Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/121

Rh ELECTROLYSIS 111 wards; it has been investigated by Wiedemann (Fogy. Ann., Ixxxvii. 321), and Quincke (Pogg. Ann., cxiii. 513). The former worked with a porous cell, and estimated the effect either by the quantity of the electrolyte which passed through the wall of the cell, the pressure remaining constant, or by the rise of pressure in the porous cell measured by a mercury manometer. A current of moderate intensity through distilled water caused 1777 g. of the electrolyte to pass through the diaphragm towards the cathode in a quarter of an hour, and with a 19 per cent, solution of CuS0 4, a pressure of 176 &quot;5 mm. was observed in the cell containing the cathode, due to the current of a battery of Danell s cells. Quincke, however, employed, instead of a porous cell, a capillary tube without diaphragm, open at one end, and connected with a reservoir at the other containing one electrode, while the other electrode consisted of one of several pieces of platinum wire, sealed into the tube in various positions. His current was obtained from either a Leyden battery or 40 to 80 Grove s cells. The two ways of experimenting gave concordant results, and showed that the pressure on the cathode vessel varies as the electromotive force between the elec trodes, and so diminishes with the resistance if the current be kept constant. It is also, in Quincke s apparatus, inversely proportional to the square of the diameter of the tube, and, for tubes of the same sectional area, is greatly increased by increasing the perimeter. The direction of motion is, as stated above, usually towards the cathode, and is immediately reversed on a reversal of the current, and stops when the circuit is broken. The rate of transfer is increased by coat ing the tube with shellac ; it is different for different fluids, and with certain specimens of absolute alcohol, and with turpentine oil, the direction is reversed, unless in the latter case the tube is coated with sulphur, when the direction is as before.
 * ionof Intimately connected with those phenomena is the motion of
 * i&amp;lt;rn solid particles contained in fluids of high resistance. Faraday

tide*, observed the motion of silk threads in water, and Jiirgensen made many experiments on the subject with a capillary tube in the form of three sides of a rectangle with bulbs at the two corners which contained the electrodes; in one was a porous diaphragm as well. Quincke (I.e.) used a similar apparatus to this, as well as the one described above, and observed by means of a microscope a double motion of particles of starch contained in water subject to the action of an electric machine. Near the sides of the tube the particles moved towards the negative electrode, but in the middle in the opposite direction ; on turning the machine more quickly the particles near the sides gradually lost their velocity, and then began to move towards the positive electrode in common with those in the middle. So that it is highly probable that near the sides the particles arc in the first instance carried along by the motion of the fluid there, but on increasing the current the friction of the liquid in contact with the tube prevents its velocity increas ing so fast as that of the particles in the opposite direction, and ultimately the motion of the particles in that direction becomes apparent. Similar phenomena are observed .with many finely divided bodies suspended in water, as gold, copper, graphite, silica, felspar, sulphur, lycopodium, &c., as well as minute drops of liquid, as Cf* 3 and oil of turpentine, and bubbles of oxygen, marsh gas, &c. All these are urged in water towards the positive electrode, but in oil of turpentine the direction is reversed except in the case of par ticles of sulphur; the direction is also reversed for silica in carbon disulphide. raday s Considering now our first equation W = KE established, for K being, as stated, dependent only on the nature of the electrolyte, we proceed to examine the constant K and its value for different electrolytes. The primary investigation is due to Faraday, who found that if A and B be two electrolytes, and if a quantity E of electricity decomposes a mass X of A and Y of B, then X and Y are chemically equivalent, that is, are the amounts of A and B which would take part in a double decomposition between them. According to this view we have for any electrolyte W = ^.eE, where /A is the amount of the electrolyte chemically equi valent to 1 gramme of water, and c is the number of grammes of water decomposed by a unit of electricity, and is called the electrochemical equivalent of water. This appears to be always true, but the law as usually stated refers to the amounts of the ions separated. The most general statement which the facts allow is the following, known as Faraday s law: In any electrolytic decomposition whatever, the mass w of one at least (usually of each) of the iam, simple or complex, separated I;/ the passage of a quan tity of electricity E, is chemically equivalent to the amount of hydrogen separated by (he same quantity of electricity in a voltameter, and hence w = ruhE, icfiere m is the chemi- Ferent ctro- es. cal equivalent of the ion, and h tlie electrochemical equivalent of hydrogen. Since water contains -Jth its weight of hydrogen A = Jc. Faraday admitted as electrolytes only bodies containing an equal number of equivalents of their components, and accordingly found that the amount of either ion was equi valent to the hydrogen evolved in a voltameter included in the circuit. The seventh series of Experimental ResearcJie.s was devoted to proving this most important law. Two methods were adopted (1) by collecting and measuring the products of decomposition, a voltameter being included in the circuit, and (2) by introducing an anode with which the anion could combine (as for instance a Pb anode in fused PbCl 2, a silver one in fused AgCl), and determining the loss in weight of the anode. By these means the law was proved for simple fused electrolytes, such as the chlorides, ike. Daniell extended it to oxygen salt solu tions, and showed that they were decomposed into a metal and a complex ion, this last splitting up into oxygen and an anhydride which united with water to form the corre sponding acid, e.y., Matteucci and E. Becquerel added a large amount of evi dence in defence of the law, which was demonstrated with great accuracy (to i per cent.) by Soret (Ann. de Chim. et de Phys., [3] xlii. 257) for a series of copper salts; and by Buff for great variations of current strength with silver compounds. So long as we confine ourselves to normal salts there is little difficulty about the statement of the relation ; even with such compounds as the series of phosphates, the double cyanides, &c., which are decomposed as in the following table, the amount of either ion may be considered equi valent to the H of the voltameter. Electrolyte. Aniin corresponding to H in voltameter. Cation. Observer. / Na 1V&amp;gt; 4 32 Xa. Hittorf. 3 4 XaP0 3 P 2 4 I 2^2 Xa. V t-, Xa 4 P 2 7 ^ o 4^2 Xa, E e NaoIIPO, ... / I V!s+ I !*P+ 2. Xa. Daniel! and Miller, 3 t - I 4 &quot; 4 / 3 f*n tV. Trd TIS^ 1 XaHXH 4 P0 4 ... Ho(XH,) 2 P.,0 ? , 1 Xa. 1844, p. 1 2 &quot; T K 4 FeCv 6 FeCy 1 Cy K. KAgCy 2 4 AgCy+Cy ^ K. K 2 A1 2 4(S0 4 ) 4(Al r 3S0 4 )-t-5- 3 +^- K. Hittorf. Faraday s law is nearly always true for both ions, but there are, as Bequerel * before stated, examples of elements forming two series of electroly- modin- sable salts, especially when dissolved in water. In these cases the tion. electronegative ion is usually equivalent to- the H of the voltameter, or we may consider that the chemical equivalent of the positive ion varies, while that of the negative ion remains unchanged in the dif ferent combinations ; so that fenic chloride may be regarded as a di chloride with formula feCl s, where fe = HFe; cuprous chloride cuCl 2, where cu = 2Cu, and s&quot;o on. Considerable confusion, too, arose from the arbitrary numbers for chemical equivalents which formerly obtained, and&quot; which caused such compounds as AljCIg, SbCl 3, AuClj, to appear anomalous, and warranted K. Becquerul (Ann. de Chim. ct de Phi/s. [3] t. xi. p. 178) in considering that generally the amount of electro-negative inn alone was equivalent to the II of the voltameter. 3 This was borne out by his electrolysis of 2X,0,, 7PbO, SILO, and N.j0 4, 2PbO, H. 2 O, which gave |- and an equivalent of Pb at the cathode respectively ; but the law as thus modified fails in the case of K.Cr fl 7, which gives K + (CrO 3 -f -L both in the melted and dissolved state, and in that of J-ajS,, which gives Na + (S 2 + 8), and also for basic acetate of lead. 1 This oxygen is set free. 5 The well-known deposit of silver in electro-plating is due to second- ary action of the K. 3 The chemical equivalents of Al, Sb, Au were taken to be 13 &quot;5, 61 98 respectively, instead of 91, 40 6, 65 5 as now.