CORRESPONDENCE
SIR: The article “A New Method of Generation of Gases a t Parts per Million Levels for Preparation of Standard Gases” (ES&T 1980 1 4 , 4 1 3 ) describes a simple method for generation of test gases a t low levels. I t should be pointed out that, for three out of the five systems described (NO2, SOz, and HzS), the solute is subject to oxidative degradation and it is unlikely that a constant concentration of the desired gas will be maintained in the effluent stream over a long period of time. This is, of course, what Figure 7 shows when NazS03 is used as a solute: the SO2 concentration decays rapidly with time. In this context, the authors consider the greater oxidizability of S032-,as opposed to HS03-, to be responsible. Yet, the legend associated with the figure states that both the NaHS03 and Na2S03 solutions were maintained a t p H 5 . If the buffer employed had had sufficient capacity to maintain a constant pH, it would have mattered little whether Na2S03 or NaHS03 was put in solution insofar as the relative distribution of S032-/HS03- is concerned. The S032-/HS03- ratio is uniquely governed by the final p H of the solution and not by the form of the salt used; S0a2- is reportedly protonated with a rate constant of 1011L mol-1 s-1 ( I ) . There are other problems associated with NO, generation by this technique. Nitrous acid is sufficiently stable in the vapor phase to constitute an appreciable fraction of what is being measured as NO,. In our studies with vapor- and aerosol-phase nitrous acid, we encounter a very similar problem; HNOz is measured totally as NO2 by the Saltzman procedure and it decomposes to NO and NO2 in the chemiluminescence-type analyzers. The reliability of the data presented in Figure 4 is, therefore, open to question. The method is, however, attractive for situations where chemical instability, either for the solute in the solution or for the gas in the effluent stream, is not a problem. If fresh solution is pumped in with a peristaltic pump a t an adequate rate, constant solution composition and thus constant gas composition in the effluent stream can be assured. Oxidative degradation may, of course, be eliminated by using N2 instead of air.
hand, NaHS03 and NazS for SO3 and H2S are not stable, so that it is a problem that the decrease of solute due to oxidative degradation occurs in the long-time gas generation over 1day. We tried to resolve this problem and now we have succeeded in stabilizing the concentration of solute for 1 week by the addition of 1mM EDTA to the solution. Trace metal ions in the solution which work as catalysts of the oxidation are masked by EDTA, so that the solute is kept stable for a long time, and it is possible to generate SO2 and H2S of constant concentrations. It is also effective for the stability of solute to lower the temperature of the solution, although the temperature was kept a t 25 “C in this experiment. The Saltzman and chemiluminescence methods have generally been used for the measurement of NO,. However, if nitrous acid is sufficiently stable in the vapor phase, the problem described by Dr. Dasqupta could occur in both methods. In this experiment, we did not count nitrous acid vapor in the generated gas because of its unstability, so that we do not know how much nitrous acid vapor exists in the gas prepared by our technique. We should try to check its existence. It is a good suggestion to cycle fresh solution in the gas generator or to use Nz instead of air in order to generate gas for a long period by preventing the oxidation of solute. We have already tried to test these points. We attached an extra supply tank containing the gas-generating solution to the gas generator, and it was successful in generating continuously NO, a t 0.86 ppm for 1week within a variation of a few percent. Moreover, we also observed that the use of N2 instead of air prevented the oxidation of NO generated to NO2.
Yoshikazu Hashimoto Shigeru Tanaka Department of Applied Chemistry Keio University Hiyoshi 3-14-1, Kohoku-Ku Yokohama 223, Japan
Literature Cited (1) Erickson, R. E.; Yates, L. M. Atmos. Enuiron. 1977,11,813.
Purnendu K. Dasgupta California Primate Research Center University of California Davis, California 95616
SIR: As for the question of Figure 7 that shows the relationship between SO2 concentration generated and the stability of solute in the gas-generating solution, we agree with Dr. Dasgupta’s opinion. I t is true that the chemical form of sulfite in the solution depends on the p H of the solution as he pointed out. In this case, NaHS03 and NazS03 solutions were maintained a t p H 5 , so that most of the sulfite in both solutions could not be S0s2- but HS03-. Therefore, it cannot be theoretically considered that the difference of stability of solute in the solutions is caused by the form of salt used. However, we really observed that SO2 concentration from N a ~ S 0 3solution decreased gradually with time while that from NaHS03 solution was stable, as shown in Figure 7. So our conclusion is that we had better use NaHS03 for SO2 generation, although the reason is not well understood yet. In this method, the stability of solute in the solution is a very important factor for generating the gas of stable concentration for a long period. The generating solutions such as KCN, and NH4C1for HCN and NH3 are generally stable. On the other 600
Environmental Science & Technology
SIR: A recent article ( I ) describes computer simulations of photochemical production of ozone in the vicinity of Norfolk, VA. Data for comparison are taken from the southeastern Virginia urban plume study of the summer of 1977, which was instituted to determine whether the urban plume of this area would be a suitable test site for the evaluation of remote sensors. Data for one day are cited, a day on which there were “substantial ozone concentrations downwind of Norfolk in the afternoon, presumably due to photochemical aging of the urban plume.” Having no information about initial concentrations (Le., 8 a.m. concentrations) of nonmethane hydrocarbons, the authors proceeded to use their model to estimate what concentration would be consistent with the observed afternoon ozone, assuming the model and all other input assumptions were correct. They note that the resulting initial NMHC concentration, 250-350 ppbC, is “within the expected range” and therefore suggests that the kinetic mechanism used in the model is consistent with reasonable simulations. I want to point out one aspect of this simulation that the authors have not seen fit to mention. If their simulation is indeed a correct representation of the atmospheric events of August 4, 1977, then it follows that the ozone observed downwind of Norfolk in the afternoon was definitely not due to photochemical aging of the urban plume of the Norfolk area. I t could have been caused only by reactions of contam0013-936X/81/0915-0600$01,25/0 @ 1981 American Chemical Society
inants which were already in the air before it arrived in the Norfolk area. Two lines of evidence lead to this conclusion. First, the source strengths cited by the authors are so small that they could generate only a very minor contribution to the ozone precursor concentration in the simulated air parcel. For NMHC, the authors estimate this contribution at 2.22 pg m-3 h-l. Since 1ppbC is equivalent to 0.67 pg m-3, this contribution amounts to 3.3 ppbC h-l or, over a 3-h period, a total of 10 ppbC. This is less than 5% of the initial NMHC concentration demanded by the model! To get the required NMHC input from the Norfolk area, the simulated air parcel would have to stagnate there for 60 h or more before starting on its trip at 8 a.m. on August 4. The second line of evidence is in a comparison of the simulation results shown in Figures 4 and 5 of the article. These figures show profiles of ozone concentration against time for simulated air parcels at two different wind speeds and several levels of initial NMHC concentration. The slower speed takes the parcel over the Norfolk urban area 2.5 h later than the faster speed. Examination shows that the same level of initial concentration gives nearly identical ozone profiles, regardless of the wind speed. A delay in the production of ozone would be expected in results for the slower parcel, if the effect of the Norfolk emissions were appreciable. Since no ozone delay can be detected, it follows that the effect of the Norfolk plume, according to the model, is negligible.
Literature Cited (1) Wakelyn, N. T.; Gregory, G. L. Enuiron. Sci. Technol. 1980,14, 1006-8. Lowell G. Wayne Pacific Environmental Services, Inc. 1930 Fourteenth Street Santa Monica, California 90404
SIR: Having read the articles “Adsorption on Carbon: Theoretical Considerations” ( I ) and “Adsorption on Carbon: Solvent Effects on Adsorption” ( 2 ) ,and having found much to disagree with, I question why ES&T chose to give so much prominence to the research and the (at times controversial) views of a relative newcomer to the field. Professor Belfort’s published contributions to date on activated carbon appear to consist of one published paper ( 3 )and a second contribution ( 4 ) which has been presented before an ACS symposium (chaired by Professor Belfort) but which does not yet have a citation indicating acceptance by a reputable journal. One would think it appropriate to let this latter contribution go through customary review procedures, unless of course one could establish some compelling reason for immediate publicity. The cited material from this manuscript ( 4 ) does not disclose any such compelling reason; indeed, I find some features that should give pause to a knowledgeable reviewer, for example: (1) The theory at best does not aim a t estimating a broad-ranged adsorption isotherm, but only a single “adsorption capacity”. Its objective is therefore quite modest. Alternatively put, it does not take into account the energetic heterogeneity of the carbon surface. (2) The manuscript refers to linear correlation coefficients ( r > 0.918) as “excellent”, when in fact one can readily obtain such coefficients either with poor data or with strongly nonlinear dependence. For example, a linear fit of 11 equally spaced points ( x = 0, 1, . . . , 10, y = (10 - X2)1/2)on the quadrant of a circle of radius 10 gives r = 0.88; omitting the 0013-936X/81/0915-0601$01.25/0
@ 1981 American Chemical Society
last point gives r = 0.93, and omitting the last two points gives r = 0.94. Thus, the points in Figure 5 do not look as though they belong on a straight line at all, and the points on the blue line of Figure 7 show considerable scatter even for a homologous series, in which it takes some imagination to find properties that do not have some linear trend. (3) The choice of molar units to express adsorbability (Figure 6) is inappropriate for a supposed monotonic linear relationship, because the mass of adsorbate per unit mass of carbon in a homologous series approaches a limiting value with increasing molecular weight (or some variable that is correlated with it), and the number of moles must therefore go through a maximum and then decrease. We can see the approach to a limit in points 4, 5, and 6 in the plot for alcohols in Figure 6, and the attainment of a maximum with points 12, 13, and 14 in the plot for ketones, for which the extension of the line beyond the experimental points is clearly misleading. Moreover, the data from Table I of Giusti et al. ( 5 ) (from which the capacities appear to have been taken) could never have shown an “adsorbability” (in mass units) greater than 0.200 g/g of carbon because this was all that they added to their flasks. This does not, of course, mean that the hydrocarbon surface area is not necessarily a good property to look at, but its superiority as a correlating variable remains to be demonstrated. Consider now the first article ( I ) . Of a number of misstatements therein on the Polanyi model, I here pick as an example the suggestion that our work on competitive adsorption of binary and ternary solids (6, 7) was “empirical”. The fact is that, once the individual isotherms were determined, the theoretical curves for competitive adsorption of binary and ternary solids (including the prediction of the existence and the location of slope discontinuities) were calculated with no adjustable parameters at all. These very nonempirical findings, incidentally, are well outside the scope of alternative models, including that of Professor Belfort. There seems to be no need to belabor further the point that your coverage was, a t the very least, premature. As for the Polanyi model, I can state that a significant number of users (not all of whom have published their results, and not all of whom are former research associates) appear to be very happy with it. Interested readers are invited to look at our published work (8) and find out why, pending my submission of a more detailed exposition.
Literature Cited (1) Miller, S. Enuiron. Scz. Technol 1980,14,910. (2) Miller, S. Enuzron Sci. Technol. 1980,14, 1037. (3) Belfort. G. Enuzron Sci. Technol 1979.13.939. (4) Belfort, G., cited in ref 2. ( 5 ) Giusti, D. M.: R. A. Conwav: Lawson. C. T. J . Water Pollut. Control Fed. 1974,46, 947. (6) Rosene, M. R.; Manes, M. J . Phys Chem 1976,80,953. (7) Rosene, M. R.; Manes, M. J Phys Chem. 1977,81,1646. (8) Manes, M. In “Activated Carbon Adsorption of Organics from the Aqueous Phase”; Suffet, I. H., McGuire, M. J., Eds.; Ann Arbor Science Publishers: Ann Arbor, MI, 1980; Vol. I, Chapter 2. This article gives citations to earlier work. I
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Milton Manes Chemistry Department Kent State University Kent, Ohio 44242
SIR: In answer to Professor Manes’ correspondence regarding the two recent interviews ( I , 2 ) , several comments seem appropriate. (1) A seminal objective of our research is to present a comprehensive formalism of aqueous-phase adsorption including fundamental formulations of all known interactions between solute, solvent, and sorbent. Another objective is to use this formalism to predict a priori a ranking order of adVolume 15, Number 5,
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