484 M. C. Lea — Nature of Certain Solutions 



4. Acid Sulphates. 



At one time it was considered to be an established fact that 

 the acid salts of the alkaline bases were completely decom- 

 posed by solution in water into neutral salt and free acid. In 

 a review of this subject Berthelot quotes Andrews, and Favre 

 and Silbermann as expressing this opinion positively and with- 

 out question.* His own investigation led him to the conclusion 

 that the separation was never complete, that a certain propor- 

 tion of acid sulphate was always present, diminishing in 

 proportion to the amount of water present ; increasing in pro- 

 portion to the amount of free acid. The formation of acid 

 sulphate was always endothermic, was accompanied by the 

 absorption of 1*05 cal. in the case of JSTaELS0 4 .f He especially 

 remarks that when a very small proportion of acid is added 

 the tendency is to a complete combination in the form of acid 

 salt. 



This last mentioned result is contrary to that which I have 

 been able to observe and I will therefore mention my reac- 

 tions with some particularity. 



I invariably found that when a trace of sulphuric acid was 

 added to the solution of a neutral alkaline sulphate it reacted 

 as free acid. The matter was examined especially in the fol- 

 lowing manner. To 40 cc of distilled water 2 drops of sulphuric 

 acid were added, the liquid was divided into two equal parts 

 in one of which was dissolved a gram of neutral sodium sul- 

 phate. These two liquids were then examined with the test 

 solution and the detection of the acid was found to be fully as 

 easy in the presence of the large excess of sodium sulphate as 

 in the case of the acid alone. 



It is true that this result is open to the following criticism. 

 Alcohol extracts sulphuric acid from solid acid sodium sul- 

 phate showing thereby, as Mendeleef remarks, the extremely 

 weak affinity which unites it to the neutral salt. In the 

 present examination the test can only be used in alcoholic solu- 

 tion. It may be said that the alcohol first precipitates the 

 solid acid sulphate and then abstracts free acid from it. There 

 is force in this objection but I think it may be answered in 

 the following manner. We first reduce sulphuric acid nearly 

 to the limit of dilution and quantity at which it can be de- 

 tected. We then find that the addition of a comparatively 

 large quantity of neutral sulphate makes absolutely no change 

 in the reaction. It seems therefore legitimate to conclude that 

 no combination has taken place because otherwise it would be 

 necessary to admit that alcohol can abstract all the acid united 

 with the solid neutral sulphate, which is highly improbable. 



*Mech. China., ii, 318. f Mech. Ch., ii, 323. 



