ELECTROLYTES AND THEIR ACTION 219 



the salt is highly dissociated, so that a number of ions make their appearance. 

 So far, -then, as the properties of a substance are those of its ions, the salts of 

 weak acids with weak bases are more powerful agents than the substances from 

 which they are formed. 



If the percentage dissociation of aniline acetate be calculated from measurements of the 

 migration rates of its ions and of its degree of hydrolytic dissociation, it is foujid that, at a 

 dilution of 1 molecule in 13 '75 litres, it is electrolytically dissociated to the extent of 45 per 

 cent. , whereas hydrogen acetate is only 5 per cent, dissociated. Aniline acetate is hydrolytically 

 dissociated to about 32 per cent. , so that about 25 per cent, is not dissociated in either way. 



There are two practical points of interest in connection with this question. 



In the first place, the fact gives us a very convenient means of following the 

 course of a tryptic digestion. The weak amino-acids produced, when they combine 

 with the ammonia used to give the requisite degree of alkalinity, or with diamino- 

 acids, acting as bases, give rise to a considerable increase in the conductivity of 

 the mixture. 



The conductivity of leucine in O'Oo molar strength at 22 is only about 3 reciprocal megohms, 

 that of ammonium hydroxide in the same conditions is 232 reciprocal megohms, together 235. 

 When mixed, the salt formed is fairly highly dissociated and the solution has a conductivity 

 of 1,548 reciprocal megohms. This may be compared with aniline acetate ; aniline in O'Oo 

 molar solution has a value of 13, acetic acid in the same concentration is 330, together 343 ; 

 while aniline acetate, O'Oo molar, is 1,518. 



In the second place, we obtain some information as to the relative strengths 

 of an acid and a base. An acid which is weak towards a strong base may be 

 relatively strong towards a weaker base. 



For example, salicylic acid, which has a dissociation constant of 102 x 10~ 5 , when combined 

 with ammonium hydroxide, gives an increase of conductivity, that is, it is a weak acid towards 

 the base ammonium hydroxide ; when combined with aniline, on the other hand, there is &Jall 

 in conductivity, that is, it is .a relatively strong acid towards the very weak base, aniline. 

 Maleic acid (dissociation constant = 1 170 x 10~ 5 ) is a strong acid to both bases and acetic acid 

 (dissociation constant = 1 '8 x 10~ 5 ) is weak to both bases. The mono -amino-monocar boxy lie 

 acids are too weak as bases to combine with acids as weak as acetic acid. On the other hand 

 the diamino-mono-carboxylic acids are sufficiently strong as bases to combine with acids as 

 strong as the mono-amino-dicarboxylic acids. For example, I found that diamino-propionic 

 acid, 0'17 molar, had, at 40, a conductivity of 1,672 reciprocal megohms, glutamic acid, 0'095 

 molar, had a conductivity of 950 on the same scale, together 2,622 ; a solution containing both 

 in the same concentration as before had a conductivity of 5,142 reciprocal megohms, showing 

 that combination had taken place (Bayliss, 1909, 2). 



It is to be noted that the use of the words " weak " and " strong " in the above connection 

 is to be taken only as referring to their relative power of combining with weak acids or bases 

 respectively. It does not conflict with the expression as used in reference to the electrolytic 

 dissociation of their solutions, which is an absolute measurement of their strength as compared 

 with one another. 



AMPHOTERIC ELECTROLYTES 



There is an important class of substances, already referred to incidentally in 

 connection with the colloidal properties of proteins, which can act either as acids 

 or bases ; that is, they dissociate with the formation of H* and OH' ions. We 

 have seen that water is a member of this class and we have now to turn our 

 attention to a very important series of substances, the amino-acids. These owe 

 their nature as both bases and acids to the fact that they contain one or more 

 NH 2 groups, together with one or more COOH groups. 



Amino-acetic acid, or glycine, exists in water as : 



CH., NH 3 OH 



COOH. 



For convenience, we may call the radicle which is combined with H arid OH, R, 

 which is : 



CH2-NH 3 - 

 I 



COO- 

 in the case of glycine. Then, according to the investigations of Bredig (1899) and 

 of J. Walker (1904), the solution contains the following molecules and ions: 

 H-, OH', HR', ROH', HROH, and R. 



