216 



HYDROGEN ION CONCENTRATION 



membrane mechanically through the osmotic differences between 

 the inner and the outer fluids. When the concentration of the 

 CUSO4 only was varied, no definite constant change in the E.M.F. 

 was observed. But when some KCl was added to a constant con- 

 centration of the CUSO4 the resulting E.M.F. values were observed 

 to be definitely and irreversibly dependent upon the concentration of 

 the added KCl, as shown by the average figures given in table 34. 



But these are the exact values for a KCl concentration chain 

 (0.040 volts). 



The same results were obtained with NaCl, HCl, NH4CI, when 

 the concentrations were varied, and on comparing chains containing 

 different chlorides of the same concentration differences were again 



TABLE 34 



Difference 



5*0 mol. 



CUSO4 + 



1 



^Q mol. CUSO4 + uJo 

 mol. CuSO^ 4- Yo^oo 

 j\ mol. CUSO4 + h* 



1 



JO 



mol KCl 

 mol KCl 

 mol KCl 

 mol KCl 



0.020 volts 

 0.061 volts 

 0.098 volts 

 0.12 volts 



* An E.M.F. of =» was theoretically to be expected in this case. But small 

 amounts of K-salts (K2SO4) diffuse through the membrane into the solution 

 (see text below), hence the finite, but uncertain and irregular values for the 

 E.M.F. observed. 



found. In brief, — this form of chain is in every respect analogous 

 to the chain. 



Salt solution 

 I 



Salicylic aldehyde 

 + salicylic acid 



Salt solution 

 II 



and it represents, for any cation, a reversible chain. The anions 

 were shown to be without any effect, just as a chain whose middle 

 phase consists of an acid oil. The analogy with such oil chains is 

 so complete that it is permissible to apply the same theory to both. 

 This theory may be stated as follows: 



The formation of the cupric ferrocyanide occurs according to the 

 equation: 



2 CUSO4 + K4Fe(CN)6 = 2 K2SO4 + Cu2Fe(CN)6 



