Voltammetry" is the name of a group of electrochemical techniques where current is studied as a response to potential.
These techniques have a broad range of applicability in modern chemistry and provide chemists with information about the
thermodynamics and kinetics of chemical reactions, and they can be used to identify and quantitate different species in solution.
Instrument
The heart of an electrochemical system is the potentiostat. A potentiostat is a device which will apply a potential (or voltage)
across a pair of electrodes and simultaneously measures the current which flows
through a solution of an analyte . The electrodes are called the working electrode and the auxiliary (or counter) electrode. The most common type of
potentiostat incorporates a third electrode, called a reference electrode. No current flows through the reference electrode, it simply
provides a potential which can be defined as "zero." As shown in Figure 1, the reference and the auxiliary are connected by
electronics (labeled "V") which impose a voltage on the auxiliary electrode. Since the reference is defined as "zero", the
working electrode must assume an equal but opposite voltage from the auxiliary. The current which flows through the working
electrode can be measured by a separate system labeled "A." This roundabout way of doing things allows for very fine control
of the potential of the working electrode and it simplifies the electronics of the entire potentiostat immensely. A potentiostat can
range in sophistication and price from custom-built devices (<$200) to computer-controlled research instruments ($20K).
Linear Sweep Voltammetry (LSV)
The simplest type of voltammetry is called Linear Sweep Voltammetry (LSV). In
LSV, the potential of the working electrode is varied linearly as a function of time, and the current response is recorded. The scan rates are relatively slow, i.e. <5 mV/s, which allows time for
fresh analyte to get to the electrode so that the electrode is always in equilibrium with the bulk solution. For example, consider
A and B which are related by a 1-electron reduction, and which obey the Nernst equation.
A + e- = B; E° = 0.00 V
E is the potential applied to the working electrode, E° is the standard reduction potential of A. If we start with a solution containing only A, at an
electrode potential of 500 mV, then a miniscule amount of current will have to flow to convert a tiny bit of A to B so that the
Nernst Equation is followed at the electrode surface. The amount of current increases as the potential approaches E°. When E
= E°, then [B] = [A] at the electrode surface. At E°, half of all A reaching the electrode is converted to B, so the current is
halfway to its limiting value. As E is lowered further, all A is converted to B as soon as it reaches the electrode.
LSV gives both qualitative and quantitative information. The value of
E1/2 can be used to identify unknown species, and the height of the limiting current can be used to determine concentration. Even so, there are improvements which can be made. An
important feature of this technique is that the largest change in slope occurs around E°. Fancier techniques such as Differential
Pulse Voltammetry (DPV) and Square Wave Voltammetry (SWV) take advantage of this fact to give improved signal to noise.
A variation on the theme of LSV is Cyclic Voltammetry. In CV, the potential is ramped up to some pre-determined value and then returned at the same rate to the
starting potential. The scan rates in CV are much faster than in LSV, which leads to local depletion of analyte around the
electrode surface. Once the analyte is consumed around the electrode, the current diminishes to a value determined by the rate
at which new analyte gets to the electrode surface from the bulk solution. On the return sweep, a peak is observed as part of
the "cloud" of material generated in the forward sweep (if still present) is converted back into starting material.
Polarography
At this point, it is good to introduce the Dropping Mercury Electrode (DME). The word
"polarography" is reserved for the
DME and "voltammetry" is used when the working electrode is made of anything else. The DME was introduced in 1922 by
Heyrovskı, who received a Nobel prize for this invention in 1959. Mercury is popular among electrochemists because
H2 formation from the reduction of water is slow on mercury, and so does not interfere with
measurements as it does with platinum or carbon electrodes. Mercury also has a clean, atomically smooth surface, so
experiments are highly reproducible. Unfortunately, mercury can be messy and unpleasant to clean up, especially because of the known hazards of long term
exposure to mercury vapor. In these experiments, we will use only small amounts of mercury which will be kept under aqueous
solutions to be recycled when we are done.
The basic idea of the DME is that the working electrode is a mercury drop, which is replaced about once per second during a slow LSV scan.
This method is also called "DC Polarography." The flow of mercury ensures that there is always a clean surface for A to be
reduced to B. Modern instruments, such as the one we will be using, control the flow by extruding mercury and knocking off
the old drops. Since the electrode area is no longer constant, the current fluctuates with the lifetime of each drop within some
range as shown in the accompanying figure. This problem can be alleviated by measuring the current once at the end of the
lifetime of each drop, a method called "Sampled DC
Polarography" Sampled DC polarograms look just like the LSV scan
on the previous page.
Pulse techniques
Fancier ways of varying the potential with time can be also used. These methods are called "pulsed methods," and are in
common use today. In Sampled DC, compound A is always being reduced to compound B, even though the current measurement happens only at a single instant in the lifetime of each drop. A
higher signal can be obtained if the potential is applied only briefly before each current measurement. The current is higher
because there is more of A around each drop of mercury to be converted into B. This method is called
Normal Pulse Polarography (NPP). NPP is somewhat more sensitive than sampled DC and regular polarography. Again, the data obtained
have the same shape as a regular LSV.
Differential Pulse
A modification on NPP is Differential Pulse Polarography (DPP). In this method, the current is measured twice during the lifetime of each
drop, and the difference in current is plotted. The result is a peak-shaped feature, shown below, where the top of the peak
corresponds to E1/2, and the height of the peak depends on
concentration. This shape is the derivative of the regular LSV data. DPP has the advantage of sensitive detection limits and
discrimination against background currents. Traditionally, metals in the ppm range can be determined with
DPP.
Square wave voltammetry
Another pulsed voltammetry technique called Square Wave Voltammetry
uses a potential waveform as shown below. The advantage of square wave voltammetry is
that the entire scan can be performed on a single mercury drop in about 10 seconds, as
opposed to about 5 minutes for the techniques described previously. SWV saves time, reduces the amount of mercury used per scan by a factor of 100. If used
with a pre-reduction step, detection limits of 1-10 ppb can be achieved, which rivals graphite furnace AA in sensitivity.
The data for SWV appear much the same as data for DPP, although the height and width of the wave depends on the exact
combination of experimental parameters (i.e. scan rate and pulse height), unlike DPP. This feature makes the running of
standards important, as with any analytical technique. Like DPP, the current at the beginning of a pulse is subtracted from the current at the end of a pulse.
Stripping
voltammetry
In this technique, the electrode will be held at a potential sufficient to reduce any metal ion it encounters for about 60 seconds
before the scan begins. This process will have the effect of reproducibly concentrating the analyte in the vicinity of the electrode.
Some analytes coat the electrode surface, others actually dissolve in the mercury. During the scan, all the reduced analyte at the
electrode surface will be re-oxidized, thereby giving a much larger signal. This strategy is called
anodic stripping and it is one of the most widely used strategies for the electrochemical determination of metal ions in waters used today.
If the potential of electrode was held to cause analyte species
oxidize and accumulate at electrode surface before potential is scanned
to more negative direction, this technique is called "cathodic
stripping voltammetry".
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