NATURE 



[August 31, 191 1 



lies the fundamental difference between these two classes 

 of solutions? Two kinds of explanation may be put for- 

 ward. First, the ionised proportion may not be given 

 accurately for strong electrolytes by the rule of Arrhenius ; 

 or second, the strong electrolytes do not obey the otherwise 

 general law of active mass, which states that the activity 

 of a substance is proportional to its concentration. The 

 first mode of explanation has been practically abandoned, 

 for other methods of determining ionisation give values for 

 strong electrolytes in sufficient agreement with the values 

 obtained by the method of Arrhenius. The other explanation 

 is that for some reason the law of active mass is, apparently 

 or in reality, not obeyed by some or all of the substances in 

 a solution of a strong electrolyte. An apparent disobedience 

 to the law of mass-action would, for example, be caused by 

 the formation of complexes such as Na,Cl,, or NaX'l+ or 

 NaCl 2 ~ in a solution of sodium chloride. Mere hydration, 

 e.g. the formation of a complex NaCl, 2H,0, would not 

 affect the mass-action law in dilute solution, and the elec- 

 trolyte would obey the dilution law in solutions of the con- 

 centration usually considered. A somewhat similar explana- 

 tion, which takes into account the properties of the solvent, 

 is that the ionising power of the solvent water undergoes a 

 noticeable change when the concentration of the ions in it 

 increases beyond a certain limit. 



I should wish now to draw attention to a point of view 

 which has not, so far as I am aware, been fully considered. 

 To begin with we may put to ourselves the question : Is it 

 the ions in the solution which are abnormal or is it the 

 non-ionised substance? A simple consideration would point 

 at once to it being the non-ionised portion. We have, for 

 example, in acetic acid a substance which behaves normally, 

 so that the ions 11 + and Ac" as well as the undissociated 

 molecule HAc are normal. Similarly in ammonium 

 hydroxide the ions NH + and OH - as well as the non- 

 ionised Ml, and XH 4 OH all behave normally. When we 

 mix the two solutions there is produced a substance, ammo- 

 nium acetate, which behaves abnormally. Now, on the 

 assumption that the equilibrium we are now dealing with is 



NH/tAc"— NH.Ac, 



which of these molecular species is abnormal in the relation 

 between its concentration and its activity? Probably not the 

 ions Ml 4 and Ac~, because these were found to act 

 normally in the solutions of acetic acid and ammonia. The 

 presumption is rather that the abnormal substance is the 



d < tab '1 ammonium acetate, for this occurs only in the 

 abnormal acetate solution, and not in the normal acetic acid 

 and ammonia. This view, that it is the non-ionised portion 

 I the electrolyte which exhibits abnormal behaviour, and 

 not the ions, lias been reached on other grounds by Noyes 

 and others, and I hope in what follows to deduce reasons 

 in its support. 



One is .apt. because the ions are in general the active 



stituents ol an electrolyte, to la} too much stress on their 



behaviour in considering the equilibrium in an electrolytii 

 on. We are justified in attributing the fact that acetic 

 acid is a weak arid, whilst trichloracetic acid is a powerful 

 oni . rather to the properties of the un-ionised substances than 

 to the properties of the ions. The divergence of trichlor- 

 acetic acid from the simple dilution law ma\ similarly 1>< 

 due to an inherent property of the un-ionised arid, a single 

 cans,- bring not improbably at the bottom of both the great 

 tendency to split into ions in watei and also the abnormal 

 in- towards dilution. 



However thai maj be, I think the following reasoning 

 goes far to show thai the non-ionised portion of the elec- 

 trolyte is that which is primarily abnormal in its behaviour, 

 the ions acting in every way as normal. The dilution 

 formulae of Ostwald or of van" 'i Hoff is essentially equili- 

 brium formulae. Oni side of the equilibrium represents the 

 ini' 'i a. tioi J 1 1.- ions in form the non-ionised substani e, the 

 its 1I1- splitting up of the non-ionised 

 ■ to fix mi: id. a. , w i- may 



der a salt which obej th pirical dilution-formula 



"t van 't Hoff. If r„ represents the molai concentration 



mi ionised portion, ami c, the molar concentration of 



•ail 1 according to van 't Hoff's empirical formula, 



If the law of mass-action were obeyed we should have, on 

 the other hand, Ostwald's dilution formula, 



According to this last formula, the activity of each sub- 

 stance concerned varies directly as its molar concentration, 

 and a normal result is obtained on dilution. According to 

 van 't Hoff's formula as stated above, the activity of none 

 of the substances concerned varies directly as its concen- 

 tration ; but since the constancy of the expression is the only 

 test of its accuracy, there are obviously other methods of 

 stating the relation which will throw the abnormal behaviour 

 either on the ions or on the non-ionised substance. Thus, 

 if we write the equivalent form 



17? 



. I — = const., or - — = const , 



the un-ionised substance is here represented as behaving 

 normally, and the ions abnormally ; whilst if we write the 

 formula in the form 



the ions are represented as behaving normally, and the 

 non-ionised substance abnormally. Now it is very important 

 that a choice should be made amongst these three expres- 

 sions, all equivalent amongst themselves so far as the mere 

 constancy of the expression is concerned, as tested by 

 measurements of electrolytic conductivity. Looked at from 

 the kinetic point of view we have in the first form, 



—=kc? 



dt 



dx ,, o 



both direct and reverse actions abnormal. In the second 

 form, we have 



dt 



■ ''''V,,, 

 dt 



the ionisation being normal, the recombination abnormal. 

 And in the third form we have 



=k'c,} 



NO. 2183, VOL. 87] 



the ionisation being abnormal and the recombination normal. 



Now, if it were possible to measure directly the velocity 

 of either ionisation or recombination, we should at once 

 be able to select the equilibrium formula which was really 

 applicable. Unfortunately such velocities are so high as to 

 be beyond our powers of measurement. Yet it seems 

 possible i" sr. k ind obtain an answer from reaction 

 velocities whii urable, One assumption must be 



made, but it seems to me so inherently probable that few 

 will hesitate to make it. It is this, if a substance in a given 

 solution has normal activity with respect to one reaction, it 

 has normal activity with respect to all reactions in which 

 ii can take part in that given solution. Similarly, if a 

 substance in a given solution exhibits abnormal ai 

 with respect b n, it will exhibit abnormal activity 



wiih 1 espec! to all. 



Granting this assumption, we have then to find a reaction 

 in which either the ionised or un-ionised portion - 

 abnormal electrolyte is converted into a third substance 

 with measurable velocity. Such a reaction exists in the 

 transformation of ammonium cyanate into urea in aqueous 

 and aqueous-alcoholic solutions, which was investigated 



some years ago bj mysell and my collabor; -. and found 



to proceed al rates which could easily be followed experi- 

 mentally. First of all .ones the question: Is the 

 formed directly from the ions or from the un-ionised 

 cyanate? As Wegscheidei pointed out, ii i- impossible 

 from reaction-velocity alone to determine which portion 

 passes directly into urea, if the velocities of ionisation and 



