B A 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,1 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 IO 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 ееее 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 E 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. CuSO4. 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'o 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 Bolton' (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. that by which it is led out the cathode. The liquid is the electrolyte, and the reaction that occurs when a current is passed through is the decomposition of water into its elements, oxygen and hydrogen. Now, the leading peculiarity of this, as of all electro-chemical as distinguished from ordinary chemical reactions, lies in the appearance of the products of reaction at the electrodes only; not, as usual, throughout the mass of the reacting material. In the case chosen as example, all the oxygen appears at the surface of the anode, all the hydrogen at that of the cathode. It is therefore convenient to consider the amount of material evolved, or in the case of a solid deposited, at either electrode, and in order to arrive at the true nature of electrolysis, it is first necessary to find on what this amount depends. This was accomplished by Faraday, and the laws in which he formulated his observations are commonly known by his name. They are (i.) The amount of any substance deposited is proportional to the quantity of electricity which flows through the electrolyte. (ii.) The amounts of different substances deposited by the same quantity of electricity are proportional to their chemical equivalent weights. What is implied by the first of these is, essentially, that the rate at which the electricity flows is of no consequence, provided the total quantity be the same. If we follow the usual exposition of electrical science, and regard the current-defined by means of its magnetic action-as fundamental, we may say that the total quantity of electricity is measured by the product of the current into the time that it flows. Current is measured in amperes, and the meaning of Faraday's first law is illustrated by saying that five amperes will in two minutes effect exactly the same amount of electro-chemical reaction as ten amperes in one minute. It is consequently more natural for our purpose to look upon quantity of electricity as fundamental; the unit in which this is measured is called the coulomb, the connection between the two being that (a) a coulomb is the quantity of electricity conveyed by a current of one ampere in a second, or (b) an ampere second. is a flow of electricity at the rate of one coulomb per The most exact measurements on the mass of substance deposited by the current have been made on silver, and it appears that one coulomb deposits o'0011175 grm. of that metal. This quantity is known as the electro-chemical equivalent of silver, and by means of Faraday's second law we may calculate from it the electro-chemical equivalent of any other substance, e.g. that of oxygen. The atomic weight of silver is 107'93, and its equivalent weight the same; the atomic weight of oxygen is 16, but, being a divalent substance, its equivalent weight is 8. Hence the weights of silver and of oxygen liberated by the same quantity of electricity are in the ratio 107′93: 8, and the electro-chemical equivalent of oxygen is 8 X 0.0011175 = 0'00008283 grm. per coulomb. other numbers, see table, p. 255. 107.93 For Faraday's two laws may be conveniently summed up in one statement. In accordance with the second law a gramequivalent (the chemical equivalent weight taken in grams) of any substance must require the same quantity of electricity to deposit it. This quantity may be calculated from the data for silver just given; to deposit one gram-equivalent of silver will require 96580 coulombs. (As the fourth significant figure has not been settled with certainty we shall adopt the approximate value 96600.) Therefore 107.93 0.0011175 = 96600 coulombs are required for the deposition of one gram equivalent of any substance. This fundamental quantity of electricity, which occurs constantly in all writings on electro-chemistry, is called by the Germans a "faraday,"1 a term which we in England may very well adopt. Faraday's laws have been found to hold exactly in all cases that have been satisfactorily measured-that is to say, the passage of one faraday through an electrolyte is always accompanied by the appearance at the anode of one gram-equivalent of new material, and at the cathode of one gram-equivalent; 'To be carefully distinguished from the unit of electrostatic capacity known as a "farad." |