85 



according to Notes' formula. For tliis the values of k\- must be 

 multiplied by [k.^ -\- c -{- H) k\. 



This correction onlv atfects the dilntioiis 16, 32 and 64. 



Thus, the following values aie found: 



v= 16 32 64 128 256 512 



mean value, 

 suiphoacetic acid ^-, = 9.1 8.9 8.8 11.5 9.4 10.3 9.7 



sulphopropionicacid /l, = 7.2 7.1 7.0 5.5 4.5 4.7 6.0 



Little differences in the coloiimetric detei-minations of p have in 

 this method a ;i;reat influence on the value of k^. 



In a simpler wa}' the second dissociation constant of a dibasic 

 acid may be measured by examining a mixture of a neutral and 

 an acid salt '). 



If a" is the degree of dissociation of the neutral salt Na^A and 

 a' the degree of dissociation of the acid salt NaHA, then the second 

 dissociation constant of the acid may be represented by : 



_ «" X [Na^A] 

 '~ [a' y NaHA]' 



Since these degrees of dissociation for salts are not much smaller 

 than 1, the factor «"/«' may be neglected in a first approximation. 



For the sake of simplicity a solution was taken containing an 

 equal number of molecules of the acid and of the neutral salt, so 

 that k = H, and this solution was examined at various dilutions. 



The concentration of hydrogen ions was again determined by 

 means of the indicator method. 



In next table v is the number of litres containing one molecule 

 of the neutral salt together with one molecule of the acid salt. 



The variations of the constant due to dilution are not consider- 

 able, but it is remarkable that they are all in the same direction. 

 By dilution the degree of acidity of the solution decreases. 



This behaviour indeed agrees with the theory, since for the 

 sodium salt of a dibasic acid the dissociation on diluting increases 

 more than for the sodium salt of a monobasic acid. Therefore the 

 value of et'/a', which for infinite dilution must amount to 1. is 

 smaller for the greater concentrations. 



The value of «" follows from the conductivity of the neutral 

 sodium salt at various dilutions, published in the previous paper, 

 and for u' the above mentioned values may be taken. 



') In this way I. M. Kolthoff lias measured the second dissociation constants 

 of a number of dicarboxylic acids. iDer Gebrauch von Farbenindicaloren, p. 102). 



