Chemical Affinity. 193 



shows that his method yields results identical with those ob- 

 tained by Thomsen by means of the thermochemical method. 



Ostwald has also determined the coefficients of refraction of 

 a series of normal solutions of acids and of bases, and the coeffi- 

 cients of refraction of the liquids produced by mixing these ; 

 and in this way he has arrived at results concerning the che- 

 mical action between the acids and bases in question, which 

 corroborate those obtained by the specific- volume method. 



When aqueous solutions of acids and bases are mixed in 

 equivalent quantities, the volume of the product is different 

 from that of the sum of the volumes of the constituents. This 

 change of volume varies with the acid, the base, the tempera- 

 ture, and the degree of concentration. The two latter con- 

 ditions being kept constant, a value is obtained for each com- 

 bination of acid and base. The normal temperature employed 

 was 20° ; the normal concentration 1 equivalent (in grams) 

 of the acid or base, in 1000 grams of the solution. 



In the general reaction A + B=A' + B', let A and A' be 

 the acids, let B' be the neutral salt of the acid A, and B the 

 neutral salt of the acid A! with the same base ; then the coeffi- 

 cient of affinity may be defined, for this reaction, as the pro- 

 portion in which the base divides itself between the two acids 

 when the three substances mutually react in equivalent quan- 

 tities. The amount of base taken up by each acid is a measure 

 of the affinity of the acid for the base; the coefficient of 

 affinity expresses the relation between these affinities, or, in 

 other words, the relative affinity of the acids for the base. 

 The relative affinity of the acids is a function of their absolute 

 affinity, and must be studied under those conditions which 

 influence the latter. These conditions are nature of the base, 

 temperature, and perhaps pressure. The latter was constant 

 throughout the investigations to be described. 



Let the changes of volume occurring when the acids A and 

 A' combine with the same base C be expressed by v and v f . 

 Let the acid A withdraw a portion x of the base, A' will with- 

 draw 1—x. Then the resulting change of volume v will be 



v =xv + (l — x)v f , 



supposing, that is, that no change of volume is brought about 

 by secondary reactions ; 



. . <X . 



V —V 



The greater the differenca (v — v') between the changes of 

 volume occurring in the neutralization of each of the acids 



