Environ. Sci. Technol. 1988, 22, 1415-1418
Henry’s Law Constants of Some Environmentally Important Aldehydes Eric A. Bettertont and Michael R. Hoffmann*
Environmental Engineering Science, W. M. Keck Laboratories 138-78, California Institute of Technology, Pasadena, Callfornia 91 125
rn The Henry’s law constants of seven aldehydes have been determined as a function of temperature by bubble-column and by head-space techniques. The compounds were chosen for their potential importance in the polluted troposphere and to allow structure-reactivity patterns to be investigated. The results (at 25 “C) are as follows (in units of M atm-l): chloral, 3.44 X lo6;glyoxal, 1 3 X lo6;methylglyoxal, 3.71 X lo3; formaldehyde, 2.97 X lo3;benzaldehyde, 3.74 X 10’; hydroxyacetaldehyde, 4.14 X lo4; acetaldehyde, 1.14 X lo1. A plot of Taft’s parameter, Ca*, vs log H* (the apparent Henry’s law constant) gives a straight line with a slope of 1.72. H* for formaldehyde is anomalously high, as expected, but the extremely high value for hydroxyacetaldehyde was unexpected and may indicate that a-hydroxy-substituted aldehydes could have an unusually high affinity for the aqueous phase. The intrinsic Henry’s law constants, H , corrected for hydration, do not show a clear structure-reactivity pattern for this series of aldehydes. Introduction In order to model the partitioning of aldehydes between the gas phase and the aqueous phase, it is necessary to have numerical values of their Henry’s law constants. Through our interest in aldehydes found in the tropopshere, particularly those found in polluted urban environments, we have noted that there is a paucity of data on the Henry’s law constants for all but a few aldehydes of interest. For example, when we began this work there was only one published value (dating from 1925) (1) for the Henry’s law constant of formaldehyde. Since then one more value has been measured (2) for this important carbonyl compound. Our intention in this work is to measure the Henry’s law constants of a series of aldehydes chosen so that a structure-reactivity pattern can be established which will allow us in the future to estimate the Henry’s law constants of aldehydes that may not have been studied and yet may be potentially significant pollutants. We selected the series of aldehydes based on their Taft Ea*parameters, numerical values of which are available (3, 4 ) for a large number of substituents R (in RCHO). Ea*is a measure of the ability of substituents to withdraw electron density from the carbonyl group and thus further + a*p for the polarize it. Ea* is given by the sum of carbonyl R1R2C0 (5). Since aldehydes are hydrated in water to a greater or lesser extent (6),a distinction needs to be made between the apparent Henry’s law constant H* and the intrinsic Henry’s law constant H. The experimentally determined value is usually the apparent Henry’s law constant, and this includes a term for the hydration constant Khy&The three constants H*,Hand Khydare related by the following equations.
H = [RCHOIaq/[RCHO],
(M/atm)
(1)
‘Department of Atmospheric Sciences, University of Arizona, Tucson, AZ. * Author to whom correspondence should be addressed. 0013-936X/88/0922-1415$01.50/0
Khyd = [RCH(OH)2I/ [RCHOlaq
(3)
In these equations [RCHO], refers to the concentration of free, unhydrated, aldehyde dissolved in the aqueous phase. The total aldehyde concentration in the aqueous phase [RCHOIt is the sum of [RCHO],, and the concentration of aldehyde present in the gem-diol form, [RCH(OH),]: [RCHOIt = [RCHO],, + [RCH(OH),] (4) Rearrangement of eq 1-3 gives a direct relationship between H*, H, and Khyd: H* = H(1 + Khyd) (5) When Kbd >> 1, i.e., when [RCH(OH),] >> [RCHO],, then H* E [RCH(OH)2]/[RCHOIg = HKhyd. The bubble-column and head-space techniques used here (see Experimental Section) yield values of H* and therefore, if Khydis known, H may be calculated. The bubble-column technique was found useful for measuring H* for all but one of the aldehydes, and two approaches were used to gather experimental data. First, for volatile species such as acetaldhyde and benzaldehyde, it was most convenient to use the method described by MacKay et al. (7). Here the loss of volatile material from the aqueous aldehyde solution is determined, and only the concentration ratio C,/Co need be known as a function of time to calculate H* (C, = solute concentration at time t and Co = initial solute concentration). The method is ill-suited to nonvolatile species however, and this led to the second approach in which the rate of accumulation of the relatively nonvolatile aldehyde in an aqueous absorber solution was determined. The apparatus used was the same in both methods, which differed only in the choice of which solution (stripped or absorber) would be analyzed. Experimental Procedure Materials. AR grade reagents were used except where indicated, and deionized water (18 M a cm) was obtained from a Millipore MilliQ system. High-purity nitrogen gas was used in the bubble-column experiments. Chloral hydrate (Mallinckrodt; USP),methylglyoxal (Sigma; 40% aqueous solution),glyoxal (Aldrich; 40% aqueous solution), hydroxyacetaldhyde (Sigma; glycolaldehyde; laboratory grade), and acetaldehyde (Aldrich;laboratory grade) were used as received. All aqueous aldehyde stock solutions were standardized iodometrically (8),and the diluted experimental solutions were allowed to equilbrate in the dark for 24 h before a run. Aldehyde solutions were discarded if they were more than 1 week old. Formaldehyde (Baker 37% AR; 10-15% methanol) was freed of methanol stabilizer by fractional distillation (6). A 2-L round-bottom flask was fitted with a 40-cm fractionating column and condenser, and 500 mL of -2 X M aqueous formaldehyde was refluxed for at least 5 h while maintaining the temperature at the column head at 65-70 O C . Analysis of the remaining aqueous formaldehyde by
0 1988 American Chemical Society
Environ. Sci. Technol., Vol. 22, No. 12, 1988
1415
GAS INLET
GAS EXIT
ABSORBENT [ABSORBER COLUMN)
AQUEOUS ALDEHYDE (STRIPPED COLUMN 1
WATER (CONDITIONING COLUMN)
Flgure 1. Schematic of the apparatus used in the bubble-column method. Each column was approxlmately 50 X 3 cm i.d.
GC (Hewlett-Packard 5880A; 2 m X 2 mm packed glass column; Supelco Carbopack B/3% SP-1500; 110 "C; FID) showed that after the reflux period the concentration of the residual methanol was
