Determination of Water Vapor Pressure Over Corrosive Chemicals Versus Temperature Using Raman Spectroscopy as Exemplified with 85.5% Phosphoric Acid

摘要

A method to determine the water vapor pressure over a corrosive substance was developed and tested with 85.5 +/- 0.4% phosphoric acid. The water vapor pressure was obtained at a range of temperatures from approximate to 25? to approximate to 200? using Raman spectrometry. The acid was placed in an ampoule and sealed with a reference gas (either hydrogen or methane) at a known pressure (typically approximate to 0.5 bar). By comparing the Raman signals from the water vapor and the references, the water pressure was determined as a function of temperature. A considerable amount of data on the vapor pressure of phosphoric acid are available in the literature, to which our results could successfully be compared. A record value of the vapor pressure, 3.40 bar, was determined at 210?. The method required a determination of the precise Raman scattering ratios between the substance, water, and the used reference gas, hydrogen or methane. In our case the scattering ratios between water and reference (1) Q-branches were found to be 1.20 +/- 0.03 and 0.40 +/- 0.02 for H-2 and CH4, respectively.