OF ARTS AND SCIENCES. 201 



YIII. 



CONTRIBUTIONS FROM THE CHEMICAL LABORATORY OF 

 HARVARD COLLEGE. 



A THERMOCHEMICAL ANALYSIS OF THE REACTION 

 BETAVEEN ALUM AND POTASSIUM HYDRATE. 



By a. Y. E. Young. 



Communicated by the Corresponding Secretary, June 10, 18S5. 



PART I. 



In the course of a thermochemical study of potassic aluminate I 

 desired to use as oue elemeut of calculation the amount of heat 

 evolved in the following reaction, 



Al^CSOJg . K,SO, + 6 KOH = 4 K.SO, + Al(OH)e, 



assuming in accordance with Thomson's statement that the alum is 

 equivalent, for this purpose at least, to alumiuic sulphate. This 

 quantity is given by Thomson* as 30528 calories. He determines it 

 directly by precipitation witii potassic hydrate in the calorimeter, and, 

 as a control, determines the heat of precipitation with three molecules 

 of baric hydrate. The results of the two methods agree fairly well. 

 This value is made by the same author the basis for the indirect 

 determination of the formation heat of the chloride and several other 

 compounds of aluminum, and this, notwithstanding the fact that he notes 

 the comjilete precipitation of the alumiuic oxide by five molecules of 

 potassic hydrate instead of six, which would appear to indicate the for- 

 mation of a basic precipitate. He evidently assumes that the addition 

 of six molecules of the hydrate leaves the precipitate free of sulpluiric 

 acid, and that the agreement with the result obtained when baric 

 hydrate is used affords confirmation of the assumption. Indeed, in 

 other instances, e. g. sulphates of copper, cobalt, and cadmium, when 

 the two methods do not give concordant results, he adopts tlie result 

 ))y baric hydrate, and rejects that by potassic hydrate as being untrust- 

 worthy by reason of the formation of a basic precipitate. It would 

 seem unsafe to assume, without experimental verification, that baric 



* See Therniocheiiiische Untersuchungen, i. 36G. 



