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



lar weight ; this gives the number of molecules of the salt 

 needed to extinguish the reaction in one molecule of sulphuric 

 acid. In order to make it possible to compare acids of dif- 

 ferent basicities, the figures thus obtained must next be modi- 

 fied to correspond with the basicity of the acid used. If the 

 acid is bibasic no change will be needed. If monobasic the 

 figures obtained must be divided by 2. If tribasic they must 

 be multiplied by f etc. Finally as the quantity characteristic 

 of the acid is the excess of the quantity found, over the 

 amount equivalent to one molecule of sulphuric acid, unity is 

 deducted from the amount obtained and the residue thus found 

 represents the comparative affinity of the acid and may be 

 called its index. 



This may be rendered more clear by one or two instances. 



In the case of hydrochloric acid there was needed as a mean 

 of many determinations 29 '37 gram molecules of sodium 

 chloride to extinguish the reaction in one gram molecule of 

 sulphuric acid. At this point the solution necessarily con- 

 tained, 



Na 2 S0 4 + 2HC1 + 27-37NaCl. 



This is proved beyond question by the fact that the solution 

 no longer gives a trace of reaction of free sulphuric acid. 

 The quantity 27 - 37 gram molecules of sodium chloride is the 

 proportion of undecomposed sodium chloride that must re- 

 main in the solution in order that the sulphuric acid may be 

 completely converted into sodium sulphate and may remain as 

 such in the solution in a condition of equilibrium. 



This number 27*37 therefore represents the strength of the 

 affinity of hydrochloric acid for sodium. But in order to 

 compare acids of different basicities it is convenient to refer 

 them all to bibasic sulphuric acid and therefore the number 

 just found must be divided by 2. Therefore 13*6S may be 

 taken as the index of the affinity of hydrochloric acid in com- 

 parison with those of other acids determined in like manner. 



Similarly with pyrophosphoric acid. The mean value found 

 for the quantity necessary to extinguish the free sulphuric acid 

 in one gram molecule of sulphuric acid was found to be 0'963 

 gram molecules of sodium pyrophosphate. At this point the 

 liquid contains 



Na 2 S0 4 +i(H 4 P 2 7 ) + -463(Na 4 P a 7 ) 



in equilibrium. The number 463 therefore represents the 

 comparative affinity of pyrophosphoric acid except that as the 

 acid is quadrobasic the number found must be multiplied by 2 

 in order to bring it into comparison with bibasic acids. There- 

 fore the index of pyrophosphoric acid is "926. This acid be- 

 ing quadrobasic half a molecule contains the quantity of 



