Rate of Vaporization of Arsenic Single Crystals and the Vaporization Coefficient of Arsenic

Abstract

Rates of vaporization from (111) cleavage faces of arsenic single crystals have been measured over the temperature range 508°–580°K with a recording vacuum microbalance. Expressing the rates as Langmuir vapor pressures ${\mathrm{(P}}_L)$ , the results are $R\ln P_L(\mathrm{atm}\text{)\hspace{0.17em}=\hspace{0.17em}−(43.99\hspace{0.17em}±\hspace{0.17em}1.47)\hspace{0.17em}/\hspace{0.17em}T\hspace{0.17em}+\hspace{0.17em}(37.98\hspace{0.17em}±\hspace{0.17em}2.72)}$ . At 550°K, $P_L{\text{\hspace{0.17em}=\hspace{0.17em}6.60\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−10}}\mathrm{atm}$ . Comparison with literature equilibrium data yields a vaporization coefficient, ${\alpha }_{\upsilon }{\mathrm{\hspace{0.17em}=\hspace{0.17em}P}}_L{\mathrm{\hspace{0.17em}/\hspace{0.17em}P}}_E{\text{\hspace{0.17em}=\hspace{0.17em}4.6\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−5}}$ at 550°K, which increases with temperature, ${\alpha }_{\upsilon }\mathrm{\hspace{0.17em}=\hspace{0.17em}}\exp \text{\hspace{0.17em}(−10921\hspace{0.17em}/\hspace{0.17em}RT)}$ . The results are consistent with a vaporization mechanism in which the slow step involves formation of As₄ molecules and occurs prior to surface diffusion and desorption. Ancillary measurements on vaporization of wrapped arsenic crystals, powdered arsenic, and Knudsen‐effusion experiments are presented. These data and previous vaporization studies on powdered arsenic are correlated using the concept of effective vaporizing areas. Some implications of the results for the equilibrium vapor pressure of arsenic, the interpretation of Knudsen‐effusion experiments, and the absolute entropy of As₄(g) are discussed.