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

What is it about?

The water vapor pressure is not easy to measure over corrosive substances. Especially at higher temperatures. We have found a way to determine the humidity over 85.5% (concentrated) phosphoric acid at a range from 25 C to 200 C using Raman spectrometry. The acid was placed in a sealed ampoule with 0.5 bar of a reference gas (hydrogen or methane). By comparing the Raman signals from water vapor and the references, the pressure was determined at the temperature. Our results compare successfully with data in the literature. For the first time the value of the vapor pressure, 3.40 bar, was determined at 210 C. Ratios between water and reference Raman bands were found to be 1.20 and 0.40 for H2 and CH4, respectively.

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