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.
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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 of different electrode processes by means of 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 in order 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 the passage of 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.
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.