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Construction and Working of Dropping Mercury Electrode & Rotating Platinum Electrode and Applications

Dropping mercury electrode - Construction, Working, Rotating platinum electrode - Construction, Working, Rotating Platinum Electrode advantages.

Dropping mercury electrode

The dropping mercury electrode (DME) is a silver electrode used in polarography, with its capillary tubes used to continuously feed mercury into the polarization solution from a reservoir (internal diameter 0.3 mm to 0.05 mm).

From a resistance-glass capillary (0.05-0.08 mm in diameter and 5-9 mm in length) it is released in small, uniform drops under polarisable micro-electrodes with a head of 40-60 em of mercury.

These are the advantages of the mercury electrode that drops.
  • The continuous renewal of its surface makes it reproducible, smooth, and eliminates the risk of passivity or poisoning.
  • After a change in the applied potential, the diffusion current takes on a steady value, which is reproducible.
  • The highly electronegative hydrogen overpotential on mercury makes it possible to deposit substances that are difficult to reduce, e.g. alkali metallic ions, aluminum ions, and manganese (II) ions. With a platinum microelectrode, current-potential curves for these ions cannot be obtained.
  • From the weights of the drops, we can calculate the surface area.
In an environment of +0.4 volts or higher, mercury dissolves and produces an anodic wave along with mercury ions. Applying the electrode to a surface between +0.4 and -2.0 volts will give mercury ion oxidation.

Acid solutions undergo visible hydrogen evolution at potentials lower than 1.8 volts us SCE, and the typical electrolytes begin to discharge.

Supporting electrolytes that possess higher reduction potentials than the alkali metals are capable of extending the range to about - 2.6 volts us SCE. Tetra-alkyl ammonium hydroxides or their salts can be used for this purpose.

Construction

Three pieces make up the apparatus
  1. Capillary
  2. The Mercury reservoir vessel
  3. Standing tube with an adjacent stopcock. In the capillary tube, 5.5 centimeters of Corning marine barometer tube are joined to 6 millimeters of soft glass. This reservoir vessel contains mercury that is connected to an electrical system through tungsten contact mercury wells.

Working

Mercury is assumed to be acting as a counter electrode, i.e., either cathode or anode, with DME acting as the cathode. Counter electrodes cannot be polarized. An electrolyte solution, such as KCl, is added to the analyte solution, producing concentrations 50-100 times greater than the sample concentration. Oxygen will be expelled (removed) from the solution by bubbling pure nitrogen or hydrogen gas through it. The polarographic cell is provided with a voltage and the current is measured. An inverse graph of voltage versus current is drawn. Consequently, Polarograph is this graph and Polarogram is its apparatus.

Rotating platinum electrode

The glass rod consists of platinum wires made through mercury at an approximately 600 rpm speed. The current is measurable based on the potential.

But why platinum?

The positive potential of its oxidation prevents mercury from being used as an electrode. Platinum rotating electrodes are used.

What's the point of rotating?

When platinum is used as an electrode, the diffusion current at a steady state is slow. After each addition of reagent, one must wait a considerable amount of time. Platinum electrodes are therefore rotated at 600 revs.



Construction

From the figure, we can see the structure of the platinum electrode that rotates. Mercury seals are used to construct the electrode. Platinum wire of about 5mm diameter that passes through a small hole under standard mercury seal contains about 0.5mm of diameter. It is powered from the source through a wire attached to the mercury seal. Essentially, the electrode stem is made of tubing that rotates at 600 revolutions per minute (rpm).

Working

Indicator electrodes are made from rotating platinum electrodes. A supporting electrolyte such as KCl or dilute salt solution is added to the analyte solution to make it more electrolyte-stable. Oxygen dissolved in the solution is removed by bubbles of nitrogen gas. Titration is started by applying an electric potential between the electrodes. Graphs are plotted between the total volume of addition of solution and diffusion current, and endpoints are determined.

Rotating Platinum Electrode advantages

The design is straightforward. Hence, it can be used at a positive potential where the mercury electrode may be used since it has a wider range of workable voltage on the positive side. Since the electrode is rotated, diffusion current increases just as much as the value of polarograms.
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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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