130 Mixter — Combustion of Silicon and Silicon Carbide. 



Art. XII. — The Heat of Combustion of Silicon and Silicon 

 Carbide • by W. Gr. Mixter. 



[Contributions from the Sheffield Chemical Laboratory of Yale University.] 



The direct determination of the heat of combination of 

 silicon with, oxygen presents many difficulties. The reaction 

 occurs only at high temperatures; the product is not volatile 

 and hence encloses silicon and prevents complete oxidation. 

 Troost and Hautfeuille* from the thermal effect of the reac- 

 tion of silicon tetrachloride with water obtained 227000° for 

 the formation of silicon dioxide from crystalline silicon. Ber- 

 thelot'sf result by the same method was 179600° for the forma- 

 tion of gelatinous silicic acid from crystalline silicon. Ostwald^: 

 states, " die Bildungswarme sammtlicher Siliciumverbindungen 

 ist noch sehr unsicher." 



Preliminary tests made by burning a mixture of silicon and 

 carbon in the bomb showed that more than half of the former 

 could be burned. The method would give good results if the 

 solid products contained only silica, silicon insoluble in hydro- 

 fluoric acid and silicon carbide. The last was always present; 

 hence its heat of formation was determined, as described later, 

 in order to have the data needed for the work on silicon. 



The writer is indebted to the Carborundum Company, 

 Niagara Falls, for an abundant supply of carborundum powder 

 and silicon for the investigation. Crystallized silicon was pre- 

 pared as follows : One part of silicon was dissolved in four 

 parts of molten aluminum and the mixture allowed to cool 

 slowly : the metal was dissolved out with hydrochloric acid, and 

 the silicon, which was in small crystals, was digested with hot 

 hydrofluoric acid. The product at this stage contained 2*8 per 

 cent of aluminum. Treatment with molten potassium pyro- 

 sulphate reduced the aluminum content one-half. Next the 

 crystals were pulverized in an agate mortar and the finer por- 

 tions separated by levigation for one minute in water. Finally 

 the powder was digested with a mixture of hot sulphuric and 

 hydrofluoric acids, washed and then heated to drive off the acid 

 not washed out. Thus prepared, the silicon contained a little 

 aluminum, a trace of carbon and 99 - 95 per cent of silicon. The 

 last was determined by dissolving the powder in a hot concen- 

 trated solution of pure sodium hydroxide and making the usual 

 separation of silica. The carborundum was purified by means 

 of molten pyrosulphate and, after washing, it was floated in 

 water and only the finer portion retained. It was next treated 

 with nitrohydrofluoric acid to remove silicon and silica, 40 



*C. E., lxx, 252. 



+ Thermochimie ii, 125, Ann. de Chimie et Phys. [5], xv, 213. 



% Grandriss der allgemeinen Chemie, dritte Auflage, 268. 



