Electrode mercurous sulphate

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. 

The described electrodes, and especially the silver chloride, calomel and mercurous sulphate electrodes are used as reference electrodes combined with a suitable indicator electrode. The calomel electrode is used most frequently, as it has a constant, well-reproducible potential. It is employed in variously shaped vessels and with various KC1 concentrations. Mostly a concentration of KC1 of 0.1 mol dm-3, 1 mol dm-3 or a saturated solution is used (in the latter case, a salt bridge need not be employed) sometimes 3.5 mol dm-3 KC1 is also employed. The potentials of these calomel electrodes at 25°C are as follows (according to B. E. Conway) ... 

The mostly used Weston cell consists of mercurous sulphate and cadmium amalgam electrodes ... 

Although there exists a concentration gradient of mercurous sulphate from the anode to the cathode it is unlikely that this causes an appreciable alteration in the electrical potential throughout the bulk of the electrolyte. Hence without serious error the applied E.M.F. V may be equated to the difference in the values of V at the two electrodes and since F at the unpolarisable electrode remains unaltered the variation of a with V is observed at the cathode. With the aid of thermo-dynamic reasoning, Lippmann deduced the relation... 

Fig. 7. Continuous recording of the limiting current Oxidation of fAreo-l,2-diphc-nylethyleneglycol by periodate. Decrease in the periodic acid concentration with time. 1M Acetate buffer of pH 4.3, 5 x 10 5 M-KIO4, 5 x 10 8Af diol. Current recorded at 10.4 V (mercurous sulphate reference electrode) at 15° C. Figures on the abscissae give the time in minutes the galvanometer zero and the current before the addition of periodate and after addition at / = 0 are marked. Full scale sensitivity 0.8 [zA...

Fig. 10. Oxidation of fAteo-l-phenylpropane-l,2-diol by periodate. 1M acetate buffer pH 4.7. 1 X 10 5 periodate, 1 x 10diol, 25° C. Time (in minutes) after which the recording of each curve was started is noted. Curves starting at 0 V, 200 mV/absc., 450 mV/min., mercurous sulphate reference electrode, h = 50 cm, full scale sensitivity 0.6 [ A...

Platinum (0.03 cm2), gold (0.07 cm2) or vitreous carbon (0.07 cm2) disk electrodes are used as working electrode (Radiometer or Metrohm), platinum grid (Good Fellow) as counter-electrode and a saturated mercurous sulphate reference electrode Hg/Hg2S04/K2S04sat (MSE E — 0.656 Y/SHE) connected to the sample by a Luggin capillary. 

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... 

Froment and co-workers " have employed REFLEXAFS (vide supra) for studying passive films on iron and nickel. Their early studies were concerned with demonstrating the applicability of the REFLEXAFS technique to electrochemical systems. Most recently, they have used this technique to study the structure of passive films on Ni and on Ni-Mo alloy electrodes. For the Ni electrodes, they performed studies after reduction at — 700 mV (vs. saturated mercurous sulphate electrode) as well as in the passive (-l-3(X)mV) and transpassive (-1-800 mV) regions. The Fourier transforms for the films in the passive region have a Ni—O peak at a distance that corresponds closely to that in bulk nickel oxide. However, no Ni-Ni interactions were observed. These investigators interpreted these results as consistent with a model that postulates an amorphous hydrated polymeric oxide. ... 

Experiments were carried out at room temperature on (lOO)-oriented p-Cdo.95Zno.05Te with a dopant density of 10 cm"3. Gold ohmic contacts were obtained at room temperature by electroless deposition. A classical three-electrode setup was used, with a p-Cdo.95Zno.05Te working electrode, a platinum counter electrode, and a saturated mercurous sulphate reference electrode (MSE = +0.64 V vs SHE). XPS surface analysis was carried out using a VG... 

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

W Let US consider a copper electrode immersed in an aqueous solution containing Cu ions with a concentration of 10 mol L l Its open-circuit potential is -eO.25 V/she (close to the thermodynamic vaiue). if this open-circuit potential Is measured by means of an SCE, the value found is -eO.01 V/SCE- whereas choosing a mercurous sulphate electrode Hg2S04/Hg with a saturated K2SO4 solution would yield the value of -0.39V/msE. A measurement error of about 1 mV on these voltages would thus yield a relative error of 10% in the first case, and of 0.3% in the second case. But this is meaningless. ... 

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 ... 

In polarographic practice the most important reference electrodes are separated calomel electrodes, a mercurous sulphate electrode, or, especially for small volumes, a silver chloride electrode immersed into an electrolysed solution containing OT M chlorides. This electrode proved satisfactory over the pH-range 1-13 when sodium or potassium chloride was added to the buffer solutions. Measurements in solutions forming complexes with silver e.g. glycine, veronal or ammonia buffers are precluded. The use of this electrode eliminates the uncertainty concerning the liquid junction potential. 

For nonaqueous solutions, several reference electrodes have been suggested, e.g. a mercurous sulphate electrode in concentrated sulphuric acid for measurements in solutions of sulphuric acid calomel, mercurous sulphate and sodium acetate electrodes in glacial acetic acid for work in this solvent, etc. A graphite rod is an excellent reference electrode in glacial acetic acid containing strong acids, whereas in aqueous solutions its application as a reference anode cannot be recommended. 

Thus of the separated reference anodes, the mercurous sulphate electrode is even somewhat superior to the calomel electrode because of its lower polarizability. As reference cathodes, amalgam electrodes have proved to be best. 

The potentials of these reference electrodes can be checked against a standard half-cell, but another method must be used when the reference electrode is not separated from the electrolysed solution, when the liquid junction potential is unknown or when the potential of the reference electrode is not very reproducible— as with the mercurous sulphate electrode. In this method the unknown half-wave potential is measured against the half-wave potential of a standard substance, whose half-wave potential against S.C.E. or N.C.E. in the used supporting electrolyte is accurately known. 

Fig. 45. Oxidation of tAreo-l-phenylpropane-l,2-diol by periodate. 1 M Acetate buffer pH 4-7, 1x10 M l-phenylpropane-l,2-diol, 25 °C. The curves were recorded after elapsing of the time marked on the beginning of the curve on polarogram, starting at 0-0 V, mercurous sulphate electrode, 200 mV/absc., 325 mV/min, tj = 2-8 sec m — V9 mg/sec, full scale sensitivity 3-2/lA.

Alternatively the total chlorine may be titrated electrometrically in buffer solution (0-2M to sodium acetate and 0-2M to acetic acid), with silver nitrate using a silver/silver chloride electrode and a mercurous sulphate reference electrode. 

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 ... 

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

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