M. C. Lea — Relative Affinities of certain Acids. 445 



Art. LV. — New Method of determining the relative Affini- 

 ties of certain Acids • by M. Carey Lea. 



[Read before the National Academy, April, 1894, by Prof . Ira Remsen.] 



This method of measuring affinities is based on the princi- 

 ple that the affinity of any acid is proportionate to the amount 

 of base which it can retain in the presence of a strong acid 

 selected as a standard of comparison for all acids. The 

 standard acid being in all cases kept exactly at the same dilu- 

 tion. 



An example will make this clearer. Sulphuric acid is here 

 taken as the standard and its presence or absence in the free 

 state is ascertained by means of the herapathite test (described 

 in this Journal, June, 1893). For simplicity we will suppose 

 that the quantity taken is always a gram molecule at a fixed 

 rate of dilution. It is evident that two gram molecules of 

 sodium hydroxide would exactly saturate it. If now we take 

 a given acid we may find that a quantity of its sodium salt 

 corresponding to three gram molecules of sodium hydroxide 

 will exactly extinguish the reaction of a gram molecule of 

 free sulphuric acid. With still another acid we may find that 

 a quantity of its sodium salt corresponding to four gram mole- 

 cules of sodium hydroxide are needed to extinguish the sul- 

 phuric reaction. Then the affinity of the second acid is 

 exactly twice as great as that of the first. At the point where 

 the free sulphuric acid reaction was extinguished the second 

 acid under examination retained twice as much sodium as the 

 first and this quite independently of any question of com- 

 parative basicity. 



Throughout the series of determinations here to be described 

 the sulphuric acid was used invariably at the same degree of 

 dilution, otherwise the results would not be strictly compara- 

 tive. Having obtained normal acid by titration with pure 

 sodium carbonate this was further diluted to -g and 50 to 100 cc 

 were found a convenient quantity to employ. The salt to be 

 tested was finely powdered and thoroughly dried at 100°, or at 

 whatever higher temperature it could support. It was then 

 placed in a weighing bottle and cooled in a dessicator and 

 kept there except for a few moments at a time. By using the 

 dry salt the dilution of the acid was kept constant, When the 

 point of extinguishment seemed to be reached, at least four 

 final crystallizations were made. Great care is necessary to 

 seize the exact point of extinguishment. The quantity of the 

 salt found is then reduced to correspond with one gram mole- 

 cule of sulphuric acid. It is next divided by its own molecu- 



