156 



Prof. J. Walker. 



[Feb. 3. 



For acids of moderate strength the value of a may be obtained from 

 measurements of the electrical conductivity of the solutions, and it has 

 been found that the results are then generally expressible by means of 

 Ostwald's dilution formula, 



a- = k a u, 



h a being the dissociation constant of the acid. For all substances 

 accessible to direct electrical measurement, the two expressions are in 

 practice identical, for K at 25° has the value 1*2 x 10" u , whilst the 

 product Jc a u has at least the value 10~ n . As far then as conductivity 

 measurements are concerned, Ostwald's simple formula may be used 

 instead of the strict theoretical formula. 



When we consider the state of an amphoteric substance such as 

 glycine, amino-acetic acid, NHo.CFL.COOH, in aqueous solution, it is 

 apparent that if it acts both as acid and as base it must give rise 

 to both hydrion H + and hydroxidion OH - , the relative concentrations 

 of which are regulated by equation (1), which holds good for all dilute 

 aqueous solutions whatsoever. Further, if the substance is capable of 

 behaving as acid and as base simultaneously, the acid portion must 

 neutralise the basic portion and form a salt. There are two probable 

 alternatives for the mode of neutralisation, first, the acid and the basic 

 portion of one molecule may neutralise each other, thus 



CH.> . NHa CH., . NH 3 



I = I V , 



COOH COO 



or, second, the acid portion of one molecule may neutralise the basic 

 portion of another, thus 



CHo . NH, HOOC CHo . NH 3 . OOC 



I + I - i ! 



COOH H,N . CH> COO . NH 3 . CH 2 



As the former alternative is more simple than the latter, and is in 

 accordance with known facts regarding the molecular weight of such 

 substances in solution, we shall adhere to it in the following 

 deductions. 



Since we are unable by the methods here under discussion to 

 distinguish between the unionised isomeric forms NHo.CH 2 .COOH and 



NH 3 .CH 2 .COO, and since by the law of mass action they must always 

 exist is solution in invariable proportions, provided the temperature is 

 constant, Ave may for present purposes treat them as being one and 

 the same substance. Similarly with regard to the hydrated form 

 HO.NH 3 .CH 2 .COOH formed from either of the anhydrous forms by 

 addition of water, the mass action law leads to the conclusion that the 

 proportion of it relatively to the total unionised glycine in the solution 



