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

Marion Rodier, Qingfeng Li, Rolf W. Berg, Niels J. Bjerrum

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Abstract

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 ∼25 ℃ 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 ∼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 ν1 Q-branches were found to be 1.20 ± 0.03 and 0.40 ± 0.02 for H2 and CH4, respectively.
Original languageEnglish
JournalApplied Spectroscopy
Volume70
Issue number7
Pages (from-to)1186-1194
Number of pages9
ISSN0003-7028
DOIs
Publication statusPublished - 2016

Keywords

  • Corrosive substance
  • Raman
  • Gas phase
  • Phosphoric acid
  • Scattering cross-section
  • Vapor pressure

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