Cross Determination of the Vapor Liquid Equilibrium of Formaldehyde Aqueous Solutions by Quadrupole Mass Spectrometry and Infrared Diode Laser Spectroscopy ADRIANA OANCEA, † BENJAMIN HANOUNE, ‡ CRISTIAN FOCSA, † AND BERTRAND CHAZALLON* ,† Laboratoire de Physique des Lasers, Atomes et Molécules (PhLAM), UMR CNRS 8523, CERLA (FR CNRS 2416), Universit ´ e des Sciences et Technologies de Lille, 59655 Villeneuve d’Ascq Cedex, France, and Physico-Chimie des Processus de Combustion et de l’Atmosphére (PC2A), UMR CNRS 8522, CERLA (FR CNRS 2416), Université des Sciences et Technologies de Lille, 59655 Villeneuve d’Ascq, France Received July 26, 2008. Revised manuscript received November 4, 2008. Accepted November 7, 2008. Quantitative measurements of the partial vapor pressure of formaldehyde are performed above aqueous H 2 CO solutions of different concentrations (from 10 -5 to 0.3 molar fraction) using mass spectrometry and IR diode laser spectroscopy. Both experimental techniques allow direct probing of the gas phase concentration collected at equilibrium above the aqueous solutions. A correlation is observed between the polymerization processes occurring in the solution and the partial pressure of H 2 CO measured at vapor liquid equilibrium (VLE). A similar correlation is observed from total pressure measurements for which the equilibrium vapor pressure decreases as [  H 2 CO VLE ] liq is increased. A saturation regime of the H 2 CO partial pressure is reached as the dissolved fraction of formaldehyde increases above ∼0.15 mol frac. Henry’s law constants are derived at 295K for the diluted solutions. Introduction Formaldehyde plays an important role in chemical industry, where it is a key reactant in many technological processes (1). It is a significant component of many manmade materials (solvents, plastics, resins, paper, fertilizers, etc.). The de- scription of the vapor liquid equilibrium (VLE) behavior of the system formaldehyde-water is thus of great importance for the chemical industry. Furthermore, the study of VLE and the determination of thermodynamic properties of formaldehyde solutions have been the focus of several research groups over the last 80 years (1-6). Most of these studies were carried out at relatively high temperatures (313-373K) and total pressures between 20 and 760 Torr. However, large discrepancies exist for the gas phase data above concentrated aqueous solutions at lower temperature (293 K), with partition coefficients differing by a factor of 30 (5). These discrepancies may illustrate possible experimental pitfalls when the gas phase concentration is measured indirectly, i.e., after recondensation into a liquid. Formaldehyde is also of great current interest in atmo- spheric chemistry, as it is a key constituent involved in several important processes occurring in the atmosphere, for instance in the production of HO x radicals that governs the tropospheric ozone cycle (7). It can be directly emitted in the atmosphere by anthropogenic and natural sources, but can also be formed as an intermediate product of the atmospheric hydrocarbons photo-oxidation. Previous studies showed that the reaction of formaldehyde with nitric acid in sulfuric acid aerosols could provide a pathway for HNO 3 conversion to NO x (8, 9). It was shown that the solubility of formaldehyde can be greatly enhanced in such cold acid droplets typically found in the free troposphere, thus efficiently removing H 2 CO from the gas phase and increasing in significant levels the formaldehyde content in rain, snow and fog (10, 11). More recently, it was demonstrated that the heterogeneous chem- istry of formaldehyde is involved in the snowpack chemistry (12). The snowpack was found to be photochemically active, producing and emitting formaldehyde into the atmosphere at elevated concentration in coastal and ice-cap sites of Polar regions. The mechanism at the origin of the formaldehyde production remains, however, controversial as the incor- poration/partitioning of formaldehyde in ice crystals has to be determined first. The partitioning of formaldehyde between the gas phase, the liquid and the solid phases is thus an important parameter in both the atmospheric and industrial context. Indeed, the phase in which formaldehyde exists can significantly influ- ence its physical and chemical properties with respect to its behavior and fate in the environment. In previous studies, we investigated the interaction of small organic compounds (formaldehyde, ethanol) with water or ice in many different concentration and temperature regimes using Raman spec- troscopy (13-15). Thin doped ice films can be produced by simultaneous condensation of the vapor phase sampled at VLE over formaldehyde aqueous solutions. In order to derive meaningful data for the influence of composition on the structure of the condensed film, it is important to determine the concentration of H 2 CO in the collected vapor. We investigate here, quantitatively, the VLE of formal- dehyde aqueous solutions of different concentrations at room temperature by directly probing the gas phase above the solutions. The cross determination of the gas phase con- centration is performed using mass spectrometry and infrared diode laser spectroscopy techniques in two completely independent set-ups. Vapor pressures at atmospherically relevant formaldehyde concentrations are investigated to derive Henry’s coefficients. Experimental Methods Preparation of Aqueous Formaldehyde Solutions. Aqueous formaldehyde solutions of different concentrations (between 10 -5 and 0.3 mol frac.) are prepared according to the classical procedure described in refs 1 and 13. Briefly, solid paraform- aldehyde (Sigma-Aldrich) is depolymerised in double distilled and deionized water (resistivity 18 MΩ cm -1 ) produced by UHQ PS (Elgastat). The appropriate amounts of paraform- aldehyde and distilled water are heated at 373 K during 10 min. Droplets are formed on the cap placed at the top of the flask. This method avoids too much water loss during boiling. To help the depolymerisation, three drops of 0.1 M sodium hydroxide solution are added. After cooling down to room temperature the formaldehyde solutions are reweighted and * Corresponding author phone: +33 (0)320336468; fax: +33 (0)320336463; e-mail: chazallon@phlam.univ-lille1.fr. † Laboratoire de Physique des Lasers. ‡ Physico-Chimie des Processus de Combustion et de l’Atmosphe `re (PC2A). Environ. Sci. Technol. 2009, 43, 435–440 10.1021/es8020588 CCC: $40.75  2009 American Chemical Society VOL. 43, NO. 2, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 435 Published on Web 12/15/2008