ELECTRO-CHEMISTRY. 



ELECTRO-CHEMISTRY. 



810 



sulphates are analogous with those of hydrochloric acid and the chlo- 

 rides, namely the appearance of hydrogen or metal at the negative pole, 

 and oxygen or chlorine at the positive one, it is not difficult to suppose 

 the two to be similarly constituted ; and that sulphates, nitrates, &c., 

 are made up, like the chlorides, of two parts, one electro-positive the 

 other electro-negative : 



+ + 



H Cl H SO t 



Cu Cl Cu S0 4 



Cu NO 6 



This hypothesis of the constitution of oxygen salts is supported by 

 the law of equivalent electrolysis, which we shall discuss directly. It 

 requires, however, the assumption of hypothetical substances, S0 4 , NO B , 

 &c., which are not separated as such, like chlorine, but must be decom- 

 posed, as soon as formed, into oxygen, which appears, and the anhydrous 

 acid, SOj, NO S , &c., which finds water with which to combine. It is 

 these hypothetical molecules, S0 t , N0 , &c., which must be supposed 

 to be successively liberated by the liberated atoms of hydrogen or 

 metal from the molecules of the salt, in the same manner as is the 

 case with the halogens in binary compounds. However, the necessity 

 for this hypothesis cannot be considered as very hostile to the haloidal 

 constitution of oxygen salts, for many instances are known where the 

 actual ultimate products of electrolysis (or ions) appear, by the strongest 

 possible analogies, to be products due to the chemical action between 

 the initial ions and the unaltered electrolyte in contact with them. 

 Thus, if instead of chloride of copper, chloride of sodium be submitted 

 to electrolysis, chlorine as before is evolved at the positive pole, while 

 at the negative not sodium but hydrogen is evolved. Now Cu Cl and 

 Nad are certainly chemically similar. Further, if the anhydrous 

 salt be employed in the fused state, sodium is actually liberated : 

 finally, if sodium were set free in an aqueous solution at the negative 

 pole, the products would be the same as they are actually found to be, 

 for water would be decomposed, soda formed, and hydrogen evolved. 

 Again, by altering the nature of the positive electrode, we may affect 

 the nature of the ion appearing there. Thus, if the electrolyte be 

 dilute sulphuric acid, and the positive electrode be of zinc amalgam, 

 the oxygen which, had the positive electrode been platinum, would 

 have been liberated there, combines with the zinc and dissolves as 

 sulphate of zinc in the sulphuric acid. Or, further, even by altering 

 the strength of the electrolyte (sulphuric acid), the products may be 

 affected ; for unless the acid be dilute, a part of the oxygen, instead 

 of escaping as such, combines with the water to form H0 a , which 

 dissolves in the electrolyte. It is, indeed, on the power we have of 

 altering the nature of the ions, that the construction of constant 

 batteries depends. In Daniell's for instance, the hydrogen, which 

 would collect at the negative pole (which is of copper) and check the 

 current by interposing a partial cushion of a non-conducting gas, is 

 made, by placing a solution of sulphate of copper round this pole, to 

 precipitate an equivalent (chemical and electrical) of metallic copper, 

 itself being converted into water and combining as such with the sul- 

 phuric acid of the sulphate of copper ; whereby the electrode continues 

 to be coated with fresh portions of metallic copper. In Grove's and 

 Bunsen's batteries, again, the hydrogen which would be liberated at 

 the negative pole is there instantly oxidised to water by the nitric 

 acid which surrounds it. 



In organic compounds, where the electrolyte ia often of a very 

 complex nature, it is frequently difficult or impossible to recognise the 

 primitive ion, and to distinguish it from the products of its action upon 

 the electrolyte in contact with it. 



It ia in considering the products of electrolysis, or ions of an elec- 

 trolyte, that the atomic theory receives prominent illustration; or, 

 conversely, it is from the atomic hypothesis that the clearest light is 

 thrown upon the process of electrolysis. 



Suppose the current from a battery to pass by one electrode into a 

 vessel of water : let the other pole be immersed in a vessel of chloride 

 of copper, and let an arc of platinum be placed with one limb in each 

 vessel. The current from the battery will enter the water by the 

 positive electrode P,, will traverse the water and escape by the negative 

 electrode N, ; passing along the arc, it will enter the solution of copper 

 by the positive electrode P 2 , traverse it, and escape by the negative 

 electrode N, : thus regaining the battery after having traversed both 

 Teasels. Now from what has been said before, it is self evident that 

 for every atom or equivalent of oxygen liberated at P, there must be 

 an equivalent of hydrogen liberated at N,, that is, if the whole of each 

 ion set free escapes. Further, that for every atom or equivalent of 

 chlorine liberated at P,, there must be an atom or equivalent of copper 

 deposited at N, (under the same restriction). In other words, the 

 hydrogen and oxygen will be in the proportion of 1 to 8, and the copper 

 and oxygen in that of 1 to 0'25. So much is in accordance with the 

 purely chemical law. But it is further found that the quantity of 

 hydrogen in the first vessel is to that of copper in the second as 

 1 to 32, and therefore to that of chlorine as 1 to 35'5, so that the 

 quantities of all four ions are in the proportion of their equivalents. 

 The name is true, however many electrolytes a current has to decompose 

 . and thin constitutes the great law of equivalent electrolysis. 

 It i* easy to understand that the same law is essentially true when 

 the primitive products of decomposition undergo further change in 

 presence of the electrolytes. Thus, if a current traverses two neutral 



solutions, the one of sulphate of potash the other of phosphate of soda, 

 each of which is divided into two portions by a porous diaphragm in 

 such manner that the sulphuric and phosphoric acids, freed at the 

 positive electrodes, shall not mix with the potash and soda at the 

 respective negative poles ; then, however long the current has passed 

 through, not only will the liberated phosphoric acid exactly neutralise 

 the liberated soda of the same cell, it will also neutralise the caustic 

 potash of the other one ; and the same is true mutatis mutandis of the 

 sulphuric acid. For the metals potassium and sodium, initially sepa- 

 rated, being equivalent, will by their oxidation give rise to equivalent 

 quantities of potash and soda. The meaning then of the law of 

 equivalent electrolysis is, that the power which a current possesses of 

 effecting chemical change is exerted throughout its course in equal 

 degrees wherever such chemical change is effected. 



It follows that by ascertaining the whole amount of one ion libe- 

 rated at any electrode of a current which passes through any number 

 of electrolytes, we ascertain the quantity of every other ion liberated 

 by the same current (or, of its equivalent derived by secondary change). 

 A practical application of this is exemplified in the construction 

 and use of the voltaic electrometer or voltameter. This instrument is, 

 in fact, nothing more than a cell containing acidulated water, which is 

 introduced into the current and decomposed by its means between 

 two platinum electrodes. By a simple arrangement, the amount of 

 hydrogen liberated at the negative electrode is collected and measured. 

 Or the oxygen and hydrogen from the positive and negative poles are 

 collected together. In the latter case, of course, for every 9 parts by 

 weight of the mixed gases, every other ion freed in the circuit has 

 been liberated in quantity proportional to its equivalent. The hydro- 

 gen liberated at the negative, and the oxygen liberated at the positive 

 pole exhibit all the powerfully developed affinities of these elements in 

 the nascent state. Thus many oxygenated or chlorinated organic 

 bodies which refuse to part with their oxygen or chlorine under 

 ordinary conditions, and which may refuse to conduct the current 

 when pure, will give up their electro-negative elements to the hydro- 

 gen at the negative pole of a galvanic current when, by mixture 

 with an acid and aqueous solution, they are made to conduct, and 

 hydrogen is furnished. Under the same conditions many substances 

 may be oxidised by the oxygen liberated at the positive pole. Thus 

 by mixing chloroform with alcohol and dilute sulphuric acid, and 

 submitting the mixture to electrolysis, the hydrogen at the negative 

 pole replaces the chlorine of the chloroform, and hydride of methyl, 

 or marsh gas is formed . Again, if a solution of caustic potash be 

 submitted to the action of a current, the positive electrode of which is 

 metallic iron, the iron becomes peroxidised to ferric acid, and ferrate 

 of iron is formed at this pole. So, a solution of cyanide of potassium 

 gives when similarly treated, cyanate of potash at the positive 

 pole. The same powerful affinity is exhibited by other electro- 

 negative ions; thus, for instance, when chloride of ammonium in 

 solution is electrolysed, chloride of nitrogen is formed at the positive 

 pole. It will, however, be borne in mind, that a part of the oxidising 

 action of electrolytic oxygen may be due to the formation of binoxide 

 of hydrogen, as before mentioned, or perhaps, to that of a still higher 

 oxide (ozone). That the electrode itself may undergo change by the 

 action of the ion liberated upon it, has already been exemplified. An 

 instructive instance is furnished by the electrolysis of fused chloride 

 of silver, when the electrodes are of silver. In this case silver is 

 deposited in crystals upon the negative electrode, while the chlorine 

 at the positive one combines with the silver of which that electrode is 

 formed. Thus the amount of chlorine in the electrolyte remains 

 unchanged, but fresh portions of silver are continually being trans- 

 ferred through it, from the positive to the negative electrode. 



If a current be led through a solution of various metallic salts, the 

 metal precipitated at the negative pole is not an alloy of all the metals 

 present. Some metals are precipitated in preference to others, and 

 the whole quantity of one metal present is precipitated before the 

 second commences to be thrown down. Thus, from a solution of 

 copper and iron salts, the copper ia precipitated before the iron, from 

 a mixture of iron and zinc salts the iron is precipitated before the 

 zinc ; finally, from a mixture of copper, iron, and zinc, the copper 

 is precipitated first, then the iron, and lastly the zinc. Zinc is thus 

 said to be more electro-positive than iron, iron more electro-positive 

 than copper, &c. Compared with regard to their electro-positive 

 characters the metals are related to one another as follows : 



Potassium, 

 Sodium, 



Zinc, 



Cadmium, 



Iron, 



Nickel, 



Cobalt, 



Lead, 



Tin, 



Copper, 



Silver, 



Mercury, 



Palladium, 



Platinum, 



Rhodium, 



Iridium, 



Gold. 



The first being the most positive, the last the most negative. 



It ia by the use of compounds free from water, and rendered 

 conductors by fusion, that the most easily oxidisable metals are 

 reduced from combination by means of the electric current. Thus, 

 fuued caustic soda yields oxygen at the positive pole, and metallic 



