Dropping Mercury Electrode and Saturated Calomel Electrode

Polarography determines the quantitative and qualitative reductions and oxidation of electroactive substances. An electrode that is a working electrode for a polarographic analysis is known as a dropping mercury electrode. The principle of dropping mercury electrodes is that the mercury drops are weighed to determine the substance qualitatively and quantitatively. This article will read more about dropping mercury electrodes and calomel electrodes used in Polarography. 

Dropping Mercury Electrode 

DME or Dropping Mercury Electrode is used in Polarography to get quantitative and qualitative analyses of substances. In this electrode, the mercury is constantly dropped into the solution from a reserve with the help of capillary tubes. The diameter of these tubes is 0.03- 0.05 mm. 

Most analyses recommend a drop interval of 1 to 5 seconds. The use of the DME has the distinct advantage of eliminating the impacts of electrode poisoning due to the continuous regeneration of the electrode surface revealed in the test solution.

Polarography makes use of DME. It has several benefits as well as drawbacks. When using the DME, exercise caution. Only valid and doubly distilled mercury must be used in DME. When not in use, the tip of the DME should always be immersed in water and cleaned by dipping it in nitric acid. Holding the assembly in a vertical position can help to eliminate the vibrations. 

Construction of DME

• A mercury reservoir is included in the set.
• It comprises fine capillaries with bore sizes ranging from 20 to 50 millimetres and lengths ranging from 10-15 centimetres.
• Rubber tubing joins the capillary to the mercury reservoir.
• The unknown solution is placed in a small glass electrolysis cell.
• The mercury reservoir’s height allows for a drop time of 1-5 seconds.

Working of the Dropping Mercury Electrode

• This electrode can be polarised and used both as a cathode and an anode. 

• If the DME acts as a cathode then the mercury pool will act as Anode and vice versa. It is a counter electrode to the mercury pool. 

• The counter electrode is not polarisable.

•  Electrolyte, such as KCl, is added to the analyte solution at a concentration of 50-100 times that of the sample.

• To remove oxygen from the circuit, pure nitrogen or hydrogen is bubbled in the solution. 

• For example, if the analyte solution contains cadmium ions, those ions are discharged at the cathode.

• The polarographic cell is then subjected to a gradually increasing voltage, and the current is measured.

• The graph depicts the relationship between applied voltage and current. 

• Polarogram is the apparatus, and Polarograph is the graph obtained. 

• The microelectrode in direct current (dc) polarography is a dropping mercury electrode (DME). It is made up of slight measurable declines of mercury flowing from the aperture of a capillary tube attached to a mercury reservoir.

• The most popular method of the model electrode is a saturated calomel electrode (SCE) with a greater surface area. Only a meagre amount of current flows as the cell’s voltage increases until the substance under assay develops reduction or oxidation.

Saturated Calomel Electrode

The saturated Calomel Electrode is another electrode used in Polarography. It is created through the reaction of inorganic Mercury with mercury chloride. 

The saturated Potassium chloride mixture in water is the aqueous solution associated with mercury chloride and mercury. (Hg2Cl2, practically insoluble salt, can be referred to as calomel). A porous frit connects the electrode that is submerged in the solution. This porous frit forms a salt bridge.

The SCE has the benefit of keeping the Cl– content, and thus the electrode potential, consistent even if the KCl solutions partly evaporate. The SCE has the significant disadvantage of being temperature-sensitive in KCl solubility. Higher temperatures (above 80°C) increase the concentration of Cl– while decreasing the electrode’s potential. 

Electrodes carrying unsaturated KCl solutions have less temperature dependence, but their potential changes as the concentration of KCl rises due to evaporation. A silver chloride electrode (SCE), which is an extra reference electrode, is often used in place because the use of liquid metal in a reference electrode can be problematic. 

Working Of Calomel Electrode

• Calomel electrode can behave as both an anode and a cathode, but it depends on the electrode or the electrode with which it is coupled. 

• Suppose the calomel electrode is used as an anode. In that case, the anodic reaction is as follows: 2HG+2Cl– will give you HG2CL2 plus 2 electrons, indicating that mercury is oxidised to liberate two electrons.

• Mercurous Chloride, which is just like calomel Hg2Cl2, will accept two electrons and undergo reduction to give you two Hg2+Cl–, when the calomel electrode acts as the cathode. All electrodes carry potential and the Nerst equation can be used to find the electrode potential of the calomel electrode. 

Conclusion

Polarography is a chemical analysis of substances that makes use of various electrodes. Two such electrodes are the dropping mercury electrode and calomel electrode. Dropping Mercury Electrode does the research by counting the dropping mercury from the capillary tube. This article contains more about these electrodes and their functioning.