What is the difference between voltammetry and amperometry

The voltammetry method that is used for electrochemical measurements, in which a series of regular voltage pulses are superimposed on stairsteps is known as differential pulse voltammetry. The Ring disk electrode is a type of an electrode which is in shape of a ring, that is kept in isolation from the center of the disk. A rotating disk electrode refers to a double working electrode that is used in the hydrodynamic voltammetry.

This type of electrode rotates during the experiments. The difference between RDE and the ring disk electrode is that in the RDE, there is only one disk whereas in the ring disk electrode, there are more than one electrode present.

RDE stands for a rotating disk electrode. In RDE electrode, a single disk is used. In ring disc electrode, a second ring shaped electrode is also used. This electrode is also alienated from the center of the disk. When an electroactive species is started upon the disk, the disk gets swept past the ring and undergoes another chemical reaction of the cell. The electrolyte and the electrode together form a capacitor. The capacitance of this capacitor is called double layer capacitance.

The capacitance and the resistance that acts together on the surface of the electrodes of the cell is referred to as the faradaic impedance. Double layer capacitance includes the entire orientation of the charged species at the interface of the solution of the electrode. Faradaic impedance is used to model an electron-mass transfer process that takes place in the electrochemical cell.

These processes are dependent upon the frequency. The Double layer capacitance takes into consideration, the complete arrangement of charged species at the electrode solution interface. On the other hand, the Faradaic impedance models only the electron-mass transfer process, that takes place inside the cell. It starts acting as a capacitor when the potential across the interface is altered or changed.

The current that is caused due to the diffusion of the charge carriers is referred to as the diffusion current whereas the limiting value of the faradaic current is known as the limiting current. Limiting current does not depend over any chemical potential which is applied, while the diffusion current always dependson the chemical potential gradient. Limiting current is linearly proportional to the reactants concentration and also depends upon the rate of transfer of the reactant to surface of the electrodes.

Limiting current is known as diffusion current when the transport of reactant takes place with the help of diffusion or migration except for convection. In turbulent flow, there is always an irregular and uneven motion and flow of the individual fluid layers.

This results in non-constant velocity of the flow of fluid. Laminar flow is a type of fluid flow in which the layers of the fluid follow a smooth, even, organized and uniform motion or path thereby never intersecting one another. In this flow, the velocity of the fluid is constant at every point within the fluid.

Laminar flow is a type of flow in which the layers of the fluid glide over one another in a regular fashion and organized orientation or manner. On the other hand, the turbulent flow is a type of flow in which the layers of the fluid flow in an irregular fashion or manner. Turbulent flow is the flow in which the fluid layers flow in an irregular manner whereas laminar flow is a flow in which fluid layers slide over each other in a regular manner.

In laminar flow, the movement of the layers remains parallel to surface of the electrode. He also got Nobel prize for this invention in Moreover, the measurement in polarography is a response that is only determined by diffusion mass transport. Polarography simply involves the study of solutions of electrode processes by means of electrolysis using two electrodes.

One of the electrodes is polarizable while the other electrode is unpolarizable. The polarizable electrode is a dropping mercury electrode. The category to which the polarography falls is the general category of linear-sweep voltammetry in which the electrode potential is altered in a linear fashion from the initial potential to the final potential. Due to the effect of having linear sweep methods that are controlled by diffusion mass transport, polarographic experiments have sigmoidal shapes.

Voltammetry is an analytical technique in which the properties of an analyte are determined by measuring the current as the potential is varied. It is important in analytical chemistry and in various industrial processes. In voltammetry, we investigate the half-cell reactivity of an analyte.

Moreover, it is the study of current as a function of applied potential. The curve that we get from the voltammetric analysis is named voltammogram. For the voltammograms in Figure The resulting voltammogram, shown in Figure Earlier we described a voltammogram as the electrochemical equivalent of a spectrum in spectroscopy. In this section we consider how we can extract quantitative and qualitative information from a voltammogram.

For simplicity we will limit our treatment to voltammograms similar to Figure When we apply a potential that results in the reduction of O to R , the current depends on the rate at which O diffuses through the fixed diffusion layer shown in Figure To determine the value of K O we can use any of the standardization methods covered in Chapter 5.

We will do this in several steps. Because the concentration of [ R ] bulk is zero—remember our assumption that the initial solution contains only O —we can simplify this equation. Now we are ready to finish our derivation. When the current, i , is half of the limiting current, i l ,. If K O is approximately equal to K R , which often is the case, then the half-wave potential is equal to the standard-state potential.

In voltammetry there are three important experimental parameters under our control: how we change the potential applied to the working electrode, when we choose to measure the current, and whether we choose to stir the solution.

Not surprisingly, there are many different voltammetric techniques. In this section we consider several important examples. The first important voltammetric technique to be developed— polarography —uses the dropping mercury electrode shown in Figure Although polarography takes place in an unstirred solution, we obtain a limiting current instead of a peak current.

When a Hg drop separates from the glass capillary and falls to the bottom of the electrochemical cell, it mixes the solution. Each new Hg drop, therefore, grows into a solution whose composition is identical to the bulk solution. The limiting current—which also is called the diffusion current—is measured using either the maximum current, i max , or from the average current, i avg. Normal polarography has been replaced by various forms of pulse polarography , several examples of which are shown in Figure Normal pulse polarography Figure The current is sampled at the end of each potential pulse for approximately 17 ms before returning the potential to its initial value.

The shape of the resulting voltammogram is similar to Figure As a result, the faradaic current in normal pulse polarography is greater than in the polarography, resulting in better sensitivity and smaller detection limits. In differential pulse polarography Figure The difference in the two currents gives rise to the peak-shaped voltammogram. The voltammogram for differential pulse polarography is approximately the first derivative of the voltammogram for normal pulse polarography.

You may recall that the first derivative of a function returns the slope of the function at each point. The first derivative of a sigmoidal function is a peak-shaped function. Other forms of pulse polarography include staircase polarography Figure For example, suppose we need to scan a potential range of mV. At this rate, we can acquire a complete voltammogram using a single drop of Hg! We also can use polarography to study organic compounds with easily reducible or oxidizable functional groups, such as carbonyls, carboxylic acids, and carbon-carbon double bonds.

In polarography we obtain a limiting current because each drop of mercury mixes the solution as it falls to the bottom of the electrochemical cell. If we replace the DME with a solid electrode see Figure We call this approach hydrodynamic voltammetry. Hydrodynamic voltammetry uses the same potential profiles as in polarography, such as a linear scan Figure The resulting voltammograms are identical to those for polarography, except for the lack of current oscillations from the growth of the mercury drops.

Because hydrodynamic voltammetry is not limited to Hg electrodes, it is useful for analytes that undergo oxidation or reduction at more positive potentials.

Another important voltammetric technique is stripping voltammetry , which consists of three related techniques: anodic stripping voltammetry, cathodic stripping voltammetry, and adsorptive stripping voltammetry.

Because anodic stripping voltammetry is the more widely used of these techniques, we will consider it in greatest detail. Anodic stripping voltammetry consists of two steps Figure The first step is a controlled potential electrolysis in which we hold the working electrode—usually a hanging mercury drop or a mercury film electrode—at a cathodic potential sufficient to deposit the metal ion on the electrode.

This step serves as a means of concentrating the analyte by transferring it from the larger volume of the solution to the smaller volume of the electrode. During most of the electrolysis we stir the solution to increase the rate of deposition. Near the end of the deposition time we stop the stirring—eliminating convection as a mode of mass transport—and allow the solution to become quiescent.

Typical deposition times of 1—30 min are common, with analytes at lower concentrations requiring longer times. In the second step, we scan the potential anodically—that is, toward a more positive potential. Monitoring the current during the stripping step gives a peak-shaped voltammogram, as shown in Figure Because we are concentrating the analyte in the electrode, detection limits are much smaller than other electrochemical techniques.

An improvement of three orders of magnitude—the equivalent of parts per billion instead of parts per million—is routine. Anodic stripping voltammetry is very sensitive to experimental conditions, which we must carefully control to obtain results that are accurate and precise. Key variables include the area of the mercury film or the size of the hanging Hg drop, the deposition time, the rest time, the rate of stirring, and the scan rate during the stripping step.

Anodic stripping voltammetry is particularly useful for metals that form amalgams with mercury, several examples of which are listed in Table Source: Compiled from Peterson, W. November , —; Wang, J. May , 41— The experimental design for cathodic stripping voltammetry is similar to anodic stripping voltammetry with two exceptions. For example, when Cl — is the analyte the deposition step is.

Table In adsorptive stripping voltammetry, the deposition step occurs without electrolysis. During deposition we maintain the electrode at a potential that enhances adsorption. For example, we can adsorb a neutral molecule on a Hg drop if we apply a potential of —0. When deposition is complete, we scan the potential in an anodic or a cathodic direction, depending on whether we are oxidizing or reducing the analyte. Examples of compounds that have been analyzed by absorptive stripping voltammetry also are listed in Table In the voltammetric techniques consider to this point we scan the potential in one direction, either to more positive potentials or to more negative potentials.

In cyclic voltammetry we complete a scan in both directions. In this example, we first scan the potential to more positive values, resulting in the following oxidation reaction for the species R. When the potential reaches a predetermined switching potential, we reverse the direction of the scan toward more negative potentials.

Because we generated the species O on the forward scan, during the reverse scan it reduces back to R. Cyclic voltammetry is carried out in an unstirred solution, which, as shown in Figure The voltammogram has separate peaks for the oxidation reaction and for the reduction reaction, each characterized by a peak potential and a peak current.

The peak current in cyclic voltammetry is given by the Randles-Sevcik equation. Scanning the potential in both directions provides an opportunity to explore the electrochemical behavior of species generated at the electrode. This is a distinct advantage of cyclic voltammetry over other voltammetric techniques. At the faster scan rate, At the slower scan rate in Figure One explanation for this is that the products from the reduction of R on the forward scan have sufficient time to participate in a chemical reaction whose products are not electroactive.

The final voltammetric technique we will consider is amperometry , in which we apply a constant potential to the working electrode and measure current as a function of time.

Because we do not vary the potential, amperometry does not result in a voltammogram. One important application of amperometry is in the construction of chemical sensors. One of the first amperometric sensors was developed in by L.

Clark to measure dissolved O 2 in blood. A thin, gas-permeable membrane is stretched across the end of the sensor and is separated from the working electrode and the counter electrode by a thin solution of KCl. The working electrode is a Pt disk cathode, and a Ag ring anode serves as the counter electrode.

Although several gases can diffuse across the membrane, including O 2 , N 2 , and CO 2 , only oxygen undergoes reduction at the cathode. The result is a steady-state current that is proportional to the concentration of dissolved oxygen.

Another example of an amperometric sensor is a glucose sensor. In this sensor the single membrane in Figure The outermost membrane of polycarbonate is permeable to glucose and O 2. The second membrane contains an immobilized preparation of glucose oxidase that catalyzes the oxidation of glucose to gluconolactone and hydrogen peroxide. The hydrogen peroxide diffuses through the innermost membrane of cellulose acetate where it undergoes oxidation at a Pt anode.

Note that O 2 serves a mediator, carrying electrons to the electrode. By changing the enzyme and mediator, it is easy to extend to the amperometric sensor in Figure For example, a CO 2 sensor has been developed using an amperometric O 2 sensor with a two-layer membrane, one of which contains an immobilized preparation of autotrophic bacteria [Karube, I.

Trends in Anal. As CO 2 diffuses through the membranes it is converted to O 2 by the bacteria, increasing the concentration of O 2 at the Pt cathode. Voltammetry has been used for the quantitative analysis of a wide variety of samples, including environmental samples, clinical samples, pharmaceutical formulations, steels, gasoline, and oil.

The portability of amperometric sensors, which are similar to potentiometric sensors, also make them ideal for field studies. Pulse polarography and stripping voltammetry frequently are interchangeable. Detection limits for normal pulse polarography generally are on the order of 10 —6 M to 10 —7 M, and those for differential pulse polarography, staircase, and square wave polarography are between 10 —7 M and 10 —9 M.

Because we concentrate the analyte in stripping voltammetry, the detection limit for many analytes is as little as 10 —10 M to 10 —12 M. On the other hand, the current in stripping voltammetry is much more sensitive than pulse polarography to changes in experimental conditions, which may lead to poorer precision and accuracy. We also can use pulse polarography to analyze a wider range of inorganic and organic analytes because there is no need to first deposit the analyte at the electrode surface.

Stripping voltammetry also suffers from occasional interferences when two metals, such as Cu and Zn, combine to form an intermetallic compound in the mercury amalgam. The deposition potential for Zn. During the stripping step, zinc in the intermetallic compounds strips at potentials near that of copper, decreasing the current for zinc at its usual potential and increasing the apparent current for copper. It is possible to overcome this problem by adding an element that forms a stronger intermetallic compound with the interfering metal.

The residual current, in turn, has two sources. One source is a faradaic current from the oxidation or reduction of trace interferents in the sample, i int. We can minimize the faradaic current due to impurities by carefully preparing the sample. For example, one important impurity is dissolved O 2 , which undergoes a two-step reduction: first to H 2 O 2 at a potential of —0. Removing dissolved O 2 by bubbling an inert gas such as N 2 through the sample eliminates this interference.

After removing the dissolved O 2 , maintaining a blanket of N 2 over the top of the solution prevents O 2 from reentering the solution. The cell in Figure There are two methods to compensate for the residual current. This is the method shown in Figure One advantage of extrapolating is that we do not need to acquire additional data.

An important disadvantage is that an extrapolation assumes that any change in the residual current with potential is predictable, which may not be the case. A second, and more rigorous approach, is to obtain a voltammogram for an appropriate blank. The analysis of a sample with a single analyte is straightforward using any of the standardization methods discussed in Chapter 5.

The initial potential is set to —0. The peak currents for a set of standard solutions, corrected for the residual current, are shown in the following table. What is the concentration of As III in a sample of water if its peak current is 1. Linear regression gives the calibration curve shown in Figure The concentration of copper in a sample of sea water is determined by anodic stripping voltammetry using the method of standard additions.

The analysis of a After adding a 5. For the analysis of the sample before the standard addition we know that the current is. Solving each equation for K and combining leaves us with the following equation. Voltammetry is a particularly attractive technique for the analysis of samples that contain two or more analytes.