Research Potentiostat Manual
The ACM Research Potentiostat is a high performance laboratory instrument. This
technical manual will describe operation of the instrument for use as a
Potentiostat (two and three electrode), Galvanostat, and ZRA. It is recommended
that an operator obtains a reasonable voltmeter such as a Fluke 70 and a range
of counter resistors ranging from 10 ohms to 1MOhms in decade steps. It is not
necessary to purchase a current meter or ammeter. Usually a good understanding
of Ohms Law will assist the probing Electrochemist. Much of the research done
into Electrochemistry is a form of Ohms Law plus jargon.
Ohms Law:
Resistance = Voltage / Current or R = V/I
where R = Resistance in Ohms,
V = Voltage in Volts (or mV if mA used)
I = Current in Amps
INDEX:
1/ General Specification.
2/ Description of Front Panel Connectors and Switches.
2.1 Positive / Negative switch
2.2 4V - 1V switch.
2.3 Knob - Zero switch.
2.4 Graduated dial.
2.5 Iso - Run switch.
2.6 Ext In.
2.7 Potential buffer and Stat/Ref switch.
2.8 Current buffer.
2.9 OP (Potentiostat output).
2.10 AE (Auxiliary electrode terminal).
2.11 RE (Reference electrode terminal).
2.12 WE (Working electrode terminal).
2.13 Back panel.
3/ General Use of Potentiostat.
4/ Specific Applications.
4.1 Three electrode potentiostatic/potentiodynamic techniques.
4.2 Two electrode potentiostatic/potentiodynamic techniques.
4.3 Zero resistance ammeter techniques.
4.4 Galvanostatic/galvanodynamic techniques.
5/ Further Details.
6/ Manufacturers.
1/ General Specification.
Output Voltage ±13.5 Volts
Output Current 325 mA max
Reference Electrode input current <10 pA
Voltage Buffer output 5 mA, ±13V
Current Buffer output 5 mA,±13V
Internal offset knob linearity ±3 increments
Internal offset ranges 1 V and 4 V
Frequency response Approximately 0-17 KHz
2/ Description of front panel connections and switches.
2.1/ Positive/Negative switch. This switch is marked + and -, in the + position
an increase in the potential of the dialled knob will polarise a working
electrode, in the three electrode Potentiostat configuration, Anodically. Whilst
in the - position an increase in the potential of the dialled knob will polarise
a working electrode, in the three electrode Potentiostat configuration,
cathodically.
2.2/ 4V/1V switch. The switch marked 4 V and 1 V sets the range of the internal
potential that is adjusted by the dialled knob. In the 4V setting, the range is
0-4 Volts, in the 1 V setting the range is 0-1 Volts.
2.3/ Knob/Zero switch. In the KNOB position, the internal offset is indicated by
the dialled knob. The internal offset is the potential imposed by the
Potentiostat at the RE input when in the run mode, provided that in normal
operation the maximum output voltage of the Potentiostat has not been exceeded.
The Knob position is the normal position for three or two electrode
Potentiostatic and Galvanostatic use. In the ZERO position the internal offset
is set to zero mV. The position of the 4V/1V switch, +/- switch and the
graduated dial is immaterial when this switch is set to ZERO. This is the normal
position for two electrode ZRA use.
2.4/ Graduated Dial. This knob is used in conjunction with the three switches
described above. It has a ten turn graduated dial reading 1 mV per division on
the 1V setting and 4 mV per division on the 4V setting. A setting lock is
provided on the dial.
2.5/ ISO/RUN. This switch either isolates the Electrochemical cell or connects
it to the instrument. On ISO, the Reference Electrode and the Auxiliary
Electrode are electrically isolated from each other and the Potentiostat
circuit. This is the position to use during initial setting up of the
Potentiostat. When setting up is complete, throwing the switch to RUN will
couple the electrodes to the Potentiostat/Galvanostat circuit and start the
polarisation.
2.6/ EXT IN. These two 4mm sockets (red-high, black-low) allow the addition of
an external signal to the internal offset generated by the graduated dial. If
the internal offset is not required, throw the Knob/Zero switch to Zero. The
maximum voltage input is ±15 V.
2.7/ Potential and Stat/Ref Switch. The Potential output is used in conjunction
with the Stat/Ref Switch. In the Stat position the Potential of the Potential
Output is the sum of the internal offset voltage, set by the dialled knob, +/-
switch, 4V/1V switch or Knob/Zero Switch and external offset, or voltage input
put into the External Input. In the Ref position, the Potential of the Potential
Output is the same as the potential of the Reference Electrode input with
respect to the Working Electrode input. Provided the instrument is switched to
iso, this is usually known as the Rest Potential, or the potential of a metal at
rest without any external polarisation with respect to an other electrode or
Reference Electrode. The Reference Electrode is often a Saturated Calomel
Electrode (SCE), which happens to hold a fairly stable potential. Other
electrodes may be used instead of a SCE electrode, such as a rusty nail or an
other type of Reference Electrode. A rusty nail or a similar object, may be more
susceptible to drifts in its own potential which could have some effect on the
outcome of the results. Different working electrodes have a range of different
rest potentials. Sometimes, checking this potential between the RE Input to the
WE Input either directly or via the Potential output switched to Ref will give
different results. This is usually because the voltmeter has a low input
impedance and its connection across the cell will to some extent couple the
reference electrode and working electrode. In the 2 or 3 electrode mode, it is
possible to check if the Potentiostat is supplying sufficient voltage whilst in
the run mode by connecting a voltmeter to the potential output and switching
between Stat and Ref. The magnitude of the output should be about the same, but
of opposite sign.
2.8/ CURRENT. The Current Output, is actually a voltage output. it represents
the voltage between terminals OP to AE. If a known resistor (counter resistor)
is used between these terminals such as a 1 Ohm Resistor, then the current
flowing through that resistor is calculated using Ohms Law, I = V/R. Where R is
the magnitude of the counter resistor (1 Ohm) and V is the voltage output from
the output labelled current.
2.9/ OP. This terminal is the output of the Potentiostat.
2.10/ AE. In the standard three electrode mode, this terminal is connected to
the Auxiliary Electrode or Secondary Electrode.
2.11/ RE. In the standard three electrode mode, this terminal is connected to
the Reference Electrode.
2.12/ WE. In the standard three electrode mode, this terminal is connected to
the Working Electrode or Test Electrode.
2.13/ Back Panel. This contains the IEC mains socket, the ON-OFF switch and the
mains fuse (500 mA). The serial number is also shown.
3/ General Use of Potentiostat.
To use this instrument two important accessories are required:
1/ A good quality voltmeter.
2/ A collection of counter resistors, e.g. 1_ to 10M_ in decade steps.
It is in the selection of the correct count resistor that often causes most
errors in the use of a Potentiostat in the usual three electrode mode. The best
counter resistor for a given experiment is usually a compromise between
resolution and output voltage.
Resolution: If a counter resistor has a very low ohmic magnitude, then there is
no danger of exceeding the output voltage of the Potentiostat across it, but the
voltage available for measurement of the polarisation current is low, with
corresponding loss of measurement resolution. For instance using Ohms Law the
potential across a 1 Ohm resistor carrying a 1µA current is 1µV. Any variation
in the current may prove difficult to measure with low resolution.
Output Voltage: If the counter resistor is of too high a Ohmic value for the
magnitude of currents flowing through the cell, the voltage across it will
quickly reach the maximum output voltage of the Potentiostat and all measurement
ability will be lost. It will also be noted that the Potentiostat will not be
able to hold the required potential at the reference electrode. Use the
potential output and the Stat/Ref switch to check this, the potentials should be
of the same magnitude but of opposite sign.
For example, suppose a sample of 304 SS of 1 cm2 area is immersed in a tap water
solution at ambient temperatures, and a Linear Polarisation sweep of ±20 mV is
performed about the rest potential, the expected currents would be low (say <1
µA). This would (from experience or prior experimentation) lead the experimenter
to use a large counter resistor (perhaps 1 M_) in anticipation that the output
voltage would not be exceeded. In this case if the maximum current was +0.6 µA
the maximum voltage measured at the current output would be 0.6 V, which is both
easy to measure with good precision and within the maximum output voltage.
However if a counter resistor of only 1 k_ is used, the voltage measured at the
current output would be 0.0006 Volts, which is too low to be measured with any
great accuracy.
At the other extreme if a cell of 1 cm2 mild steel is immersed in 25% sulphuric
acid and polarised to an overpotential of 300 mV with respect to its rest
potential, the expected current might be 100 mA. If the 1 k_ resistor is used in
this case, the theoretical voltage from the current buffer would be 100 V, which
is beyond the 13.5 V maximum voltage output of the instrument. Instead, perhaps
a 10 _ resistor should be used. In this case the output of the current buffer
would be 1 Volt. This is within the output voltage and of a measurable
magnitude. Voltage drop in the solution is also important in this respect. In
the above example, if the solution resistance, which restricts the current
flowing between the Auxiliary Electrode to the Working Electrode, between the
Auxiliary Output to the Reference Electrode sense point is 2 Ohms and the
current flowing is 100 mA the voltage drop is 200 mV. The overall output voltage
burden is in this case 1 Volt across the counter resistor plus 0.2 V through the
solution or 1.2 V in total, again within the overall voltage output of 13.5V.
4/ Specific Applications.
4.1/ Three Electrode Potentiostatic/Potentiodynamic techniques.
Figure 1 shows the basic wiring arrangement for this method together with a
typical circuit diagram.
The mode of measurement is as follows;
Ensure ISO/RUN switch is set to ISO. Plug in the Working Electrode (WE),
Auxiliary Electrode (AE), Reference Electrode (RE) and select using a best guess
technique based on the magnitude of currents you may expect for the cell under
test, an appropriate counter resistor for insertion between OP to AE. See
section 3 if you have difficulties with this, or select a 10 Ohm resistor and
see how you get on.
Measure the Potential of the Potential Output with the adjacent switch switched
to Ref. This is the potential of the Reference Electrode with respect to the
Working Electrode. The sign of the output needs to be reversed in order to
obtain the potential of the Working Electrode with respect to the Reference
Electrode.
Switch to Stat and Knob, then using the Dialled Knob, +/- Switch and 4V/1V
Switch, reproduce the inverse of the Potential obtained at the Potential Output
in the Ref position. For example you may obtain -345 mV at the potential output
whilst in the Ref position and it is necessary to dial in +345 mV in the Stat
position. The Potentiostat has been adjusted such that once the instrument is
switched to Run, the Potential of the Working Electrode will not be altered
significantly with respect to the Reference Electrode.
Switch to Run.
The cell should now be polarised to the voltage set on the graduated dial. For
fine trimming of the rest potential, connect the Voltmeter to the current output
and adjust the knob slightly until zero voltage can be read at the current
output. In this position the Potentiostat is holding the Working Electrode at
its precise rest potential. If large currents are flowing, it is probable that
the Voltage at the potential output is of the same sign in both the Stat and Ref
positions. If after small adjustments of the dialled knob, large potential
perturbations occur at the Current output, it is probable that a Counter
Resistor of lower ohmic resistance should be considered. It also follows that if
virtually no change in potential can be detected at the current output, then a
higher valued resistor should be considered. This may be the case for the
operators who selected the 10 Ohm resistor as suggested earlier.
The Working Electrode can now be polarised either by the internal Potentiostat
alone, or in conjunction with an External Input such as one generated by a Sweep
Generator.
Internal Polarisation: Adjusting the dialled knob in the positive direction will
polarise the Working Electrode Anodically. Adjusting the dialled knob in the
negative direction will polarise the Working Electrode Cathodically.
External Input: A positive external input will polarise the Working Electrode
Anodically. A negative external input will polarise the Working Electrode
Cathodically.
To isolate the cell at the end of the experiment, throw the ISO/RUN switch to
ISO.
4.2/ Two Electrode Potentiostatic/Potentiodynamic Techniques.
Figure 2 shows the basic arrangement for the recommended circuit.
The mode of measurement is as follows:
Ensure the ISO/RUN switch is set to ISO. Plug one electrode into the socket
marked WE and the other in the AE socket.
Add the counter resistor between OP and AE as in the three electrode method
above.
Connect the AE terminal to the RE terminal with a piece of wire to give a good
electrical connection.
In general the potential difference between the two electrodes is not measured,
but it may be. If it is, this potential can be backed off using the graduated
knob, otherwise disable the knob, setting the KNOB/ZERO switch to ZERO.
If the polarising potential is to be applied manually, then the dialled knob
needs to be set to the required potential and enabled.
The cell may then be connected to the electronics, throwing the run switch to
RUN.
The current flowing is measured at the current output terminals, again using
Ohms Law with the counter resistor selected.
Switch the ISO/RUN switch to ISO to isolate the cell at the end of the run.
4.3/ Zero Resistance Ammeter Techniques.
Figure 3 shows the basic arrangement for the ZRA technique.
The mode of measurement is as follows.
Ensure the ISO/RUN switch is set to ISO. Plug one electrode into the socket
marked WE and the other in the AE socket.
Add the count resistor between OP and AE as in the three electrode method above.
Connect the AE terminal to the RE terminal with a piece of wire to give a good
electrical connection.
Switching to RUN will couple the two electrodes.
The galvanic current is now measured at the current output terminals with the
Voltmeter.
Observe the same counter resistor precautions as indicated in section 3.
Terminate the run by switching the ISO/RUN switch to ISO.
4.4/ Galvanostatic/Galvanodynamic Techniques.
Figure 4 shows the basic arrangement for the Galvanostat technique.
The test method is as follows.
Ensure the ISO/RUN switch is set to ISO.
Plug the auxiliary electrode into the socket marked RE.
Plug the working electrode into the socket marked OP.
Plug the Reference Electrode into the socket marked AE
Connect the Counter Resistor between RE and WE.
To select a suitable Counter Resistor, determine the maximum galvanic current,
for example 1 mA. Then decide on a comfortable operating voltage, say
approximately 1V. This then leads to a counter resistor of approximately 1 k_ or
1/0.001, i.e. V/I. Thus for a Galvanodynamic polarisation of 2 mA, a
polarisation of 2V is required as calculated using Ohms Law or V = IR.
The Polarisation Current is monitored at the Potential output in the Ref
position and then converted into Current using Ohms Law.
The Potential of the Working Electrode with respect to the Reference Electrode
is monitored at the Current Output.
Current Sweeps can be imposed onto the Cell by using the External Input. In this
case a +ve potential will produce an Cathodic Current and a -ve potential will
produce an Anodic Current.
5. Further Details.
This is just an overview of the basic Electrochemical techniques, there are many
variations on these themes published in the literature. The above diagrams
should give an indication of how to use the ACM Potentiostat in any of these
different circumstances.
6. Manufacturers.
Designed and made in England by: