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Polarography - Principle and Ilkovic Equation

Principle, Ilkovic equation, Measurement of capillary length, It's working, Advantages, Disadvantages
Electrochemical polarography is a method of testing a solution using an electrode that can be polarised as a function of the voltage applied over an electrolytic reaction.

Principle

This method of recording current flows between two electrodes, one of which is polarisable (the mercury-dropping electrode) and the other is non-polarisable, by gradually increasing the voltage between them. The half-wave potential and diffusion current are calculated from a sigmoid shape current-voltage curve. Concentrations of substances are measured using diffusion currents. Every element possesses a half-wave potential.

Ilkovic equation

This equation expresses the relationship between the ID current and the concentration of the metal in the mercury droplet - that is, the element that is reduced/oxidized or the metal that is reduced/oxidized during the droplet's residence.

id = 607 nD1/2 m2/3 t 1/6 C

Where,
id - In microamperes, id is the diffusion current
607 - The constant 607 is the sum of a variety of numerical factors, including the Faraday constant (P), the density of mercury, etc.
n - In electrode reactions, n is the number of electrons involved,
D - The diffusion coefficient D in centimeters per second-1,
m - Weighed by mg.sec-1 of the flow of Hg through the capillary,
t - Time in seconds = Drop time,
C - The concentration C (mmol/L) is the concentration.

It is reasonable to assume that the 'drop-time' will vary between 2 and 8 seconds following the Ilkovic Equation. It is imperative that the following two critical adjustments are carefully performed to achieve this goal.

Measurement of capillary length
By manipulating the Hg pressure, the drop time could be kept within the acceptable range (i.e., 2-8sec).

Based on four major factors, the Ilkovic equation can be described as follows:
  • As the size of the capillary (the length of it) and pressure in the Hg reservoir grow, both ‘m’ and ‘t’ should evolve.
  • As the 'drop time' of Hg is solely dependent upon the pressure that is applied at the interface between DME and 'analyte' solution, the height of the Hg column must be maintained consistently.
  • In a DME component, a voltage applied to the electrode's tip is responsible for potentially changing the surface tension at the electrode tip.
  • A significant variation in temperature and viscosity disturbs the 'diffusion coefficient' most significantly, so they must be kept to a minimum.

It's working

  • Mercury electrodes may be used as anodes and cathodes. This electrode is polarisable.
  • Pools of mercury serve as counter electrodes which are anodes when DME is cathode and cathodes when DME is the anode.
  • Counter electrodes are non-polarisable electrodes.
  • Electrolytes like KCl are added to analyte solutions, i.e., 50-100 times the concentration of the sample.
  • Oxygen is emitted by bubbles of nitrogen or hydrogen gas blowing through the solution to eliminate it.
  • A solution containing cadmium ions is discharged at the cathode if that solution contains cadmium ions. Cd2+ + 2e- → Cd
  • In the next step, it is applied progressively increasing voltage to the polarographic cell and current is measured.
  • This graph shows the relationship between the voltage applied and the current. Polarograph refers to this graph, and Polarogram refers to the apparatus.
  • A diffusion current analyzer is used to measure the concentration of analytes. Each compound has its half-wave potential, and this can be used when analyzing qualitatively.

Advantages

  • The area of the surface can be reproduced.
  • It is possible to replace electrodes, which eliminates poisoning effects.
  • Various metals form amalgams (solid solution) with mercury.
  • A drop's weight can be used to calculate the area of the drop.

Disadvantages

  • The capillary wall is very thin, making it easy to block.
  • Mercury is highly toxic.
  • Every drop of mercury has a different surface area.
  • There is no use for it at higher positive potentials because of the oxidation of mercury
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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