44:8 M. C. Lea — Relative Affinities of certain A cids. 



drop be placed in each of two small porcelain basins previously 

 slightly warmed. To one of them let a single drop of distilled 

 water be added and then the herapathite test to both. In a 

 few minutes the one which has received the drop of water will 

 show well marked crystallizations of herapathite whilst the 

 other will not show a trace. 



The effect of dilution in changing the equilibrium of the 

 solution of a base with mixed acids is thus made visible to 

 the eye by a chemical reaction. Hitherto it has been a deduc- 

 tion from physical changes requiring great delicacy of measure- 

 ment. Results of a precisely similar character were obtained 

 when potassium bromide was substituted for sodium nitrate 

 and are no doubt of general occurrence. 



The applicability of this method proved to be a good deal 

 restricted owing to the tendency of many acids when set free 

 to decompose the herapathite reagent. For this reason the 

 affinities of hydrobromic, hydriodic, chloric, iodic and nitric 

 acids could not be measured with accuracy, although many 

 attempts, sometimes as many as 30 or 40 or more were made 

 to get reliable results. This work, however, was not entirely 

 thrown away. It demonstrated that chloric acid has the 

 strongest affinity for bases of any known acid. It might have 

 been expected a priori that a highly oxidized acid of chlorine 

 would have stronger affinities than chlorine hydride. It also 

 showed that the comparative affinity of nitric acid has hitherto 

 been placed somewhat too high. Taking hydrochloric acid as 

 100, nitric acid scarcely exceeds 75. 



The weaker acids, being for the most part without action 

 on the test solution, give satisfactory results. Oxalic and tar- 

 taric acids must, however, be excepted, the acid set free tends 

 to form acid salts of sparing solubility, these are precipitated, 

 thus the conditions are changed. 



The results obtained are here tabulated. 



Hydrochloric acid 



29-37 



13-68 



100 



Succinic 



1-21 



0-21 



1-54 



Acetic 



2-28 



0-14 



1-02 



Citric 



1-02 



0-53 



3-87 



Pyrophosphoric 



0-963 



0-926 



6-77 



Tungstic 



1-2 



0-2 



1-46 



The first column of this table shows the absolute number of 

 molecules of the sodium salt which must be added without 

 regard to the basicity of its acid in order that one molecule of 

 sulphuric acid may be so completely saturated with base as no 

 longer to give a reaction for free sulphuric acid. 



