Mercurous Sulphate Electrod

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) ... 

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. ... 

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. ... 

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.

M Acetate buffer pH 4-3, 5 X 10" m potassium periodate, 5 X 10 M diol. Current recorded at — 0-4 V, mercurous sulphate electrode, at 25 C. Time in minutes after mixing. Galvanometer zero and the current before the addition of periodate and after addition at t = 0 are marked. Full scale sensitivity 0-7 / A. 

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

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

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