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have studied the voltameter in itself, but their work is necessarily dependent on the electrical measurements referred to. Lord Rayleigh's form of voltameter consisted of a platinum bowl to serve as cathode, a wire or rod of pure silver for anode, suspended in the middle of the bowl, and a strong solution of silver nitrate or chlorate as electrolyte. The weight deposited on the cathode is determined: the loss of weight of the anode cannot be depended on, as partial oxidation takes place. This arrangement of apparatus is not quite free from irregularity in its action; it has been subjected to careful chemical criticism by Richards, who finds. that the irregularities are due to a subsidiary reaction at the anode, and can be eliminated by enclosing the anode in a porous pot, to prevent diffusion between it and the cathode. The apparatus, in his hands, took the form shown in Fig. 5 (actual size). E is a platinum crucible, with lip; it contains ΙΟ per cent. AgNO, solution, freshly prepared. C is the anode, of pure silver rod, suspended by a silver wire from a glass rod, A, which served to ensure good

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E

D

C

FIG. 5.

electrical insulation. Between anode and cathode is placed the

1 Zeitschr.phys. Chem., 32. 321 (1900) and 41 302 (1902), or Proc. Amer. Acad., 35. 123 and 37.

cylinder D of porous earthenware (Pukal of Berlin). This, which is only 1 mm. thick, is thoroughly cleansed with nitric acid and water beforehand, and the level of liquid inside it being a trifle lower than outside, if any diffusion takes place it is towards the anode. The strength of current used may be about o'or ampere per square centimetre of cathode surface. The silver is deposited in a crystalline form, and needs to be very thoroughly washed with water, to remove the mother liquor from the crystals; it is finally washed with alcohol and dried at 160° C. After use the silver should be dissolved. off with nitric acid and the bowl cleaned for further use. The bowl should be kept free from scratches, to ensure a good deposit.

Richards finds that Rayleigh's deposits were too heavy, the correct value being o'0011175, and considers that the improved instrument can be relied upon to one part in ten thousand

or more.

3. Copper Voltameter.-This is a convenient instrument, very simple in manipulation, and satisfactory when the highest

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degree of accuracy is not required. In its usual form it consists of a thin copper sheet for cathode, suspended between a pair of thicker sheets of copper for anode, the solution being any copper salt, commonly A the sulphate. A convenient construction is shown in Fig. 6. A is a square glass jar, fitted with an ebonite lid. The anode plates B, B are bent out of one sheet of copper, and are fastened to the terminal D. The cathode C has a lug passing through a slot in the lid, and is provided with a detachable binding screw E. The electrolyte is 10 per cent. CuSO. The anode oxidises on use, and consequently cannot be employed for measurement. The cathode, on the other hand, receives, under proper conditions, an adherent deposit of metallic copper. It is removed from the solution, washed, dried with filter paper, and weighed. The weight of copper obtained is always a little too low: 1 if the current

FIG. 6.

1 Richards, Zeitschr. phys. Chem., 32. 328.

density is high, a little hydrogen is evolved instead of copper, while if the current density is too low, some of the copper goes to form a cuprous salt instead of being deposited. The most favourable results are obtained with a current density of about o'or ampere per square centimetre of cathode. The voltameter works most accurately at a low temperature and in the absence of oxygen. Used without any special precautions, however, it should give results to within two or threetenths per cent.

4. Mercury Voltameter.

Mercurous salts are always used. If Hg(NO3)2 be taken in quantity sufficient to make a decinormal solution, shaken up with water, and enough nitric acid added just to dissolve the basic salt which forms, an electrolyte is obtained which can be used between mercury electrodes for voltametric purposes. The apparatus used by Bolton1 (Fig. 7) consists of two glass spoons of mercury supported by the lid of the containing vessel. Contact is made with the mercury by platinum wires sealed through the glass. Mercury dissolves off the anode and is deposited on the cathode: hence there is a tendency to form a denser solution at the anode, and, if too strong current be used, crystals will form there; the anode should therefore be put at the top. The mercury of either electrode is removed after the experiment, washed with distilled water, dried-best by rolling it, along a trough of filter paper —and weighed. The mercury behaves as a univalent metal, and the process at both anode and cathode is quantitative; the current density must not, however, exceed about o'005 ampere per square centimetre. The mercury voltameter, therefore, has the advantage that the loss of weight of the anode can be used as a check on the gain by the cathode; but it is only available for small currents.

FIG. 7.

Minute currents of long duration can be measured con

1 Zeitschr. f. Elektrochem., 2. 75.

veniently by a form described by Lehfeldt.1 A glass tube of about 1 to 1.5 mm. bore is graduated and calibrated, and provided with two platinum electrodes sealed through the glass at opposite ends. It is filled with mercury, except for one drop of mercurous nitrate solution in the middle, which separates the mercury into two portions, and finally sealed up. The tube is placed vertically, and the upper part made the anode. Mercury then dissolves off this and is deposited on the cathode, so that the drop of solution creeps slowly up the tube, its movement being proportional to the quantity of electricity flowing. When the drop has reached the top of the scale, the tube may be inverted and used over again.

The mercury voltameter has recently been adapted for use as a meter for domestic purposes. The anode is mercury, the cathode a platinum cone; the mercury deposited on this drops into a measure glass. The whole apparatus is enclosed in glass and sealed up, so that there is no leakage or loss by evaporation; and when most of the mercury has been deposited, it is only necessary to invert the meter to make it flow back into the anode and start afresh.

§ 2.

MECHANISM OF ELECTROLYSIS

The fact that the products of electrolysis do not appear in the interior of the electrolyte, but only at the electrodes, has from the first suggested a convective explanation. Faraday was sufficiently impressed with it to form the hypothesis of ions, i.e. of charged particles in the liquid, travelling under the action of the electric force; the movement of these electric charges would constitute the current, and on their reaching an electrode and giving up their charge to it, they would accumulate, and one might expect to distinguish in the ordinary chemical ' forms the substances of which the charged particles are made. It appears that all electrolytes break up into two parts-one travelling in one direction (called, arbitrarily, the direction of the current); this consists of "cations," positively charged and carried towards the cathode: the other of "anions," negatively

1 Phil. Mag. (6) 3. 158 (1902).

charged, carried in the reverse direction, and deposited on the anode. The former include hydrogen and the metals; the latter, most other atoms and atomic groups, such as Cl, NO, SO, CH COO, etc.

The typical electrolyte is a salt, and the dissociation is into (a) the metal; (b) the residue, described as the "acid radicle." Acids and bases are also electrolytes, the former being compounds of (positive) hydrogen ions with an acid radicle, the latter compounds of a metal with (negative) hydroxyl ions. The acid and basic properties of liquids are due merely to the presence of hydrogen and hydroxyl ions respectively. Water is slightly dissociated into hydrogen and hydroxyl, and is therefore capable of reacting either as a weak acid or a weak base.1

The combination of Faraday's laws with the accepted. atomic theory of chemistry leads at once to an atomic hypothesis as to electricity. For if, say, the silver in a solution of silver nitrate is made up of a large number of similar particles, the silver atoms, and if the amount of electricity conveyed is proportional to the amount of silver deposited on the cathode, it is natural to conclude that each atom carries the same quantity of electricity with it. Hence we arrive at the notion of an atomic charge of electricity.

Now, the masses of the different atoms are very unequal; but, Faraday found, an atom of sodium or of hydrogen carries with it the same quantity of electricity as one of silver. Here is a consideration that greatly strengthens the view that electricity is divided up into "charges" of equal magnitude, of which one may be associated with an atom of matter; and the view is only confirmed by the cases in which an atom does not carry the same charge as silver or hydrogen. For then it is found that the charge conveyed is a simple multiple of the unit, as, two by zinc, two or three by iron, three by aluminium, two or four by tin, and so on: never a fraction of the unit charge. The number, as was remarked in the previous section, is equal to the valency of the atom (or atomic group); indeed, on the electrical theory, valency comes to mean nothing else

See Mellor, "Chemical Dynamics," in this series.

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