Beyond acid rain Dear Sir: I have just finished reading ‘‘Beyond acid rain’’ by Gahey, Streit, Spall, and Hall (ESdrT June 1987, pp. 519-24). I note that the authors express Henry’s constant on page 520 as H, = [x]/P,, where Hx is stated as “mold L-atmos.” In Perry’s Chemical Engineering Handbook, 6th edition, page 3-101 states: “p/x = H or, converted to the author’s form, H, = P,/[x]. The unit of H, in Perry’s handbook is atm.mols/ mol. In chemical engineering parlance, the lower Hz becomes, the more soluble the gas is. Restated, according to Perry’s, [‘q increases as H, decreases! The authors state that “highly soluble gases, those that have Henry’s law constants > 1o00, can be critical...,” whicb leads one to believe that the p t e r H, is, the more soluble the gas is. Yet if we compare values in Table 1 of the paper, where 3-nitrophenol is listed with HI = 5 x 1CP and formic acid is listed with its H, = 6 x l@, we could incorrectly conclude that formic acid is less soluble than 3-nitrophenol. On the contrary, formic acid is totally miscible with water, while 3-nitropheno1 has a solubility of about 0.023 mols/L at 25 OC. It’s difficult to follow the logic of the article given that the authors’ treatment of Henry’s law does not agree with conventional engineering practice, nor does their Table 1 agree with known solubilities of substances. Also, H, for formaldehyde is listed as 6 x l@,while acetaldehyde has an H, = 15. The latter is infinitely soluble in water, while formaldehyde is not. Do they mean H, = 15 X 1@ for aldehyde? We are interested in the article and would appreciate clarification from the authors regarding Henry’s law and their Table 1. Robert Blevitt Robert Blevitt & Associates Santa Barbara, Calif. 93160 828
Envimn. Sci. Technol., MI. 21. NO.9, 1987
The authors reply: Dear Sir: Our use of Henry’s law and effective Henry’s law constants in units of mol L-’ am-’ is obviously inverse to that usage with which Mr. Blevitt is familiar, but is standard in the atmospheric chemistry community (I). When gas-phase partial pressure is expressed in atmospheres, the use of Henry’s law constants in these units yields liquid-phase concentrations in moles per liter. As long as one is cognizant of the units being employed and defines tbe equilibrium equation, we see no problem in converting values fromthe two inverse approaches. The actual solubility of a gas in water must take into account any aqueous reactions such as dissociation, dimerization, or protonation to determine the effective Henry’s law constants. It should be emphasized that the equilibrium relates to that of the gas in contact with atmospheric water, and although it is related to the aqueous solubility of the liquid, it is not a direct proportionality. Our value for formic acid is calculated at a pH of 4, as pointed out in the footnote to lhble 1. At this pH it is expected to be quite soluble, althougb less soluble than the 3-nitrophenol, which undergoes an acid-base reaction to increase its solubility. No gas is infinitely soluble in a solution; our value for acetaldehyde of 15 mol L-I a t i d is much smaller than that for formaldehyde, indicating it is not very soluble in water (2). Recent measurements of acetaldehyde and formaldehyde in both gas phase and in precip itation are consistent with the fact that in acidic rainfall formaldehyde is the only reasonably soluble aldehyde (35). There is a great deal of data in handbooks concerning relative solubility of gases.
It was long believed that nitrogen dioxide was highly soluble in water. Careful measurements have shown the actual solubility of N& to be very low (6). This work highlights the difficulties in determining actual solubilities, particularly for reaction gases. Thus it is not unusual to find e m r s in tabulated handbook data, and it is quite possible that the values reported in our Table 1 will be further refined as better measurements are made. The purpose of our table was to point out to the reader the wide range of p tentidy soluble species in the atmosphere and the fact that research in polluted precipitation should not ignore these possible toxins in acidic medii, and it was not intended as a definitive list of Henry’s law constants. JefFmy S. G&ey Gerald E. Streit
W.Dale SpaU John H.Hall
Los Alamos National Laboratory Los Alamos, N.M.87545
References B. 1.; pins, 1. N., Ir. I n Amspheric Chemislry: Fwdamenrnls and Experimenral Techniques: Wiley-Interscience: New York, 1986; pp. 273-75. (2) Gaffney,1. S.; Senum, G.1. Ia &-Liquid Chemisrry of Narural Waters: Newman, L., Ed.; NTIS TIC-4500,UC-Ill BNL 51757; Brookhaven National Laboratory: Upton, N.Y., 19W,pp. 5.1 105-7. (3) Lee,Y-N.; Shen, 1.; Klolz, R J. Ubrer Air Soil Pollur. 1986,30, 143-52. (4) Tanner, R. L.; Meng, 2. Environ. Sei. Technol. 1984, I S . 723-26. ( 5 ) Tanner, R. L.; Gaffney, 1. S. Paper 8416.2. Presented at the 77th Annual Air Pollution Control Association meeting, San Francisco, June 1984. (6) Lee, Y-N.; Schwartz, S. E. J . Phys. Chem. 1985.85, 8 W 8 . (1) Finlayson-Pi-,
0013-936W87M9210828$01.~0 0 1987 American Chemical Society
