J Heyrovsky’s innovated polarography, in which a falling mercury electrode is utilized in an electrochemical cell, revolutionized electroanalysis, electrode kinetics research, and adsorption investigations on mercury electrodes. Additionally, polarography has directly influenced the creation of novel and creative concepts in electrochemical methods, tools, and applications.
What is polarography?
Polarography is an electrochemical method of analysis that measures the current flow arising from the electrolysis of a solution at a polarisable microelectrode as a function of applied voltage. It is a type of voltammetry in which the working electrode is a dropping mercury electrode (DME) or a static mercury drop electrode (SMDE), which are advantageous due to their wide cathodic range and a reusable surface.
Polarography entails precisely determining the current-voltage relationship at a dropping mercury electrode under specific conditions. It has been regarded to be a potent and adaptable tool for qualitative and quantitative analysis if the substance is capable of cathodic reduction or anodic oxidation. The polarographic behavior of coordination compounds can be used to identify the degree of formation, stability constants, and compound structure.
The value of the current flowing through the cell at any applied voltage is measured using a polarograph, and the curve obtained is known as a polarogram.
The current that flows through the working electrode consists of two components:
- Faradic Current (iF), which is based on the analyte’s oxidation and reduction and contributes to the useful signal.
- Capacitive Current (iC), which is produced by charging and discharging the electrochemical double layer on the working electrode’s surface and contributes to the signal’s undesirable interference component (noise).
Principle of polarography
The basic idea behind polarography is to apply a progressively increasing negative potential (voltage) between polarizable and non-polarizable electrodes and then record the resultant current. So, the basic principle of polarography is the analysis of solutions or electrode processes through electrolysis using two electrodes, one polarizable and one non-polarizable.
a. Polarizable electrode or Working Electrode:- Dropping mercury Electrode
b. Non-polarizable Electrode or Reference Electrode:- Calomel Electrode
Polarography is a voltammetric technique that involves the oxidation (loss of electrons) or reduction (gain of electrons) of chemical species (ions or molecules) at the surface of a dropping mercury electrode (DME) at an applied potential. Qualitative and quantitative analysis can be performed using the current-voltage curve.
Types of polarography
I) Direct current (DC) polarography
In DC polarography, a dropping mercury electrode (DME) is subjected to a constantly increasing DC potential, and the resulting current is continuously recorded. The concentration of the component to be examined in the solution can be calculated by measuring the current-potential curve obtained during the electrolysis process. This approach has the advantage of allowing the electrode potential to shift very slowly.
II) Alternating current polarography
The DC voltage of DC polarography is superimposed with a low-frequency sinusoidal voltage of tiny amplitude (several to tens of millivolts). The AC polarography wave is produced by measuring the branch current of an electrolytic cell. In this approach, a constantly increasing DC potential (E) is superimposed on a constant amplitude AC potential. If such a combination is applied to DME, it generates two types of current. The total current is the sum of DC and AC. This process is not influenced by irreversible processes such as oxygen reduction.
III) Pulse polarography
The procedures in pulse techniques are based on the application of pulse variations of potential, and the current response is measured at a suitable time relative to the time of the pulse. Since diffusion and capacitive current intensities vary with time, pulse methods increase detection limits. There are three different polarography methods that work under the content of pulse polarography. They are:
A. Normal pulse polarography
B. Differential pulse polarography
C. Square wave polarography
A. Normal pulse polarography (NPP)
It is also known as large-amplitude pulse polarography. Normal pulse polarography (NPP) alters the potential by using square wave potential pulses with increasing height (pulse amplitude Ep) placed over a constant initial potential, rather than a continually increasing potential ramp. NPP maintains the DME at a constant potential before the start of the Faradaic current. Subsequently, near the end of the drop’s life, a voltage pulse is delivered, the amplitude of which steadily increases from drop to drop.
B. Differential pulse polarography (DPP)
In this approach, a constantly increasing DC potential is superimposed on a pulse of constant amplitude. The pulse is only applied at the end of drop times of 50mS. The current is measured twice during each drop. The current is measured just before and after the pulse is applied. The difference between these two currents is represented as a function of baseline potential (E).
DPP varies from NPP in that the potential is not constant before the pulse application but is replaced by a ramp voltage with DC polarographic properties. In this process, the pulse amplitude is constant. The measured current is displayed as the difference between the current recorded immediately before the pulse and the current sampled after the pulse.
C. Square wave polarography
In Square Wave Polarography, the current at a working electrode is monitored while the potential between the working electrode and a reference electrode is swept linearly across time. The potential waveform is a superposition of a conventional square wave and an underlying staircase. It has a higher sensitivity than a differential pulse that does not involve reversed current. Due to the faster scan rates, it reduces analysis time.
Types of mercury electrodes
Mercury is often used as a working electrode in polarography because it is a liquid metal that can be renewed after each droplet. A drop suspended from the end of a capillary tube is frequently employed as the working electrode. There are three different types of mercury electrodes, they are as follows:
a. Dropping Mercury Electrode (DME)
Gravity causes mercury drops from the capillary tube’s end of the dropping mercury electrode or DME. It increases continually as the mercury drips from the reservoir and has a finite lifetime of several seconds. The mercury drop is dislodged at the end of its lifetime, either manually or automatically, and replaced with a new drop.
b. Static Mercury Drop Electrode (SMDE)
To control the flow of mercury, the static mercury drop electrode, or SMDE, employs a solenoid-driven plunger. When the solenoid is activated, it temporarily lifts the plunger, allowing mercury to flow through the capillary and form a single, hanging Hg drop.
c. Hanging Mercury Drop Electrode (HMDE)
In this electrode, the mercury drop is released by revolving a micrometer screw, which forces mercury from a reservoir through a small capillary.
Instrumentation of polarography
The polarographic analysis apparatus consists of a cathode of dropping mercury electrode (DME), also known as a working or microelectrode, and an anode of a pool of mercury. Since the anode has a wide surface area, it is not polarized, which means its potential remains nearly constant in a solution containing anions capable of forming insoluble salts with Hg (Cl-, SO42-). It serves as an unstandardized reference electrode, the precise potential of which is determined by the nature and concentration of the supporting electrolyte.
The cell’s polarization is thus determined by the reactions that occur at DME. The cell has inlet and outlet tubes for expelling dissolved oxygen from the solution by passing inert gases (He or N2) before but not during an experiment, otherwise, the polarogram of dissolved oxygen will appear in the current-voltage curve. Under those conditions, the current-voltage curve is the current cathode potential curve, but it has been displaced by a constant voltage corresponding to the anode’s potential. An external anode of known potential, such as a saturated calomel electrode, is sometimes employed.
Polarography: Basic Apparatus
Image source: https://www.bdn.go.th/tp/ebook/qQqcAKt1pR9gC3q0GT5gMJq0qT5co3uw
It is a polarographic analysis graph of current versus potential.
Image source: https://soe.unipune.ac.in/studymaterial/ashwiniWadegaonkarOnline/UNIT%20VII.pdf
Factors affecting the current-voltage curve
1. Residual current Ic
In polarography, the residual current is the small charging current detected in the absence of a reactive species. This current flows in the absence of the depolarizer (due to the supporting electrolyte). This must be taken into account when analyzing polarograms.
When a current-voltage curve for a thoroughly degassed solution containing only a supporting electrolyte that does not reduce at the electrode surface is plotted, a tiny current is still recorded in the system at potentials greater than about -0.4V. This is due to the Hg drop and the solution acting as a tiny condenser; the Hg drop accumulates a -ve charge on its surface in comparison to the potential of the thin layer of the solution surrounding it. As the Hg drop descends, it takes this charge with it, resulting in a modest +ve current.
This condensing or charging current is a non-faradaic current. It does not result from electrochemical processes at the electrode.
There are two sources of residual current.
a. The first is the reduction of trace contaminants (D.O., heavy metals, etc.) that are almost always present in the blank solution throughout time.
b. The second is the ensuing charging current electrons, which charge the droplets in relation to the solution.
2. Migration current (Im)
It is caused by cation migration from the bulk of the solution to the cathode due to diffusive force, regardless of a concentration gradient. An electro-reducible or oxidizable ion in the solution can get to the DME by diffusion or migration in the absence of convection. The electrostatic interaction between the electrode and the oppositely charged ions causes migration. The migration current is also affected by the transport no.of ions.
The migration current complicates the investigation of the electrode reaction. It can be removed by introducing an excess of supporting electrolytes to the solution. The supporting electrolyte is chosen in such a way that it conducts current while not reacting with electroactive species. Since the supporting electrolyte is added in significant quantities, a state is generated in which the contribution of transport no. of the electroactive species is decreased to negligible levels and their migration is almost zero.
It is found that,
Il (cationic red”) = Id + Im
Il(anionic red”) = Id + Im
Where, Il, Id, and Im, are limiting, diffusion, and migration currents, respectively.
3. Diffusion current
Diffusion Current (id) is the difference between Residual and Limiting current.
Diffusion current is caused by electro-reducible ion diffusion from the bulk of the sample to the surface of the mercury droplet as a result of the concentration gradient.
The rate of diffusion of an ion to an electrode surface is given by Fick’s second law:
dc/dt = D d2C/ dx2
Where, D = diffusion coefficient
C = Concentration at the time ‘t’
x = distance from the electrode surface
4. conventional current (Ic )
Conventional current occurs when ions are brought to the electrode surface by mechanical processes, such as swirling in a solution affecting the limiting current.
5. Limiting current (Il)
The current reaches a steady state value when it exceeds a particular potential, known as the limiting current. The limiting current is due to the contribution of three different types of current. Thus
Il = Ic + Im + Id
where, Ic = convectional current
Id = Diffusion current
Im = Migration current
6. Kinetic current
The kinetic current, which is the rate of non-electrode reaction, might influence the limited current. The kinetic current is proportional to the rate constant and the interface volume. As a result, it is a direct function of size but is unaffected by the velocity of mercury flow. This current occurs when an electroactive species’ oxidized or reduced state is in chemical equilibrium with another material.
In 1934 Ilkovic investigated the different factors that affect the ld and developed the polarography equation that relates the diffusion current (id) and the concentration of the non-polarisable electrode.
id = 607 nD1/2 m2/3 t 1/6 C
where. ld = the average. diffusion current
n = no. of Faraday
D = a constant (Cm2S-1)
C = analyte conc (m mol L-l)
M = mass of Hg dropping per sec
t = drop time (s)
Factors affecting diffusion current
Diffusion current varies with temperature. As the ionic mobility changes with temperature, the Id changes.
Pressure controls the Id by altering the mass flow of mercury and thus its speed. This can be regulated by adjusting the reservoir’s height and speed.
Diffusion current is directly proportional to the concentration of electro-reducible ions.
4. Interfacial surface tension
The diffusion current is affected by interfacial tension at the mercury surface, i.e. there is a gap between the surface of DME and the solution phase containing active ions. As a result, the current may be reduced by reducing the size of droplets as their interfacial tension can be reduced.
Applications of Polarography
1. Polarographic analysis can be used to determine the majority of the chemical elements.
2. Metals and metal-containing drugs must be determined.
3. This approach can be used to analyze alloys and other inorganic substances.
4. The pharmaceutical application involves the determination of dissolved oxygen and peroxides.
5. It is used to investigate chemical equilibria and reaction rates in solutions.
6. The measurement of polarographic current provides a simple way of estimating the rate of numerous fast electrode reactions.
7. Polarographic methods can be used to investigate food and food products, biological materials, herbicides, insecticides, and pesticides, among other things.
8. Polarographic detectors can be used to monitor organic effluents (such as minerals and fertilizers) from an HPLC chromatographic system.
Limitations of polarography
1. It is said that the solution should not be disturbed throughout the polarographic experiment, although Hg itself disturbs the solution.
2. Capillaries are quite small and thus readily clogged.
3. Mercury is extremely poisonous.
4. Polarography cannot measure solutions with concentrations fewer than 10-5 M. Electrical noise and residual current are found at low concentrations, hence no good signal can be observed.
5. The surface area of a drop of mercury is never consistent.
6. It cannot be used at greater positive potentials due to mercury oxidation.