July 1 8, 1889] 



NATURE 



277 



constant C obtained by applying the above equation to 

 a monobasic acid represents the affinity of that acid. 

 The constant C measures the readiness of an aqueous 

 solution of the acid to conduct electricity, as also its 

 readiness to take part in chemical reactions ; the value 

 of C depends only on the nature of the acid, and is in- 

 dependent of dilution. As C has small values for strong 

 acids and large values for weak acids, Ostwald pre- 

 fers to put the equation in the form - - ,-- =/&, where 



{\ — m)v 



k = —. To avoid small fractions Ostwald multiplies in, 



and also k, by 100 ; he has determined look for more 

 than 100 monobasic acids at dilutions varying from 8 to 

 1024 litres ; finally, he expresses the most probable value 

 of look as K. I cannot here give even a selection from 

 the numerous measurements of K made by Ostwald, but 

 must content myself with drawing attention to some of 

 the conclusions he has come to regarding connexions be- 

 tween the affinities and the constitution of acids. The 

 method has been worked out chiefly for monobasic acids. 

 In the acetic series of acids, affinity decreases from 

 formic to propionic acid, and then remains nearly constant 

 until caproic acid is reached. The substitution of chlorine 

 or bromine for hydrogen in an acetic acid raises the 

 affinity, bromine causing a smaller increase than chlorine. 

 If S is substituted for O in the group COOH in acetic 

 acid, the value of K is raised from "ooiS to '0469 ; while the 

 "substitution of the group SH for H in the same acid is 

 attended with an increase in the value of K from '0018 to 

 •0225 only. The greater or less acidic character of such 

 groups as OH, OCH3, OCgHg, NOo, &c., is quantitatively 

 measured by the increase in the value of K attending the 

 substitution of one of these groups for H in an acid. In 

 the acetic acids, the change of H to OH is accompanied 

 by an increase of affinity, OCH3 is more acidic than OH, 

 and OCgHj is the most acidic of the three radicles con- 

 sidered. In studying the relations betv/een the affinities 

 of acids and their derivatives, attention must be paid not 

 only to the composition and character of the replacing 

 groups, and to the series of acids in which the replace- 

 ment occurs, but also to the position of the replacing 

 groups relatively to the other atoms of the molecules. The 

 influence of position is very marked in the affinities of the 

 isomeric oxy, chloro, nitro, methoxy, and acetoxy, ben- 

 zoic acids. The following numbers exhibit the influence 

 of the positions of the replacing groups : — 



It is seen that the group OCH3, or OC0H3O, substi- 

 tuted for H in benzoic acid, raises the affinity, if the 

 group is placed in the ortho-position, but decreases the 

 affinity if the group is placed in the para-position. The 

 influence of the position of the replacing groups on the 

 change of affinity of many acids points to some connexion 

 between the affinities of acids and the space-arrangement 

 of the atoms which form the molecules of the acids ; 

 measurements of the affinities of such acids as maleic and 

 fumaric, mesaconic, citraconic, and itaconic, confirm this 

 conclusion. Maleic acid is about twelve times stronger 

 than fumaric acid ; these acids are probably geometrically 

 isomeric, and the COOH groups are probably nearer one 

 another in maleic than in fumaric acid. Again, if the formulai 



of Wislicenus for citraconic and mesaconic acids are 

 adopted we should expect the former to be the stronger of 

 the two. These formulae represent the acids as geometri- 

 cally isomeric ; they are — 



H3CV /COOH 



>C-C< 

 IK ^COOH 



citraconic 



and 



HOOC. /CH3 



>C-C< 

 IV ^COOH 



mesaconic 



The values obtained for K are, for citraconic -34, for 

 mesaconic ^079 : the third isomeride, itaconic, is very 

 weak ; K = 012. 



Ostwald's researches open up a new path along which 

 advance may be made : they show us how to connect the 

 characteristic property of an acid, its affinity, with the 

 constitution of the acid ; they form a further and more 

 important step towards solving f/ie problem of chemistry, 

 which is to find definite and measurable connexions 

 between the properties and the composition of homogeneous 

 kinds of matter. 



But the coefficient of affinity of an acid has not yet been 

 fully analysed. What is the meaning of the constant K ? 

 What is affinity ? The value of K for a monobasic acid 

 measures the readiness of that acid in solution to take 

 part in chemical changes, and also the readiness of an 

 aqueous solution of that acid to conduct electricity. Now, 

 when a compound is electrolysed, the parts or ions into 

 which it is separated are chemically equivalent and carry 

 with them equal quantities of electricity, and the elec- 

 tricity travels only with the ions. The conductivity of the 

 electrolyte will depend on the number of molecules 

 electrolysed, and on the velocity of transference of the 

 ions across the space separating the electrodes. The 

 greater the number of molecules separated into ions, 

 and the more rapid the migrations of these ions, the 

 greater will be the conductivity of the substance. Hence 

 the value of K for an acid will be conditioned by the 

 amount of separation into ions, and the rate of migration 

 of these ions ; z>. the affinity, as well as the conductivity, 

 of the acid will depend on these quantities. The ions 

 into which a monobasic acid is separated when electrolysis 

 occurs are H and a negative radicle ; the scheme of 

 electrolysis may be represented as HR = H -f R. As 

 hydrogen moves much more rapidly than the most rapid 

 negative acidic ion, the molecular conductivity of a 

 monobasic acid in solution is chiefly conditioned by the 

 degree in which the acid is separated into its ions. The 

 affinity of the acid is sometimes dependent to a consider- 

 able extent on the velocity of the negative ion: in such 

 cases acids which are separated into their ions to an equal 

 extent will exhibit different affinities ; in other cases the 

 degree of electrolytic separation is the chief factor con- 

 ditioning the affinity. Now the fact that, so far as accurate 

 research has gone, electrolytes fully obey Ohm's law, or, 

 in other words, the fact that the smallest electromotive 

 force suffices to cause electrolysis, points to the action 

 of the E.M.F. in electrolysis as being only a directive 

 action on the ions already existing. This view of electrolysis 

 has been developed by Clausius, and recently by Arrhenius, 

 van 't Hoff, and Ostwald. The hypothesis, in its present 

 form, bids us regard an aqueous solution of an electrolyte 

 as already more or less completely dissociated ; it bids us 

 see the molecules of the electrolyte in the solution as 

 dissociated into their ions ; it says that the electrolytic 

 and the chemical activity of the solution is dependent on 

 the ratio between the number of dissociated, or " active," 

 molecules, and the number of undissociated, or " inactive," 

 molecules. This view of electrolysis, and of chemical change 

 occurring between electrolytes, regards an aqueous solution 

 of a strong acid as containing a great many free ions, which 

 are, respectively, hydrogen and a negative radicle ; it 

 looks on an aqueous solution of a weak acid as containing 

 only a few free ions. 



There are difficulties in the way of accepting the 



