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A method for measuring enthalpy of volatilization of a compound, Δ vol H , from dilute aqueous solution
Author(s) -
Wang Tianshu
Publication year - 2006
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.2641
Subject(s) - chemistry , aqueous solution , enthalpy , ion , analytical chemistry (journal) , volatilisation , atmospheric temperature range , standard enthalpy change of formation , thermodynamics , standard enthalpy of formation , chromatography , organic chemistry , physics
This study has developed a method for measuring the enthalpy of volatilization (Δ vol H ) of a compound in a dilute solution via ion‐molecule reactions and gas‐phase analysis using selected ion flow tube mass spectrometry (SIFT‐MS). The Δ vol H / R value was obtained using an equation with three variant forms either from the headspace concentration of the solution or from individual product ion(s). Under certain experimental conditions, the equation has the simplest form ${\rm \Delta }_{{\rm vol}} H/R = - \ln \left( {{{i_n /I} \over {i_n^{\rm o} /I^{\rm o} }}} \right){\rm /(1/}T - {\rm 1/}T^{\rm o} {\rm )}$ , where R is the gas constant (8.314 J · mol −1 · K −1 ), i n and I are the respective product and precursor ion count rates, and T is the temperature of the solution. As an example, a series of 27.0 µmol/L aqueous solutions of acetone was analyzed over a temperature range of 25–50°C at 5°C intervals using H 3 O + , NO + and O 2 + .precursor ions, producing a mean Δ vol H/R value of 4700 ± 200 K. This corresponds with current literature values and supports the consistency of the new method. Notably, using this method, as long as the concentration of the solution falls into the range of Henry's law, the exact concentration does not have to be known and it can require only one sample at each temperature. Compared with previous methods which involve the measurement of Henry's law constant at each temperature, this method significantly reduces the number of samples required and avoids the labour and difficulties in preparing standard solutions at very low concentrations. Further to this, if the contents of a solution were unknown the measured Δ vol H/R from individual product ion(s) can help to identify the origin of the ion(s). Copyright © 2006 John Wiley & Sons, Ltd.