PAPERS OX CHEMISTRY AND PHYSICS 287 



SUMMABY 



1. The electrical conductivity of arsenic trisnlfide hy- 

 drosol increases upon exposure to light, the rate of in- 

 crease being practically independent of the initial con- 

 centration of the sol, although increasing somewhat with 

 decreasing concentration of the coUoid. Studies were 

 made upon a very pure arsenic trisnlfide hydrosol (one 

 containing very little excess hydrogen sulfide), following 

 the conductivity during exposure to the light of a 500 

 watt nitrogen-filled lamp under constant conditions of 

 temperature and intensity of illumination. 



2, The reaction is explained, after the suggestion of 

 Freundlich and Xathansohn, as one of a two-stage photo- 

 chemical oxidation of HoS adsorbed on the As^Ss-niicelles 

 to colloidal sulfur and pentathionic acid (or tetrathio- 

 nic), followed by the reaction between H^S and the 

 thionic acid in the intennicellular liquid with liberation 

 of free sulfur, and with final precipitation of As^Ss upon 

 removal of the stabilizing H^S. The increase in con- 

 ductivity is to be explained as due to the building up of 

 a concentration of the thionic acid more than sufficient 

 to serve as the stabilizing electrolyte for the colloidal 

 sulfur, the reaction with H:>S then proceeding at such 

 a rate that the equilibrium is maintained between the 

 several components of the system, further change in 

 conductance thus being prevented. The increase of the 

 rate of change of the conductance with decreasing con- 

 centration of the sol is explained as due to the increased 

 photochemical activity per unit mass of As^Ss, brought 

 about by the greater dispersity of the more dilute colloid. 



The author wishes to acknowledge the kind assistance of Dr. J. H. 

 ilathews of the Department of Chemistry at the University of Wisconsin. 

 under ■whose direction the work described in this paper was done. 



REFERENCES. 



1) Jour. Am. Chem. Soc. 4/. 1515. (1919). 



2) Koll. Zeit.. 2S. 25S. (1921). 



3) cf. Freundlich and Xathansohn. loc. cit. 



4) Jour. Chem. Soc. 53. 2TS. (18SS). 



