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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 Molécules (PhLAM), UMR CNRS 8523, Centre d’Etudes et de Recherches Lasers et Applications (CERLA, FR CNRS 2416), Université des Sciences

et Technologies de Lille, 59655 Villeneuve d’Ascq , France 2 Physico-Chimie des Processus de Combustion et de l’Atmosphère (PC2A), UMR CNRS 8522,

Centre d’Etudes et de Recherches Lasers et Applications (CERLA, FR CNRS 2416), Université des Sciences et Technologies de Lille, 59655 Villeneuve d’Ascq, 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 (Henry’s 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 MΩ cm−1). 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

3

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 10−5− 10−3

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

COHAr

COH

PP

P

2

2

+=χ

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

4

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

2χ (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

+=χ (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 pressure COHP2

, above the formaldehyde aqueous solutions, is

calculated from the measured total pressure PVLE over the equilibrated solutions as

VLEVLECOH PP *2

χ= .

5

0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,3510

20

30

40

50

60

70

80

90

T=323K

T=313K

T=293KT=295K

Tot

al v

apor

pre

ssur

e (T

orr)

X H

2CO

liquid [mol.frac]

Brandani 1980, T=313K Hasse&Maurer 1991, T=323K Brandani 1980, T=323K This work, T=295K Hasse&Maurer 1991, T=293K Kogan et al. 1977, T=313K Kogan et al. 1977, T=323K

Figure 3 Equilibrium total vapour pressure as a function of the H2CO concentration (molar

fraction) in the aqueous solutions. Note the decrease of the vapour pressure as the content of dissolved H2CO in the solution increases (see discussion).

Figure 3 displays the equilibrium total pressure as a function of the

formaldehyde molar fraction in the liquid solution. The results of Hasse and Maurer [7], Kogan et al. [12], Brandani et al. [13] for the formaldehyde-water system, at temperatures between 293K and 323K, are also included. It can be seen that the vapour pressure decreases with increasing content of formaldehyde in the aqueous solutions. When dissolved in water, formaldehyde is almost completely hydrated into methylene glycol. Depending on the strength of the solutions, it polymerizes to form polyoxymethylene glycols. The concentration of these oligomers and their average degree of polymerization increases progressively as the dissolved fraction of H2CO increases. Consequently, the contribution of molecular formaldehyde to the vapour pressure tends to reduce significantly. This tendency is much pronounced at higher temperatures but is also noted at temperatures around 295K.

Using both QMS and IR techniques, the H2CO partial pressure at VLE can be calculated. These results are displayed in figure 4 together with the data reported by Dong and Dasgupta [6], Hasse and Maurer [7] Blair and Ledbury [14]. For the concentration range 10-5 – 10-2, we can see that our IR measurements are in good agreement with those reported by Dong and Dasgupta using a fluorometric technique. The slight difference in slope may be attributed to the temperature difference between the two sets of measurements.

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10-7 10-6 10-5 10-4 10-3 10-2 10-1

1E-6

1E-5

1E-4

1E-3

0,01

0,1

1

PH

2CO[T

orr

]

X H

2CO

liquid [mol.frac.]

This work QMS data T=295K This work IR data T=295K Blair and Ledbury T=294K Dong and Dasgupta T=293K Hasse and Maurer T=293K

Figure 4 Formaldehyde partial pressure at VLE as a function of the H2CO molar fraction in

the aqueous solutions.

For the more concentrated solutions (10-1- 0.3 mol. frac.) we obtain data that lie below those reported by Hasse and Maurer [7] and Blair and Ledbury [14] that both used a gas saturation technique and indirect measurement methods. Further work is needed to better explain these discrepancies.

The partial pressure seems to saturate at [2H COχ ]liq. ~ 20 mol%. This behaviour

can be due to the increasing amount of oligomers already widely present at concentration ~ 15 mol%. This corroborates our previous suggestions that the vapour phase becomes depleted in molecular formaldehyde as the dissolved fraction of formaldehyde in the solution increases.

The solubility of gaseous species (like formaldehyde) in aqueous water is an important property in atmospheric chemistry. Information about this property can be obtained from our data by determining the Henry coefficient at 295 K. Since the formaldehyde is hydrated to different degrees in water, an apparent Henry’s law constant needs to be introduced as:

[ ] ( )[ ] [ ]atmMP

OHCHCOHH /;222 +=∗

where [CH2(OH)2] represents the concentration of methylene glycol. For the concentration range from 10−5 to 0.01 mol frac of formaldehyde in

water a value of 3800 M/atm (at T=295K) can be derived. This value is in good agreement with data previously reported of 3140, 2530 both at 293 K and 3400 M/atm at 298K [6,15,16]. Moreover, we observe a negative deviation from Henry’s law that becomes apparent at concentrations of dissolved H2CO higher than 0.01 mol frac.. This behaviour seems consistent with the important oligomerization occurring in solution as concentration increases.

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4. Conclusion

Quantitative measurements of the partial vapour pressure of formaldehyde collected at VLE above aqueous H2CO solutions of different concentrations (from 10-

5 to 0.3 mol. frac.) have been determined at 295K using two different experimental techniques. Mass spectrometry is performed for gas samples collected above solutions between 10-2 to 0.3 H2CO mol. frac. whereas IR diode laser spectroscopy is used to analyse gas phase above dilute H2CO aqueous solutions (10-5 to 0.2 mol. frac.). For both techniques we used a direct gas collecting method and special care is taken to avoid the effects of the non-equilibrium influence. A correlation is observed between the polymerisation process occurring in the solution and the partial pressure of H2CO measured at VLE. A drop of the H2CO partial pressure is observed as the oligomer concentration increases above 15 mol%. Finally, from equilibrium data obtained at low concentration, Henry’s law constant is derived at 295K.

It should be mentioned that no information exists concerning the solubility of formaldehyde at lower temperature. Such information is desirable to better characterize the incorporation mechanism of COV compounds in ice. Our next step will be focused on the determination of the temperature dependence of solubility in the temperature regime of the cold troposphere (248 K – 273 K).

Furthermore, these results permitted to calibrate the composition of thin ice films produced by vapour deposition of gas mixtures (H2O/H2CO) collected above different H2CO aqueous solutions at VLE. The Raman analysis of such ice films has revealed important information with respect to the interaction of formaldehyde with water [5].

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