Jidy i8, 1889] 



NATURE 



-/D 



the equation. And tlie equation is confirmed by the 

 determinations, made by Berthelot and P. de Saint Gilles 

 by chemical methods, of the quantity of ethereal salt 

 formed when an acetic acid reacts with an alcohol. The 

 accuracy of the equation has also been confirmed by 

 applying it to physically heterogeneous systems consist- 

 ing of solids and liquids or gases ; Ostwald (in his 

 " Lehrbuch der Allgemeinen Chemie ") gives the necessary 

 forms of the equation for different cases. 



The law of mass-action, and the principle of the co- 

 existence of reactions, are thus amply confirmed. But 

 the expressions, reaction-velocity, or velocity-coefficient, 

 or coefficient of velocity, must be analysed. 



The ratio ' ^ 



(r^.)' 



coefficient of the reaction 



i.e. it is the same as 



/ 



acid. The ratio - 



base between the two acids. In the case of sodium 

 sulphate (Na2S04) reacting with nitric acid (HjNoOu), 



Thomsen found | = § ; therefore, the ratio -i— - = f / J = 2. 



In this case, the direct change consists in formation of 

 sodium nitrate and sulphuric acid, and the reverse change 

 consists in the re-formation of sodium sulphate and nitric 

 acid ; the square root of the ratio of the velocities of the 

 direct and reverse changes in this reaction is f/^ = 2. 

 Or, one may say that the ratio of the affinity-coefficients 

 of the acids nitric and sulphuric for the base soda is 

 %i\ = 2. These statements are identical. Two-thirds of 

 the soda combines with the nitric acid, and one-third with 

 the sulphuric acid, when equilibrium is established ; or the 

 velocity of the direct change is double that of the reverse 

 change ; or the affinity of nitric acid for soda is twice 

 that of sulphuric acid for the same base. It must be 

 remembered that the acids and the base interact in 

 equivalent quantities and in dilute aqueous solution. 



Proceeding in the way indicated by the foregoing 

 example, Ostwald determined the ratio ^^c/c', or i'/A', 

 for many acids reacting with a given base ; he stated 

 these ratios in terms of one acid taken as 100. For in- 

 stance, taking the base soda (NagO) the ratio for J^*^'^ 



H.2SO4 



was found to be 1-94, for i^-J _Ai 2-0, and for ^2^\ -07. 

 H0SO4 ' H2N2O0 ^^ 



If the affinity of nitric acid for soda is taken as 100, that 

 of hydrochloric acid for the same base, according to these 

 results is 97, and that of sulphuric acid is 50. Ostwald 

 examined many different experimental methods for 

 measuring the distribution of a base between two acids 

 in dilute solution. The experimental difficulties are great, 

 and the results obtained by one method cannot be ex- 

 pected to agree very closely with those obtained by 

 another. Secondary reactions very often complicate the 

 change which it is sought to measure. The order of the 



is called by Ostwald the partition- 



The square root of this ratio, 



is the same as the square root of the ratio 



of the velocity-coefficients of the two parts of the change, 



it is also identical with the 



ratio of the affinity-coefficients, kjk'. 



When equivalent masses of one acid and the sodium 

 salt of another acid interact in dilute solution, ^ represents 

 the number of equivalents of the salt which are decom- 

 posed, and I - I represents the number of equivalents of 

 the salt which remain unchanged, when equilibrium is 

 established ; or, to put the statement in another form, as 

 each equivalent of salt decomposed produces one equiva- 

 lent of acid and one of base, ^ represents the number of 

 equivalents of base which have combined with the second 

 acid, and i - ^ represents the number of equivalents of 

 base which have remained in combination with the first 



then expresses the distribution of the 



affinities of many acids, for a specified base, was not 

 altered by a change of method, except in a i^w cases : in 

 these cases the affinities were very small, and therefore 

 incapable of accurate measurement by any of the methods 

 tried. 



Ostwald next proceeded to examine the influence of the 

 nature of the base on the affinities of acids. He showed 

 that whethei the base be potash, soda, ammonia, magnesia, 

 zinc oxide, or copper oxide, the ratio of the affinities of 

 hydrochloric and nitric acids is the same ; but that the 

 ratio varies in the case of sulphuric and hydrochloric, or 

 sulphuric and nitric, acids. But it is known that sulphuric 

 acid reacts with its normal sodium salt to form an acid 

 salt (NaHSOj) ; Ostwald was able to explain the results 

 obtained with sulphuric acid on the supposition that the 

 affinity of this acid for a base, as measured by any of the 

 methods used by him, really represents only the affinity of 

 that part of the acid which has not combined to form an 

 acid salt. He concluded that the true relative affinity of 

 sulphuric acid, like the affinities of hydrochloric and nitric 

 acids, is independent of the nature of the base. Extend- 

 ing the investigation to other acids, Ostwald concluded 

 that the relative affinities of the acids are independent of 

 the nature of the bases with which they react, and can be 

 expressed by constant numbers. If this conclusion is 

 accepted, it follows, from the nature of the reaction ex- 

 amined, that the relative affinities of the bases are also 

 independent of the acids with which they react, and can 

 be expressed by constant numbers. From these con- 

 clusions, the further deduction is made that the affinity 

 between an acid and a base is the product of two specific 

 affinity-coefficients, one of which belongs to the acid and 

 the other to the base. 



This conclusion is of extreme importance and requires 

 rigorous examination. In order to test the accuracy of 

 the statement that each acid has a specific affinity-co- 

 efficient, Ostwald has determ.ined the affinities of a series 

 of acids by different methods, with the result that the 

 affinity-coefficients determined by one method are as 

 nearly the same as those determined by other methods 

 as could be expected, considering the errors inherent in 

 the methods themselves. If each acid possesses a specific 

 affinity-coefficient, the value of this coefficient for any 

 acid might be expected quantitatively to condition many, 

 if not all, the reactions brought about by that acid. 

 Several chemical changes brought about by acids, other 

 than those in which an acid interacts with the salt of 

 another acid, have been examined by Ostwald. Among 

 these changes may be mentioned that of acetamide to 

 ammonia and acetic acid, that of methylic acetate to 

 acetic acid and methylic alcohol, and that of cane-sugar 

 to inverted sugar. The rate of each of these changes 

 varies according to the acid added to the system ; the 

 results obtained show that the square roots of the ratios 

 of the velocity-coefficients are in the same order as, and 

 are as nearly identical as could reasonably be expected 

 with, the ratios of the affinity -coefficients of the acids em- 

 ployed, as determined by the division of a b.ise between 

 these acids. Hence the conclusion that each acid has a 

 specific affinity-coefficient is verified, and at the same 

 time new methods for determining these coefficients are 

 put into the hands of chemists. 



But none of the methods employed was found alto- 

 gether satisfactory. In every case secondary reactions 

 more or less interfered with and complicated the primary 

 change. 



There is, however, another and altogether different 

 method whereby the affinities of acids may very accu- 

 rately be determined. This method is based on the 

 relations which certainly exist between the rate of a 

 chemical change brought about by an acid and the elec- 

 trical conductivity of an aqueous solution of that acid. 

 If the electrical conductivities of dilute aqueous solutions 

 of a number of acids are stated in terms of that acid which 



