Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/118

108 day demonstrated the truth of the law in the case of dilute sulphuric acid by experiments with vessels in which the products of decomposition of the dilute acid between platinum electrodes could be conveniently collected, either separately or together, and measured (Exp. Res., 714- 728.) Such an instrument he called a volta-electrometer, and subsequently a voltameter. After demonstrating that the amount of de- , composition was /TIN independent of the size of the elec trodes, he con nected up two voltameters A and B, iu multiple arc, as in the accom panying diagram, and then passed the whole current Dia S ram sllowin S connection of voltameters. through a third C, and found that the amount of decom position in C was equal to the sum of the amounts in A and B. He therefore applied the voltameter 1 to measure quantities of electricity in other cases. Various forms of voltameter have been employed (see Wiedemann, Galvanismus, Bd. i. 317-319). The most accurate is the silver voltameter of Poggendorff, which con sists of a vertical rod of silver with the lower end immersed in a solution of silver nitrate contained in a platinum vessel ; the silver is connected with the positive, the platinum vessel with the negative pole of the battery, and the amount of decomposition is ascertained by weighing the platinum vessel with the attached silver before and after the experiment. Buff directly proved the truth of equation (1) for such an instrument by electrolysing silver nitrate solutions of different strengths between silver electrodes. The currents employed were varied for different experiments, and were measured by a tangent galvanometer, and the quantity E of electricity was deduced by observing the time of passage of the current. (Ann. d. Chem. u. Pharm., xciv. 15.) We have, then, in order to demonstrate generally the law expressed in equation (1), to measure the amount of the ions set free in any case of electrolysis, while the amount of electricity is measured at the same time by means of a voltameter included in the circuit. But the measurement of the amount of ions liberated is not always an easy task ; in the great majority of cases secondary actions (see above, p. 106) occur, the primary results of electrolysis are obscured, and in order to determine the nature and amount of the ions special apparatus and further investigation are necessary. Since the ions are liberated at the electrodes the products of secondary action will remain in the immediate neighbour hood if the action be not too long continued. We may therefore determine the ions by collecting any gaseous pro ducts, ascertaining the loss or gain in weight of the electrodes, and analysing the electrolyte in the immediate neighbourhood of the electrodes, taking care that the pro ducts at the two do not mix by gravitation, by diffusion, or otherwise. For instance, if a fused chloride (e.g., PbCl 2 ) be electrolysed with platinum electrodes, no chlorine will be 1 Many corrections have to be applied to the observations with a water voltameter in consequence of (1) the formation of ozone in the collected oxygen; (2) the formation of H s 0.j ; (3) the solution of the evolved gases in the water, varying with different strengths of acid, and greater for oxygen than hydrogen ; (4) the re-combination of the oxygen and hydrogen if in contact with platinum (see Wied. G alv., I.e.). A diagram and description of t ie water voltameter will be found in any of the numerous works on the subject. evolved at the anode, although Pb will be deposited at the cathode ; but if the liquid round the anode be analysed, for every 414 grammes of lead at the cathode will be found 339 grammes of PtCl 4 round the anode. Now the platinum must have been derived from the anode, which will be found to have lost 197 grammes in weight, consequently the 142 grammes of Cl were derived by the electrolysis from the P oCl.,, and hence PbCl 2 is electrolysed as Pb + C1 9 . In order to separate the fluids at the two electrodes, various forms of apparatus have been employed. For fused electrolytes a W-shaped tube, which can be divided after the fluid has solidified, is sufficient; with solutions, how ever, where the solvent introduces new complications the separation is more difficult, owing to the migration of the ions" and other causes which will be considered below. Daniell and Miller (Phil. Trans., 1844) used a cylindrical glass vessel separated into three compartments by porous clay diaphragms, the two end compartments containing the electrodes, and having tubes for conducting away gaseous products; while Hittorf, in a classical series of experiments (Pogcj. Ann., Ixxxix. xcviii. ciii. cvi.), used a number of bell-shaped glass vessels fitted to each other with india- rubber washers, the electrodes being inserted in the bottom and top vessels respectively. The lower end of each bell was covered with membrane to prevent mixing of the pro ducts; the whole apparatus was filled with the electrolyte to be decomposed; and the products at the two electrodes were known to be separated if the composition of the fluid in one of the intermediate bells remained the same through out the experiment. Great numbers of experiments have been made by differ ent experimenters in one or other of the ways mentioned, and they have thus proved that, whatever the electrodes, and whatever the electromotive force, the secondary action at the electrodes has no effect upon the amount of chemical decomposition,- and therefore the law of equation (1) always holds. We can give here but a few examples of secondary Se action. A very good account will be found in Wiedemann, ar Bd. i. 326-385, with, however, the drawback of the use tl( of an obsolete chemical notation. (1.) The ions themselves are set free, but separable into component parts. That this is the case with oxygen salts, which are separated into the metal and a complex anion which is resolved into oxygen and an anhydride, was pointed out by Dauiell (Phil. Trans., 1839), who gave to the S0 4 , derived as electro-negative ion from sulphates, the name of oxysulphion, and so on. Many similar cases occur in electrolyses of organic compounds. Thus potassium acetate is elec trolysed originally as KC S H 8 S K + C S H 8 ; but the anion splits up (partly at least) thus : 2C 8 H 8 S =CH 6 -|-2C0 2 . All the potassium salts of the fatty acids behave similarly, so that this becomes a gene ral method of preparing the normal paraffins. (2.) The ions appear in an abnormal molecular state. The deposit of copper in Gladstone and Tribe s ZnCu couple is a black crystalline powder (see p. 114). The most important instance, however, is the formation of ozone in the oxygen liberated at the anode by the electrolysis of acid solutions, which was recognized bv Schcinbein in 1840, although the smell and powerful oxidizing properties of the gas evolved had previously been noticed Ify Franklin and Van Marum. The amount of ozone, though very small, may be recognized by all the ordinary tests (KI, indigo, &c.) ; it diminishes with rise of the temperature at which the electrolysis takes place, and is above 2 per cent, when the electrolyte dilute H. 2 SO 4 is cooled by ice and salt, and the electrodes are platinum-indium wires (Sore t). With dilute H 2 S0 4 at 6 C., 100 c. c. of oxygen contained 00009 gramme ozone, and 00027 gramme at a mean temperature of - 9 C. * dilute H 2 Cr0 4 gave at C. 00052 gramme per 100 c.c. of oxygen (Soret). The amount varies with the different acids, solutions of chromic and permanganic acids giving the largest percentage. These points arc of importance in correcting observations by the water voltameter. 4 Of course, if the products of decomposition be allowed to accumu late until the electrode is surrounded with an envelope of liquid differ ing from the original electrolyte, the whole character of the decomposi

tion changes.