Polarography

Polarography, also known as Electrochemical Polarography, is an Electroanalytical technique that measures the reduction potential of Electroactive species. Polarographic sensors are the mainstays of field microelectrodes, but field microelectrodes can be made Polarographic too. Polarographic techniques are a subset of potentiometric techniques, which includes coulometry, anodic stripping voltammetry, and atomic emission spectrometry. The principle is based on measuring the Current or voltage drop that occurs in a polarizable electrolyte that is in contact with a polarized electrode. The ability of the electrode to become polarized is a Direct function of the activity of the substance in the electrolyte. The polarization of the electrode is expressed in mV as a difference between anodic and cathodic overpotentials.  Polarographic electrodes are widely used in electroanalytical chemistry and biochemistry for the determination of oxidation-reduction potentials, pK, the detection of electroactive contaminants in wastewaters, and the detection of chemical and biological species such as pesticides, heavy metals and microbes.

Instrumentation 

A Polarographic instrument consists of three main parts: a cell or electrochemical cell, an electrode and a potentiostat.  In the case of the instrument's probe, the potentiostat is also the electrode.  A cell is an electrochemical cell that is used to hold a solution with electrodes.  Each electrode in a cell has one or more potentials on it that are applied to the cell, and a potential is applied to each electrode relative to a second electrode in the cell.  The probe is the instrument's electrode.  An electrode can be either the working electrode or the counter electrode. 

The working electrode is the electrode that the Current is measured by or the electrode that is charged with the potential.  The counter electrode is the electrode that the Current is NOT measured by.  The reason for this is that if the working electrode was charged with potential, the counter electrode would be the electrode that measured the Current.  Since the counter electrode doesn't measure the Current, there is no Current flow on the counter electrode.  

When the Current is measured, it is the working electrode that is charged, and the potential is that which the counter electrode is held at.  The potentiostat is the electrode and instrument that applies the potential to the electrode.  It has two or more probes to measure the Current between the potential of the working electrode and the potential of the counter electrode, as well as the Current between the potential of the working electrode and the ground.  

There is one potentiostat for each electrode in a cell, with one probe connected to the working electrode and another probe connected to the ground.  A potentiostat can also have several probes connected to different potentials.  In this way, the potentiostat can be used to control how much Current goes into the cell (by setting the potential of the working electrode), as well as how much Current leaves the cell (by changing the potential of the counter electrode).

Examples 

Potentiostats are used to test the performance of cells in many applications, including photovoltaic cells, fuel cells, batteries, corrosion cells, and various chemical Analysis applications. A potentiostat may also be used to control electrodeposited materials in the form of electroplating baths and electrochemical baths for chemical deposition.

In addition to the galvanostatic charging application described above, potentiostats can be used to measure specific electrochemical reactions. The potential difference across an electrochemical cell is controlled using the potentiostat to control the Current flow and the cell is held at a constant potential. This is referred to as the galvanostatic mode of operation.

Some potentiostats may have more than one set of wires and one set may be used to control the Current while the other set is used to control the potential, to create a bridge circuit.

Advantages

Compared to other methods, the advantages of Polarographic measurements are as follows:

• Electrochemical cells used for polygraphic measurements are simpler to construct and operate than electrolytic cells because they do not require the use of a mercury or silver amalgam. 

• The concentration of a TM species is measured electrochemically; there is no need to separate the solution. 

• The Polarographic technique is sensitive, reproducible and provides a quantitative measurement. 

• The technique allows the determination of valencies in one order of magnitude.  

• It is an accurate technique and the precision may be defined by the type of equipment employed. 

• The standard deviation is typically 0.01–0.05 mV for a single Analysis. The method gives information on the number of electrons (or ions) carried by the species studied. 

• The voltammetric studies of electroactive compounds are carried out easily using Polarographic equipment. 

• The technique is well suited for the fast determination of valencies and the investigation of the reduction potential of compounds that undergo thermally activated processes. 

• In aqueous solutions, most of the organic compounds give stable voltammograms with a high signal to noise ratio. 

• The technique may be used for a wide variety of stable metal cations, weakly or moderately electrophilic, including lanthanides, actinides, uranium, vanadium, molybdenum, tungsten, etc. 

• The method may also be used for electrochemically active inorganic compounds. 

• The electrode material has to be chosen carefully. Most metal oxide electrodes are not suitable because of the irreversible oxidation of the electrogenerated ions.

Polarographic Analysis

Polarography is considered to be an electroanalytical technique that is used for measuring the Current flowing between two electrodes present in a solution. This technique is possible only in the presence of applied voltage which seems to increase gradually. The purpose of this technique is to determine the concentration of a particular solute as well as the nature of the solute, respectively. Polarography is also known as Polarographic Analysis in analytical chemistry. This technique is considered to be an electrochemical method that is responsible for analyzing solutions of reducible or oxidizable substances.

Overview

In analytical chemistry, Polarography is also known as voltammetry, and Polarography is known to be a type of voltammetry where the working electrode is considered to be the same as a dropping mercury electrode or static memory drop electrode; these electrodes are believed to be very useful as they possess a wide cathodic range and renewable face.

Voltammetry is considered to be an electroanalytical method in which varied information is obtained about the analyte when the Current gets measured as the potential. The analytic data which is meant for a voltammetric experiment gets depicted in the form of a voltammogram. Voltammogram is considered to be a polarogram in the case of Polarography. Voltammogram is responsible for plotting the Current which is produced by the electrolyte.

The simple principle of Polarography is known to be the study of solutions or different electrode processes using electrolysis in the presence of two electrodes, among which one is polarizable, and one is non-polarizable. Non-polarizable electrodes are formed when mercury regularly drops from a capillary tube.

Types of Polarography

After having a brief about the principle of Polarography, let's discuss its types in a brief and detailed way to get a clearer understanding of Polarography:-

Direct Current Polarography (DCP)

In dc Polarography, it is witnessed that a constant potential seems to be applied during the entire drop-life time. It constructs a Current-voltage curve by applying a series of potential steps; these steps are synchronized with the drop fall. In most of the instruments, however, linearly changing potential gets applied at such a slow rate that the change of potential throughout the drop lifetime is found to be in a few millivolts.

Square Wave Polarography (SWP)

In the case of Square Wave Polarography, the Current present in the working electrode gets measured when the potential between the working electrode and a reference electrode is swept linearly with time. One can view the potential Waveform in the form of a superposition of a regular Square Wave onto an underlying staircase. The Current which is measured gets sampled two times - once at the end of the forward potential and again at the end of the reverse potential Pulse. Due to this Current sampling technique, the contribution that the Current signal receives from the capacitive Current is minimal.

Normal Pulse Polarography (NPP)

In the case of Normal Pulse Polarography, the potential doesn’t get altered due to a potential ramp that seems to increase continuously but gets altered by the Square Wave potential Pulses whose height is increasing and is overlaid on a constant initial potential. The mercury drop electrode is held at a constant potential for most of its duration. During this time, no electrochemical reaction seems to take place under a given experimental condition. The limiting Current that is there in NPP is considered to be diffusion controlled.

Differential Pulse Polarography (DPP)

Differential Pulse Polarography is considered to be the most efficient Pulse method among all. In digital instruments, an increasing Direct potential which is in the shape of a staircase is present in the excitation signal. In periodic succession, small Square Wave Pulses having a constant potential get applied to this increasing Direct potential. The superimposition gets synchronized with the drop time and seems to take place when the surface of the electrode experiences no changes. The Current gets measured two times at each mercury drop, before each Pulse, and at the end of the Pulse time. The difference between the measurements seems to be plotted against the Direct potential and has the potential to produce peak-shaped polarograms.

Polarographic Cell

The Polarographic cell is considered to be used for continuous Analysis of flowing situations. When the Polarographic cell gets used for the Analysis, it should get designed in such a way that it can have a low holdup volume in respect to the flow rates that are being used so that the response to changes in the composition can be faster than before.