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Vapour-liquid equilibrium of the formaldehyde-water system

A. OANCEA1, B. HANOUNE2, C. FOCSA1, B. CHAZALLON1,

adriana.oancea@ed.univ-lille1.fr 1 Laboratoire de Physique des Lasers, Atomes et Molcules (PhLAM), UMR CNRS 8523, Centre

dEtudes et de Recherches Lasers et Applications (CERLA, FR CNRS 2416), Universit des Sciences et Technologies de Lille, 59655 Villeneuve dAscq , France

2 Physico-Chimie des Processus de Combustion et de lAtmosphre (PC2A), UMR CNRS 8522, Centre dEtudes et de Recherches Lasers et Applications (CERLA, FR CNRS 2416), Universit des

Sciences et Technologies de Lille, 59655 Villeneuve dAscq, France

Key-words: Vapour liquid equilibrium, formaldehyde, aqueous solution, Henry coefficient

1 Introduction

Formaldehyde is one of the most important carbonyl compound in the

atmosphere. It can be directly emitted in the atmosphere by anthropogenic and natural sources, but can also be formed as an intermediate product of the photo oxidation of atmospheric hydrocarbons. This compound is involved in several important processes occurring in the atmosphere, for instance in the production of HOx radicals which may influence the tropospheric ozone cycle [1]. Lately studies showed that the reaction of formaldehyde with nitric acid in sulfuric acid aerosols could provide a pathway for HNO3 conversion to NOx [2, 3]. There is now evidence that the heterogeneous chemistry of H2CO is involved in the snowpack chemistry. This latter was found to be photochemically active, producing and emitting formaldehyde into the atmosphere [4].

Formaldehyde can partition between the gas phase, the liquid and solid phases. In the atmospheric context the phase in which formaldehyde exists can significantly influence its physical and chemical properties with respect to its behavior and fate in the environment. In a previous work, we investigate by Raman spectroscopy the formation of different hydrate phases as a function of concentration in ice films obtained by vapor deposition [5]. One important parameter that influences the partitioning gas phase /liquid particle phase in the atmosphere is the gas-phase vapor pressure of formaldehyde.

Experimental data on the formaldehyde vapor pressure at atmospherically relevant concentration are very scarce [6]. On the other hand discrepancies exist between the data reported at ~293K for gas phase collected above concentrated aqueous solutions of formaldehyde [7, 8]. These controversies illustrate possible experimental pitfalls when measuring indirectly the gas phase concentration (chemical analysis of the condensed gas phase). This probably also highlights the complex behaviour of dissolved H2CO that polymerise intensively at high concentrations.

Our group is involved in the study of atmospheric pollutant gases (formaldehyde, ethanol) in interaction with ice particles. The ice doped samples are obtained by simultaneous condensation of vapor phase sampled over aqueous solutions at vapor liquid equilibrium. In order to get better insights on the composition of vapor collected above H2CO aqueous solutions and to derive meaningful data on the condensed film compositions, we investigate the vapor liquid equilibrium (VLE) of

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formaldehyde aqueous solutions of different concentrations at room temperature. This work is realized by probing directly the gas phase at VLE above the solutions. The analysis is performed using cross calibrations of mass spectrometry and IR diode laser spectroscopy. Furthermore, vapor pressures at atmospherically relevant H2CO concentrations are investigated to derive thermodynamic parameters (Henrys coefficients) at room temperature.

2. Experimental methods 2.1. Preparation of aqueous formaldehyde solutions Aqueous formaldehyde solutions at different concentrations are prepared

according to the classical procedure [9]. Solid paraformaldehyde (Sigma-Aldrich 95%) is depolymerised in double distilled and deionised water produced by an UHQ PS (Elgastat) (resistivity 18 M cm1). The appropriate amounts of paraformaldehyde and distillate water are heated at 393K during 10 min. To help the depolymerisation, three drops of sodium hydroxide solution 0.1M are added. The exact amount of formaldehyde in the liquid samples is determined by adding an excess of sodium sulfite and titrating with a hydrochloric acid solution [9]. The end-point of the titrations is detected by pHmetric measurements. Once prepared, the formaldehyde solutions are pumped down to 10-1 Torr and allowed to stand for at least 14 hours at room temperature before analysis of the gas phase. This length of time has been found necessary to obtain reproducible results. 2.2. Mass spectrometry experimental set-up

A schematic representation of the mass spectrometer experimental set-up is displayed in Figure 1.

Figure1 A. Experimental set-up used for the determination of gas phase composition of equilibrated aqueous formaldehyde solutions: 1 the intermediate pressure stage; 2 Quadrupole mass spectrometer (QMS) chamber. B. Gas mixing line preparation (H2CO/Ar, H2CO/H2O/Ar)

B 1

2

A

B

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The main parts of the system are the gas collecting assembly (or mixing line) and the mass spectrometer itself (Pfeiffer Vacuum, model Prisma QMS 200M). The evolution of the H2CO signal is followed using the MID mode, at the parent ion signal M/z=30. The response of the mass spectrometer to formaldehyde (conversion of ionic current into molar fraction) is calibrated using known mixtures of gas phase formaldehyde diluted in argon. Molecular formaldehyde vapor is obtained by heating solid paraformaldehyde at 358 K in a stainless steel oven with adjustable temperatures between 303 and 393K with 0.1K accuracy. The formaldehyde vapour is diluted with argon so as to obtain calibration point in the range 105 103 formaldehyde molar fraction.

The molar fraction of formaldehyde in the gaseous mixture is given by:

COHAr

COH

PP

P

2

2

+=

where PH2CO is the pressure of formaldehyde, and PAr the pressure of argon used to dilute the sample.

Because of the large difference of working pressure between the mass spectrometer chamber and the mixing line, gas expansion can produce substantial fractionation effects. A 4-liter round-bottomed flask was introduced as an intermediate pressure stage to prevent such effects (Figure 1).

2.3. IR experimental set-up The tunable infrared diode laser spectrometer is represented on Figure 2. It

has been described, together with the associated acquisition and analysis procedure in [10]. Since then it is used for formaldehyde quantification in ambient air and microenvironments [11].

Figure 2 The tunable infrared diode laser spectrometry experimental set-up. 1-gas sample,

2-the permeation calibration device, 3-the Herriot multipass cell, 4- laser source

1

3

2

4

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With the present setup, mole fractions down to about 1 ppb can be determined with a 10 % precision. Small volume samples can be analyzed, as only 7 L at about 12 Torr are needed for one measurement.

Formaldehyde is detected at 2829.5 cm-1. Other absorption lines can be probed between 2800 and 2840 cm-1 but this specific line has been preferably chosen due to its strong intensity and absence of interference from any other gas. The cw radiation of the LN2 cooled tunable diode laser is actively stabilized on this absorption line by way of a 10 cm long absorption reference cell filled with a few Torr of pure formaldehyde.

The gas sample is flowed through a 7.7 L multiple pass cell providing a 34 m optical path. Pumping of the cell and injection of the sample and reference gases (provided by permeation, necessary for absolute calibration of the absorption) are achieved by way of electromagnetic valves.

H2O/ H2CO gas samples are collected as described before, but are further diluted with argon so as to fall below the ppm range. Indeed, the spectrometer is designed for the quantification of low concentrations of formaldehyde, and it is necessary to avoid as much as we can the contamination by highly concentrated gas samples. The formaldehyde is recognized to adsorb onto the walls and afterwards desorbs from all components in the cell and gas handling system.

3. Results and discussion Formaldehyde partial pressure The formaldehyde molar fraction measured with the quadrupole mass

spectrometer corresponds to that in the mixtures H2CO-H2O-Ar and is given by:

ArCOHOH

COHmeasured

PPP

P

++=

22

2 (1)

The formaldehyde molar fraction of the gas phase above the aqueous solution

at liquid vapour equilibrium can be deduced from:

COH

OHVLE

P

P

2

21

1

+= (2)

where COH

OH

P

P

2

2 is obtained from the relation (1).

Similarly, in the case of infrared diode laser measurements, the molar fraction of formaldehyde in the gas phase is directly converted into molar fraction above the solution by taking into account the argon dilution factor.

The equilibrium pre