Mercury-mercurous sulphate electrodes

Table III.2.3 Electrode potentials of the mercury-mercurous sulphate electrode at different temperatures [1]...

Other examples are the silver/silver chloride and mercury/mercurous sulphate electrodes,... 

However, in the case of stress-corrosion cracking of mild steel in some solutions, the potential band within which cracking occurs can be very narrow and an accurately known reference potential is required. A reference half cell of the calomel or mercury/mercurous sulphate type is therefore used with a liquid/liquid junction to separate the half-cell support electrolyte from the process fluid. The connections from the plant equipment and reference electrode are made to an impedance converter which ensures that only tiny currents flow in the circuit, thus causing the minimum polarisation of the reference electrode. The signal is then amplifled and displayed on a digital voltmeter or recorder. 

Mercury-mercurous sulphate. This electrode is prepared in the same wayas the calomel, but the mercurous sulphate has to be specially prepared. It is a useful electrode for sulphate solutions but becomes unstable if the sulphate concentration falls below 0.1 mol dm. ... 

The most popular separated reference electrodes are the calomel electrode (usually saturated) and the mercurous sulphate electrode. The calomel electrode is made by adding a solution of potassium chloride of the desired concentration (1 M or saturated) to a layer of mercury. No calomel need be added because a thin layer of this salt is formed during electrolysis, and a thicker layer of calomel can cause an increase in the resistance. Because of the greater solubility of mercurous sulphate, a small amoimt of this salt is added to the surface of mercury in the preparation of a mercurous sulphate electrode. A solution of 1 N sodium sulphate... 

When anodic as well as cathodic waves are to be investigated and when the values of the half-wave potentials are to be determined, as in other instances where the use of the mercury pool electrode is excluded, it is necessary to have a reference electrode separated from the solution to be examined. A suitable vessel for such measurements is the Kalousek vessel, shown in Fig. 20d the solution to be investigated is separated from the electrolyte of the reference electrode by a liquid boundary. The cell consists of two compartments the solution to be examined is placed in the left compartment (Fig. 20d) the right compartment, separated by the stopcock B, contains the reference electrode. To ensure a low resistance, the stopcock B is best constructed with a wide bore and the connecting tubes on both sides of this stopcock should be as short as possible. As reference electrodes, calomel or mercurous sulphate electrodes are usually used. The procedure for a cell containing a mercurous sulphate electrode is as follows ... 

For titration of halides the solution is best buffered to about pH 5. The indicator electrode is a piece of silver wire of about 1 mm in diameter and about 3 cm long which has been curled into an open spiral the reference electrode may be a mercury-mercurous sulphate half-cell with a potassium sulphate bridge. An alternative reference electrode for use in titration of halides, and one which works well in practice and is convenient to use, is the glass electrode if a glass electrode is used in this way, however, the potentiometer must have a high-impedance, shielded input socket. The following practical details are suitable for halide titrations ... 

The potential of the standard mercurous sulphate electrode used above is -f- o 956 volt, the mercury being positive to the solution. 

An H shaped glass container has electrodes at the bottom of the lower arms connected to the external wires by sealed-in platinum wires. The negative electrode is of cadmium amalgam containing 12.5 % cadmium, while the positive electrode is formed by mercury with a layer of paste, which consists of mercurous sulphate, dispersed mercury and small crystals of cadmium sulphate. 

The mercurosulphate electrode is most suitable for work with the sulphate solutions. It is prepared analogously as the calomel electrode, with one exception that the paste is composed of mercurous sulphate, ground together with mercury and an alkali metal sulphate solution of a definite concentration. In the electrode the equilibrium is attained, according to the equation... 

Two glass vessels A and B (Fig. 59), each provided with a pair of electrodes, are to be used the upper electrode is a piece of metallic zinc, the lower mercury, covered with a layer of mercurous sulphate. The vessel A contains moist ZnSO. 7H2O the other, b, contains moist Zn SO. 6H20. Although the two hydrates in the moist state are both stable only at 39°, yet with certain precautions, like those to be attended to in working with supersaturated solutions—especially the closure (with shellac) of the two vessels at the top—it is possible, to work with both salts at temperatures other than 39°. To prepare the hexahydrate in Bit is then only necessary to warm that vessel... 

---