284 ILLINOIS STATE ACADEMY OF SCIENCE 



fully prepared, clear arsenic trisulficle liydrosol may be 

 explained npon tlie following grounds: Arsenic trisulfide 

 liydrosol sensitizes the jjliotocliemical oxidation of dye- 

 stnffs like eosin and malachite green. Dne to this photo- 

 sensitizing action of the arsenic trisulfide micelles, hy- 

 drogen sulfide produced by the hydrolysis of arsenic 

 trisulfide is oxydized to colloidal sulfur which is stabi- 

 lized by absorbed pentathionic acid. That pentathionic 

 acid is the stabilizing electrolyte for sulfur hydrosols of 

 the Oden type, has been shown by Freundlich and 

 Scholz^\ This acid is quite stable in the presence of 

 hydrogen sulfide, with which it reacts to form free sulfur 

 and water: 



5 H,S + H.SsOe = 10 S + 6 H,0, 



this reaction explaining the great sensitiveness of the 

 Oden S-hydrosols toward HoS. It is the reaction between 

 the stabilizing pentathionic acid of the S-micelles and 

 the stabilizing HsS of the arsenic trisulfide-micelles ac- 

 cording to the above equation which deprives the micel- 

 les of both sols of their stabilizing electrolyte and re- 

 sults in the coagulation of a mixture of the two sols. 



In their presentation of the above explanation of the 

 action of light upon arsenic trisulfide hydrosol, Freund- 

 lich and Nathansohn do not discuss the mechanism of the 

 formation of pentathionic acid. It is probable, however, 

 that the photochemical oxidation of H^S which results 

 in the formation of colloidal sulfur may also account 

 for the formation of the pentathionic acid, presumably by 

 the photochemical oxidation of HoS to SO2 which reacts 

 with HoS to form the acid, a reaction which is probably 

 one of many taking place in the formation of Wacken- 

 roder's solution, and which has been investigated by 

 Debus*^ and others: 



5 HoS + 15 = 5 SO2 + 5 HoO 



5 H,S + 5S02 = H2S506 + 5S + 4H20 



Assuming that a condition of equilibrium exists in an 

 arsenic trisulfide hydrosol Avith respect to adsorbed hy- 

 drogen sulfide and pentathionic acid on the one hand, 

 and intermicellular hydrogen sulfide and acid on the 



