Polarography

Polarography is an electroanalytical method used in chemical analysis. It is used to analyse qualitative and quantitative determination of substances that can be oxidised or reduced. In addition, the technique helps obtain current-voltage curves from which the concentration of many species is determined with high reproductivity at low concentrations. This analysis is done using polarographs, and it doesn’t need an initial separation step. Polarographic also uses dropping mercury electrodes as part of its analysis. This article further explains Polarography and polarogram.

What Is Polarography?

A method of analysis of a solution that is electrolysed and kept under diffusion-controlled conditions is known as Polarography. The basic idea behind this method is to pass electricity between two electrodes, one with a large surface area and the other with a small surface area. This technique is convenient for analysing inorganic, organic, and biological solutions both qualitatively and quantitatively.  

The large electrode used in this technique acts as a mercury pool at the very end of the cell. Moreover, a small electrode is the drop of mercury that enters the very finely sized tube of the dropping mercury electrode (D.M.E.). So, if we apply a steady voltage to this cell, constructing a reproducible current-voltage curve is possible.

How Does Polarographic Analysis Work? 

A dilute solution of the electroactive substance to be analysed in an appropriate mixture containing a surplus of stabilising agents is known as an electrolyte. A Polarographic cell containing a cadmium chloride solution, subjected to an external Electro-Motive Force (E.M.F.), positively charged ions in the way to solve will be drawn to the dropping mercury electrode (D.M.E.).

The amount of current through a cell is the total of these diffusive and electrical forces. A graph can be obtained by increasing the voltage applied and the current measured.

Advantages and Disadvantages of Using D.M.E.

Advantages

• Its surface area can be replicated using only one capillary.
• The weight of the drops is used to calculate the surface area.
• Mercury can form amalgams with a wide range of metals, reducing their potential.
• High hydrogen overvoltage on mercury allows for ions accumulation that would be difficult to concentrate on other working electrodes.

Disadvantages

• When the size of the drop changes, the area of the microelectrode changes.
• Mercury is easily oxidised, limiting the electrode range for use.
• Refrain from touching the capillary’s tip with any foreign material because it can clog the capillaries.  

Characteristics of Polarography

The sample investigated is dispersed in a base solution that contains a surplus of a base or assisting electrolyte and placed in a particular electrolytic cell with a pool of mercury as an anode.

In this cathode, mercury falls from a capillary at one drop every 2 to 4 seconds. 

When a slowly increasing potential is applied to the cell, and the resulting current is produced on a galvanometer, the nature and intensity of the reductive materials in the sample can be determined from the resulting current-voltage curve.

Analytical data on oxidisable substances can be obtained if the dropping mercury electrode is used as the anode.

Types of Polarography

There are two main Polarography types: differential pulse polarography and normal pulse polarography.

Differential Pulse Polarography

It is a type of Polarography that generates a graph using a range series of discrete potential steps rather than a possible straight ramp. If you want to perform differential pulse polarography, you have to plot the differential current versus the average potential.

Normal Pulse Polarography

The normal pulse polarography (NPP) step starts at almost the same value and rises in tiny steps. When Hg drop is removed from a capillary, the potential is reset to zero in anticipation of the next step.

What is A Polarogram?

A polarogram is a graph that shows current versus potential in a polarographic analysis. Identification of the compound is provided by the location of a polarographic wave in a polarogram along the x-axis (E1/2). In contrast, the density is provided by the magnitude of the limiting diffusion current (id).

Ilkovic Equation

This equation is used to find the relation between diffusion current and concentration of the electrode, which is not polarised (it is also the substance reduced and formed at dropping mercury electrode. 

ID = Constant nCm⅔t⅙D0.5

Application of Polarography

• Various new methods, such as differential, derivative, and oscillographic, have recently been implemented in this field. 
• Derivative Polarography has the potential to help determine the existence of materials with similar half-wave potentials.
• Oscillographic Polarography should find a wide range of applications in reaction kinetics.
• Polarography has many pharmaceutical applications like dissolved oxygen and peroxide, trace metals and metal-containing drugs, insecticides, antiseptics, vitamins, hormones, antibodies, etc. 

Conclusion

We have learnt that Polarography is a method used to find trace metals in pharmaceutical products and estimate medication with metals as constituents. There are two types of Polarography, namely differential pulse polarography and normal pulse polarography. An essential part of the polarographic analysis is the polarogram and polarographs. This analysis also uses dropping mercury electrodes to analyse electroactive substances quantitatively and qualitatively.