560 Mr. Carey Lea on a New Method of Determining 



determined are unreliable for want of sufficient exactitude in 

 this respect. 



To show how much precaution is needed the following 

 reactions may be mentioned. 



When 4 cub. centim. of normal sulphuric acid are added to 

 40 cub. centim. of normal solution of sodium nitrate, not a trace 

 of free sulphuric acid can be detected in the liquid. In con- 

 sequence of the large excess of sodium salt the sulphuric acid 

 has been completely taken up by the sodium, with of course 

 an expulsion of an equivalent quantity of nitric acid. 



But when, instead of 4 cub. centim. of normal sulphuric acid 

 we use 40 cub. centim. of decinormal acid, then, although the 

 quantities of acid and of salt are exactly the same, the equili- 

 brium is completely changed. The greater quantity of water 

 present by reason of its affinity for sulphuric acid counter- 

 acts to some extent the affinity of the sodium. Free sulphuric 

 acid exists in the solution, and is abundantly indicated by the 

 herapathite test. 



This difference may be even more strikingly shown in the 

 following manner : — Taking the mixture of 4 cub. centim. of 

 normal sulphuric acid and 40 cub. centim. of normal solution 

 of sodium nitrate, let a 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 crystalliza- 

 tions 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 deduction 

 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. 



