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mercurous chloride as depolariser, and immersed in a solution of potassium chloride, which may be either normal or decinormal. Potassium chloride is in most cases exceptionally favourable as to electromotive force of contact with other solutions, so the calomel electrode is usually a good one to use. It is also easy to set up, and the materials can be obtained pure without difficulty. The potential of the normal electrode is usually taken as + o'56oo (metal positive to solution) at 18°, increasing o'oooé per degree above; that of the decinormal as + o-6130 at 18°, temperature coefficient o'ooo8. A convenient construction of the electrode is shown in Fig. 49. The mercury at the bottom of the tube is covered, about

FIG. 49.

1 cm. deep, with calomel that has been several times washed with KCl solution, and shaken up with mercury to remove any mercuric chloride; the rest of the tube contains the KCl solution. It is placed with its point immersed in a beaker of the same solution, and the bent tube filled by blowing through the side tube on the right. After use a few drops of the liquid in the bent tube may be blown out, and the remainder sucked back. The other electrode whose potential is required may be made in similar form, and also allowed to dip in the beaker

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of solution. Contact is made with the mercury of the normal electrode by a platinum wire sealed through a glass tube; or, as in Clark and cadmium cells, liquid mercury may be dispensed with, an amalgamated platinum sheet being immersed in the calomel paste instead.

When made up with due care, calomel electrodes agree amongst themselves to about 1 millivolt.

The decinormal electrode is stated by Richards 1 to be more constant and less disturbed by vibration than the normal. According to Ostwald and Luther, this is not the case if the electrode be properly made up; and the normal electrode has of course the advantage of a lower resistance.

The merturous sulphate electrode referred to above, for use with accumulators, is similarly constructed. Its potential with equivalent normal acid is + 0.956 volt.

For alkaline solutions the combination Hg: HgO: in.NaOH may be used. Potential + 0-387.

Silver, coated with silver chloride or bromide, and immersed in the corresponding potassium solution, may be used instead of mercury.3

Besides electrodes of the calomel type, the most important standard that has been used is the hydrogen electrode. This, as used by Wilsmore,4 is shown in Fig. 50; the form is convenient and suitable for accurate work, and may be adopted for electrodes of other gases as well. The gas is led in through a Richardson wash-flask K ; this is filled with the same solution as is to be used in contact with the electrode, in order that the gas should be saturated with the vapour of that solution, and şo not cause any change in concentration as it bubbles past the electrode. From the wash-flask it enters the bottom of tube A. This is closed at the top by a rubber stopper, through which passes a glass tube carrying the metal sheet which is to act as electrode; the latter may be palladium, or well platinised

i Zeitschr. phys. Chem., 24. 37 (1897).

? Physiko-Chemischen Messungen (Leipzig: Engelmann), 2nd ed., 1902, 9.v. for further details of potential measurements.

3 For details see Jahn, Zeitschr. phys. Chem. 33. 555 (1900). 4 Zeitschr. phys. Chem., 35. 296 (1900).

platinum, and is arranged so as to lie half in the electrolyte, which fills the lower part of A, and half in the gas. The gas escapes through the mercury trap H which prevents air from getting backwards into the apparatus. The electrode vessel is

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in communication through B with the beaker D, which is filled with the same electrolyte, and serves to receive the other electrode. A funnel is provided, with a tap, G, for convenience of washing out the end of the gas electrode. The whole arrangement can be sunk in a thermostat. For successful working it is desirable that there should be but little electrolyte in A, so that it may be the more quickly saturated with gas To keep the gas from going through by jerks, a glass tube drawn out very fine, to serve as a resistance, may be inserted between K and A.

Wilsmore finds that such an electrode made up with hydrogen in normal sulphuric acid, reaches a constant value in about a quarter of an hour, and can be relied on to nearly the same extent as the calomel electrode, i.e. to one or two tenths of a millivolt. It gives, with the Hg : Hg,SO, electrode, also in normal acid, an electromotive force o‘6953 at 25°, with the normal calomel electrode oʻ3276, mercury being positive, in both cases.

From the mean of several combinations Wilsmore concludes that H in normal ionic solution of hydrogen ions is 0·283 negative to the mercury of the normal calomel electrode, and therefore, taking the absolute potential of the latter as +0560, the absolute potential of hydrogen in such a solution is + 0.277. This is the number quoted in the table of electroaffinities (p. 159). There is, however, much uncertainty in the reduction to normal ionic concentration, so that for practical purposes it is better to take the electrode actually used for standard-preferably the normal calomel electrode.

For further particulars as to the use of the gas electrode reference may be made to the work of Emil Bose.

Another device used by Wilsmore is to polarise a platinum point cathodically with a definite E.M.F., and use it as a potential standard. The electrode in question should be very small (a wire sealed through glass and cut off short), but well platinised ; it is used as cathode, with a large platinum anode, in normal sulphuric acid, 1'2 to 1'3 volts being applied to the combination. The point is then slightly polarised, and may serve as a standard electrode for the measurement of any other electrode immersed in the same solution. The potential of the point is as follows :

| Zeitschr. phys. Chem., 34. 754 (1900).

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$ 9. STANDARD CELLS.1 ... A standard, or normal, cell is one which, if made up with , due care, can be relied upon to possess a definite and

permanent electromotive force, so that it can be used as a standard in electrical measurements. Since all cells, when discharging, suffer a certain amount of concentration, if not also chemical polarisation, and since on account of resistance the potential difference between the terminals is reduced during discharge, it is evident that a cell can only be a completely satisfactory standard when used on open circuit. Standard cells are therefore normally used with the potentiometer : if employed to give current, it must be only minute compared with what the cell could give on short-circuiting. Thus if an accumulator (with acid of measured density) be designed to gives ten amperes, then ido ampere may be regarded as a minute current for it; it will scarcely polarise at all when giving this small current, and may be regarded as a rough standard (reliable to i per cent.) when so used. But a Clark cell, which would perhaps only give zoo ampere when short circuited, could not be treated as a good standard if yielding more than 100000 ampere.

The conditions for a satisfactory standard cell are

(i.) It must be made of materials so well defined chemically that it can be reproduced with exactness.

(ii.) When brought back to the same external conditions (especially temperature), it must give the same electromotive force.

See the monograph by Jaeger, Die Normal-Elemente (Knapp: Hallé, 1902).

T, P.C.

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