Environ. Sci. Technol. 1984, 18, 890-890
CORRESPONDENCE Comment on “Mass Transfer of Volatlle Organic Contaminants from Aqueous Solution to the Atmosphere during Surface Aeratlon”
Literature Cited Roberts,P. V.; Diindliker, P. G. Environ. Sci. Technol. 1983,
SIR This is to point out an error in the estimated value of the Henry constant for dichlorodifluoromethane used by Roberts and Dhdliker (I). The value of H for CClzF2 at 293 K used in the referenced paper, estimated from vapor pressure and solubility data as suggested by Mackay and Wolkoff (2),was 1.5 atm.m3.mol-l; this value was estimated from vapor pressure and solubility data reported by the manufacturer (3, 4). Professor Peter Liss (private communication) has suggested to us that the value of the Henry constant for CCI2F2should be on the order of 0.25 atmm3.mol-l on the basis of work in his laboratory with CC1,F; the value of the Henry constant for CC&Fat 293 K is 0.267 atmm3.mol-1 (5). Upon further examination, we conclude that the estimation procedure used by Roberts and Dlindliker (1)was based on an incorrect interpretation of the solubility data. For compounds that are gases at ambient conditions, it is customary to measure the solubility of the compound in equilibrium with the pure gas, rather than with the pure liquid; this apparently was the case with the solubility data in ref 4. Therefore, it is incorrect to estimate the Henry constant by dividing the vapor pressure by the solubility. If the solubility is measured in equilibrium with pure gas at 1atm, then the Henry constant should be estimated as H = l/Cs (atm.m3.mol), where Cs is the solubility in units of mol.m3. Using the latter approach, with the solubility data of Park et al. (6),we estimate the Henry constant for CC12F2at 293 K to be approximately 0.225 atmm3.mol-l, slightly smaller than the value reported by Hunter-Smith et al. (5) for CC13F. Hence, the value of H used for CC12F2by Roberts and Diindliker (1) overestimated the true value by a factor of approximately 6. Although this error is distressingly large, it would not have substantially affected the interpretation or conclusions of Roberts and Dlindliker’s work ( I ) because the revised value remains large enough to assure liquid-phase control of the transfer rate. By use of the revised value of H for CC12F2, the ratio of the liquid-to-gas-phase resistance (eq 5 of ref 1) is calculated to be 234, and the fraction of the total resistance accounted for by the liquid phase (Table IV of ref 1) is 0.996, which is close enough to unity to justify neglecting the gas-phase resistance. We regret this error and warn other investigators of this potential pitfall in estimating Henry constants for highly volatile compounds using solubility data. Registry No. CCl,F2, 75-71-8.
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17,484-489. Mackay, D.; Wolkoff, A. W. Environ. Sci. Technol. 1973, 7,611-614. E. I. duPont de Nemours and Co. “Thermodynamic Properties of Freon-12 Refrigerant”. Wilmington, DE, 1971, Report T-12. E. I. duPont de Nemours and Co. “Solubility Relationships between Fluorocarbons and Water”. Wilmington, DE, 1971, Report B-43. Hunter-Smith, R. J.; Balls, P. W.; Liss, P. S. Tellus 1983, 35B, 170-176. Park, T.; Rettich, T. R.; Battino, R.; Peterson, D.; Wilhelm, E. J . Chem. Eng. Data 1982,27, 324-326.
Paul V. Roberts Department of Civil Engineering Stanford University Stanford, Callfornia 94305
Correction for “Field Compatible Calibration Procedure for Peroxyacetyl Nitrate” SIR In our recent article (I)we misquoted a conclusion from the paper of Lonneman et al. (2)to the effect that the author’s procedure was not highly accurate and was most appropriate for approximate values of PAN. A closer reading of the Lonneman’s paper shows that the quote actually refers to use of their procedure in the absence of benzaldehyde. When benzaldehyde is present to control the OH radical chemistry resulting from the PAN and NO reaction, the Lonneman et al. procedure is a highly accurate means of performing PAN calibrations. This is clearly demonstrated in their paper. We apologize for our initial misreading of their article and hope that this letter will clear up any confusion caused by our error. Literature Cited (1) Holdren, M. W.; Spicer, C. W. Environ. Sci. Technol. 1984, 18, 113. (2) Lonneman, W. A.; Bufalini, J. J.; Namie, G. R. Environ. Sci. Technol. 1982, 16, 655.
Michael W. Hoidren Battelle Columbus Laboratories Columbus, Ohio 43201
0013-936X/84/0918-0894$01.50/0
0 1984 American Chemical Society
