378 PRINCIPLES OF CHEMISTRY 



acid may, however, be judged from the accumulation of many indications. 

 With such energetic alkalis as soda and potash, carbonic acid forms. 



capable of forming an acid salt NX 2 , and Y is not, the conclusion of the relative strength 

 of X and Y will be still more erroneous, because the X set free will form such a salt on 

 the addition of Y to NX. We shall see in Chapter X. that when sulphuric and nitric 

 acids in weak aqueous solution act on sodium they are distributed exactly in this way : 

 namely, one-third of it combines with the sulphuric and two-thirds with the nitric acid ; 

 but, in my opinion, this does not show that sulphuric acid, compared with nitric acid, 

 possesses but half the degree of affinity for bases similar to soda, and only demonstrates 

 the greater affinity of sulphuric acid for water compared with that of nitric acid. In this 

 way the methods of studying the distribution in aqueous solutions probably only shows 

 the difference of the relation of the acid to a base and to water. In general, it is impossible 

 to hope to be able to determine the direct relative degree of affinity of acids for bases by 

 studying aqueous solutions without taking into account the relation of the acids, ba 

 and salts to water, and by regarding the water as a passive medium, because the water itself 

 forms saline and all other compounds with substances. This refers more especially to 

 those weak solutions by means of which investigations of this kind are most often con- 

 ducted, because the weak solutions contain a large mass of water, and its influence is 

 then great even when there is but little affinity, in accordance with the law of the action 

 of masses, and from the fact that water itself is a saline oxide. 



In deference to these considerations, although the teaching of the distribution of salt- 

 forming elements in aqueous solutions is an object of great and independent interest, it 

 can hardly serve to determine the measure of affinity between bases and acids. Similar 

 considerations ought to be kept in view when determining the energy of acids by means of 

 the electrical conductivity of their weak solutions. This met hod, proposed by Arrhenius 

 (1884), and applied on an extensive scale by Ostwald (who developed it in great detail in 

 his Lehrbuch d. allgemeinen Chemie, v. ii., 1887), is founded on the fat-t that the re- 

 lation of the so-called molecular electrical-conductivity of weak solutions of various acids. 



(I) coincides with the relation in which the same acids stand according to the distribution, 



(II) found by one of the above-mentioned methods, and with the relation deduced for 

 them from observations upon the velocity of reaction, (III) for instance, according to the 

 rate of the splitting up of an ethereal salt (into alcohol and acid), or from the rate of the so- 

 called inversion of sugar that is, its transformation into glucose as is seen by comparing 

 the annexed figures, in which the energy of hydrochloric acid is taken as equal to 100 : 



I. II. III. 



Hydrochloric acid, HC1 . . 100 100 100 



Hydrobromic acid, HBr . . 101 98 105 



Nitric acid, HNO 3 ... 100 100 96 



Sulphuric acid, H 2 SO 4 . 65 41) 74 



Formic acid, CH 2 O 2 2 4 1 



Acetic acid, C 2 H 4 O 2 . . 1 2 1 



Oxalic acid, C 2 H 2 O 4 . 20 24 18 



Phosphoric acid, PH 5 O 4 7 (5 



The coincidence of these figures, obtained by so many various methods, presents a 

 most important and most instructive relation between phenomena of a different kind, but 

 in my opinion it does not permit us to assert that the degree of affinity existing between 

 bases and various acids is determined by all these various methods, because the influence 

 of the water must be taken into consideration. On this account, until the theory of 

 solution is more thoroughly worked out, this subject (which now ought to be treated 

 of in special treatises on chemical mechanics) must be treated with great caution. 

 But now we may hope to decide this question by exact methods, which we shall partially 

 touch on when speaking about the velocity of reaction. 



There is no means of determining the degree of energy of carbonic acid by any one of 

 the above-mentioned methods, 



