260 PROPERTIES OF ELECTRICALLY CONDUCTING SYSTEMS 



Kendall and Andrews 22a have recently extended the investigation of 

 the solubility of acids in the presence of weak acids. They have meas- 

 ured the solubility of acids of varying strength and solubility in the 

 presence of both strong and weak acids up to high concentrations. They 

 include hydrogen sulphide, carbonic acid, boric, oxalic, succinic, trichloro- 

 acetic, m-nitrobenzoic, 3-5-dinitrobenzoic, benzoic, picric and (3-naph- 

 thalene sulphonic acids in the presence of hydrochloric acid ; and suberic, 

 mandelic, succinic, oxalic, tartaric and boric acids in the presence of 

 acetic acids up to concentrations of 10 normal added acid. They have 

 also measured the solubility of boric, benzoic and salicylic acids in the 

 presence of nitric acid. 



The solubility of all acids on addition of a strong acid is initially 

 decreased. On addition of larger amounts of the strong acid the solu- 

 bility, with a few exceptions, passes through a minimum. At high con- 

 centrations of the added acid, the solubility increase is very marked in 

 some cases while, in a few, the minimum is lacking. The initial decrease 

 appears to be due to a repression of the ionization of the saturating acid. 

 The stronger the acid, the greater is the initial depression, while in the 

 case of very weak acids the initial depression is wanting. The minimum 

 solubility of an acid is much lower than corresponds to the concentration 

 of its un-ionized molecules in pure water. This is ascribed to the depres- 

 sion of the solubility because of hydration effects accompanying the addi- 

 tion of the strong acid. It may be noted that the maximum depression 

 of hydrogen sulphide and carbonic acids is very low, amounting to only 

 a few per cent. The final rise in the solubility curve is ascribed to the 

 formation of compounds between the two acids at high concentrations. 

 This view is supported by the results of conductance measurements which 

 indicate the formation of complexes. This accounts for the widely 

 divergent effect of strong acids on different weak acids at higher concen- 

 trations. The solubility curves for weak acids in the presence of acetic 

 acid exhibit a great variety of form. Here, the common ion effects at 

 low concentration of added acid are approximately as might be expected. 



The effect of strong and weak acids on the un-ionized fraction of weak 

 acids does not differ greatly from that observed in the case of non-elec- 

 trolytes. For example, the solubility of hydrogen in water is only very 

 slightly depressed due to the addition of acetic acid, but somewhat more 

 strongly due to the addition of hydrochloric acid. In a normal solution 

 of hydrochloric acid, the solubility depression in the case of hydrogen is 

 7 per cent and that in the case of the undissociated fraction of orthonitro- 

 benzoic acid 10 per cent. The percentage depression in the case of sali- 



" Kendall an<J Andrews, /. Am. Chem. Soc. $3, 1545 (1921). 



