.
Pin
Colour
Function
1
Brown
GND
2
Red
Working electrode (WE)
3
Orange
Counter electrode (CE)
4
Yellow
Reference electrode (RE)
5
Green
Stir control
6
Blue
Purge control

The Potentiostat LabVIEWTM Front Panel

The potentiostat LabVIEWTM front panel is shown in the screen capture above. Here, an experiment is being performed to determine trace metals in an aqueous solution. The peaks that are visible are from left to right : Zn, Cd, Pb and Bi. Detailed descriptions of the controls on this screen and explanations of the experimental procedure are given below.

Classroom Demonstration : A Step-By-Step Introduction to Stripping Voltammetry



Stripping voltammetry is a very sensitive electro-analytical technique that makes use of important concepts such as oxidation/reduction, the electrochemical series, and the use of Faraday’s law. The activities described here explore these concepts in a step-by-step fashion.

The basic idea of the technique is quite straightforward - the cation(s) present in a solution can be deposited onto an electrode in an initial reduction step. This is followed by an oxidation step in which the element(s) present come back into solution. During the oxidation step characteristic peaks for each metal appear at potentials that are predictable from the electrochemical series – and importantly for quantitative analysis, the areas of these peaks can be related to the chemical amounts of each element deposited.

The aims of this experiment are to

- demonstrate that a metal film electrode (Bi) can be prepared in-situ by controlled reduction
- show that addition of trace amounts of Pb, Cd and Zn results in additional analyte peaks from each of these elements
- explore the use of Faraday’s law to determine the actual mass of each metal that is deposited during the experiment and to compare these values to the actual amounts of each metal that are present in solution

The activities described here are ideally performed as a classroom demonstration of stripping voltammetry, prior to students conducting their own experiments following the procedure to be described later.

PROCEDURE
Our initial solution contains 2 mL of a 10 ppm Bi solution (prepared from a 1000 ppm AAS standard) and 2 mL of acetate buffer (pH 4.5) that is made up to a total volume of 20 mL using 16 mL of ultra-pure water. The final Bi concentration is thus 1 ppm.

STEPS
1. Formation of a Bi film on the working electrode
By applying a negative potential at the (glassy carbon) working electrode (WE) we can deposit a Bi film on the WE. The amount of Bi deposited depends on the deposition potential, the amount of time that the potential is held negative, and also on the amount of stirring of the solution (this affects the extent of mass transfer of cations to the electrode). Here we set the deposition voltage to -1.40V to ensure efficient electrodeposition of metals onto the WE.

Activity : Record a series of scans varying both the stirring rate (on/off) and the deposition time (try 20s to 120s).

2. Addition of Pb standard
Next, add 200 μl of a 10 ppm Pb standard (this gives a Pb concentration of 100 ppb). Record the stripping trace at a deposition time of 120 sec. A new peak should appear to the left of the Bi peak. Verify that the peak area is affected by the deposition time.

3. Addition of Cd standard
Add 200 μl of a 10 ppm Cd standard. Record the stripping trace at a deposition time of 120 sec. A new peak should appear to the left of the Pb peak.

4. Addition of Zn standard
Add 200 μl of a 10 ppm Zn standard. Record the stripping trace at a deposition time of 120 sec. A new peak should appear to the left of the Cd peak. Are the peak positions consistent with the order that is predicted from the electrochemical series ?

Some useful data :
Pb 207.2 g mol-1 rPb = 175 pm, E0 = -0.13V
Cd 112.41 g mol-1 rCd= 149 pm, E0 = -0.40V
Zn 65.39 g mol-1 rZn = 133 pm, E0 = -0.76V

(Note that the potentials reported by the instrument here are relative to that of the Ag|AgCl reference electrode ie E0 = +0.222V.

5. Data analysis
Enable file saving and record a scan at a deposition time of 120 seconds for the solution containing the bismuth and the three added metals. You will use this file for analysis. The stripping analysis program described below determines peak integrations for each metal peak. Although the peaks are displayed as current vs voltage the data is easily converted into current vs time. Integration yields the total amount of charge transferred during each oxidation process. Record the # of mol of electrons transferred for each peak.

Then performing the following steps :
(i) From the known stoichiometry, compute the number of atoms of each element that have been deposited (then removed) from the working electrode in the experiment.

(ii) From the metallic radius of each element work out the number of atoms of each element that would just make a monolayer coverage on the working electrode. .

(iii) Use your answers to parts (i) and (ii) above to determine the % fraction of the electrode surface that is covered by each element.

(iv) Work out the mass (express your answer in nanograms) of each element that have been deposited on the electrode. How does this compare to the total mass of element that is actually present in the analyte solution ? What does this tell you about the pre-concentration step in a stripping voltammetry experiment ?

6. Add additional Pb to the solution
Add an additional 200 μl of the 10 ppm Pb standard to the electrocehemical cell and observe the change in the stripping trace after a 120 sec deposition time.

A data sheet with typical results from this experiment is shown below.

Stacks Image 4928

Stripping Analysis : Post-Processing of Stripping Voltammetry Experimental Data


A separate LabVIEWTM run-time executable has been developed to process data from stripping voltammetry experiments (Stripping_Analysis.exe) and this is described here. This program performs baseline correction, peak current/potential measurements, peak integration and also allows the # of moles of electrons transferred to be computed for individual peaks in a stripping trace.

1. Double click on the icon Stripping_Analysis.exe
You will see this screen :
Stacks Image 4900
2. Enter the dwell time (in microseconds) for your run.

3. Click on the right pointing arrow button near the top left hand side to launch the vi

4. Select the file to analysed
The file will be opened and displayed in the “Raw data” window.

5. Position the pink cursor at lower extreme left and the yellow cursor at the lower extreme right of the section of raw data to be extracted and analysed. The tool palette immediately above each screen contains tools for zooming in on sections of the screen and moving the display within the screen – use these tools if necessary.

6. Click on the “EXTRACT WAVEFORM” button

7. Check that the waveform displayed in the “Baseline_Corrected” window has a baseline of about zero and contains the data to be analysed. The waveform baseline should be similar to what you see in the screen above. If necessary, repeat steps 5) and 6) to correct errors.

8. Position the blue cursor in the “Baseline Corrected” window at a peak in the data
Record the corresponding values of current (in μ amps) and voltage (in volts) from box below the window. Repeat until all the peaks have been read and recorded. The far right hand window “Moles e- transferred” shows the integration of the data in the “Baseline Corrected” window.

9. Use the blue cursor to measure the moles of electrons transferred as each analyte element is stripped from the working electrode into solution. This can be used to determine the total charge associated with each oxidation step.

10. If you are finished with this data file click on the “STOP” button

11. Repeat steps 4) to 10) until all data files have been analysed

Experiment - Square Wave Stripping Voltammetry Determination of Trace Heavy Metals


Here is a laboratory exercise using the potentiostat and its control vi to determine the levels of Pb, Cd and Zn in an unknown solution using the method known as standard addition. The experiment here uses a BAS cell stand; however it could just as easily be carried out using a sample small sample reservoir and a low cost laboratory stirrer.

1) Remove the glass cell (shot glass !) from the BAS cell stand

2) Obtain and insert electrochemical cell electrodes
A glassy carbon working electrode, a Ag/AgCl reference electrode, and a platinum counter electrode are used in this experiment. The reference electrode is stored by immersing it in a saturated KCl solution held in a small vial. This electrode is particularly fragile and expensive and care should be taken when handling it.

3) Collect solutions for the experiment
In this experiment you will require a sample for analysis, a 1M acetate buffer solution (pH 4.5) and a 10ppm Bi plating solution. During the experiment you will also need a standard solution containing 10 ppm each of Pb, Cd and Zn. Solutions should never be drawn directly from the original containers – when required, transfer a small quantity of each solution into a separate container prior to use.

4) Clean and dry the glass electrochemical cell, then weigh it on a balance before adding 16 ml of your unknown, 2.00 ml of the acetate buffer and 2.00 ml of the 10ppm Bi solution (the last two solutions should be delivered by autopipette). Perform separate weighings after each of these additions so that you can later work out the exact concentration of each of your trace metals during your standard addition experiments.

5) Connect the cell electrodes to the potentiostat.

6) Connect the USB cable at the PC.

7) Double-click on the Potentiostat runtime executable, whose vi is shown above. The potentiostat control screen will appear.

8) Set the “Experiment Type” to SWV and the parameters for this experiment to the following values :

StartV -1.30 volts
The potential at which to commence the oxidative scan after the bismuth film deposition step.

StepH (step height) 5 mV
The step height in millivolts. This defines the increments in the staircase steps of a voltage sweep.

Pulse H (pulse height) 25 mV
The pulse height in millivolts. A positive excursion from the current potential by an amount PulseH is followed by a negative excursion by the same amount.

EndV -0.35 volts
The potential at which to terminate the oxidative scan.

FS Current/+- (full scale current) 150 μA
This sets the full scale current. To get best results the measured peak current during the scan should ideally approach 80-90% of this value. If flat-topped peaks are observed an over-range condition has occurred and the FS current should be increased.

Deposition time 120 sec
The plating time (in seconds) during which Bi2+ is reduced to Bi on the working electrode. Simultaneously, analyte metals are accumulating into the bismuth film electrode (BiFE) during this time. The longer the plating time the lower the detection limit that can be obtained.

Deposition voltage -1.40 volts
The potential at which to hold the working electrode (relative to the reference electrode) during the Bi film deposition step.

Swp Rate(sweep rate) 1000 mV/sec
This determines how fast the analytical scan is performed (in mV/sec). This parameter determines the time interval per “step” in the waveform (for example a StepH of 5 mV and a sweep rate of 1000 mv/sec equates to 5 msec per step).

Rest/s 1 sec
After an oxidative sweep is completed, the potentiostat holds the working electrode at the rest voltage (see next parameter) for this number of seconds.

Rest/V 0.3 volts
The potential (in volts) at which to hold the working electrode after the analytical scan is completed. In a Bi film experiment a setting of 0.3V will strip the Bi film from the working electrode prior to the next run

Wait/%Dwell 80
The instrument will wait for this percentage of the dwell time before commencing current measurements. The dwell time is determined by the sweep rate and step height (see above). This parameter is set to minimize the effects of charging current.

Important
From this point on you should save a data file for each experiment.
Use the hand cursor (in the LabVIEWTM tools palette) to activate file saving by clicking the File? Button to “Yes”. At the end of each run a file will be saved in an Excel compatible, two column text format – potential (in volts) vs. current (in μA).

9) Note the elapsed time meter in the LabVIEWTM control program
In SWV experiments the solution is stirred continuously to assist in mass transport of analyte to the bismuth film electrode. The stirring speed must be held constant during a series of related measurements. It is normal for stirring to be switched off 15 seconds before the end of the deposition period (i.e. after 105 sec for this experiment + 15 seconds quiet time = 120 sec) to ensure a "quiet time" during which the analytical scan is performed. The instrument control program has an in-built timer that allows you to see when the "quiet time" period commences. The instrument control panel has an elapsed time meter and an audible beep is generated when stirring ceases. Control of the stirrer on a BAS cell stand is handled automatically.

10) Start experiment
Commence your run by clicking on the single-headed arrow button near the top left of the screen. This initiates the run and commences stirring. During the experiment a bismuth film is deposited onto the surface of the glassy carbon electrode making a BiFE, and metal analyte ions also form an alloy with the BiFE. After the deposition period stirring ceases, the square wave stripping waveform is applied and analyte metals are stripped from the BiFE. The BiFE is removed in the rest period. A stripping trace should appear on screen and you will then be prompted to enter a filename as described earlier.

11) Review your stripping trace
You should now review your stripping trace. During the potential scan any heavy metals that have accumulated into the BiFE during the deposition period are stripped off – each such metal gives rise to a distinct peak. Approximate stripping potentials are Zn : -1.0V, Cd : -0.70V and Pb : -0.45V.

12) Repeat steps 10 and 11 twice more to generate three replicate scans.

13) Perform a standard addition
Using an autopipette, obtain 100 μl of the standard solution containing 10 ppm each of Pb, Cd and Zn and add this volume of standard to the electrochemical cell. Then follow steps 10 through 12 above.

You do not need to remove the cell from the BAS stand – simply add the solution via autopipette through the spare inlet port in the Teflon cell top. Note: each standard addition will increase the amount of Pb, Cd and Zn in the cell by approximately 50 parts-per-billion (ppb).

14) Perform four further standard additions as described in step 13
A total of 18 stripping scans will have now been recorded after you have completed this step (6 runs x 3 replicate scans each). During these experiments you should notice a steady increase in the peak heights for the Zn, Cd and Pb peaks as the concentration of each metal will have been increased by ~250 ppb as a consequence of your standard additions.

16) At the end of the run the cell contents should be transferred to an inorganic residues waste container as the cell contains toxic heavy metals, albeit at low concentrations. The 3 electrodes should also be rinsed with milli-Q water. After doing this the reference electrode should be returned to its saturated KCl container. After filling the cell with milli-Q water it can be returned to the cell stand. Finally, the working and counter electrodes should be left immersed in water in the cell.

ANALYSIS
Using the dataset you have just recorded, prepare plots of peak height/peak area vs the volume of standard added (0-500 μl) for each element (Zn, Cd and Pb). To obtain the information for these plots use the Stripping Analysis program described earlier.

Then, to determine the concentration of each element in your unknown sample, perform linear regression and extrapolate your lines back to the volume of standard added axis, noting the (negative) intercept. The absolute value of this volume of standard (now a positive value) will contain the same amount of the element as is present in your unknown solution. This allows its concentration in the unknown solution to be determined.

Guidelines for Setting the Deposition Voltage



In stripping voltammetry, the deposition voltage should typically be set around 300 mV more negative than the lowest of the standard reduction potentials in the suite of elements being investigated.

The data shown opposite illustrates deposition of metals onto a Bi film as the deposition voltage is varied in 200 mV steps from -0.4V to -1.4V. In all cases the deposition time is 120 seconds.

At -0.4V, none of these metals are deposited (black trace), while at -0.6V (red trace) there is the first evidence of Pb deposition.

Going to -0.8V, the Pb peak is now clearly visible and some Cd is also detected (blue trace). Further lowering the deposition voltage the Pb and Cd peaks continue to increase until they plateau at -1.2V (olive green trace).

The Zn peak at far left is only visible at -1.2V and -1.4V with the latter condition (orange trace) giving a slightly improved recovery.

In this experiment a deposition voltage of -1.4V would therefore be the ideal choice.

The rising current on the right hand side of these traces corresponds to stripping of the Bi film from the working electrode. Here one notes that the formation of the Bi film is also most effective at -1.4V.