Environ. Sei. Technol. 1992, 26 1259- 1260 I
contaminated Newark Bay sediment samples. Registry NO. 2,3,7,8-TCDD, 1746-01-6; 2,3,7,8-TCDF, 51207-31-9.
Literature Cited Bopp, R. F.; Simpson, H.J.; Olsen, C. R.; Trier, R. M.; Kostyk, N. Environ. Sci. Technol. 1982, 16, 666-676. Olsen, C. R.; Simpson, H.J.; Bopp, R. F.; Williams, S.C.; Peng, T.-H.; Deck, B. L. J. Sediment. Petrol. 1978, 48, 401-418. Simpson, H.J.; Olsen, C. R.; Williams, S.C.; Trier, R. M. Science 1976,194, 179-183. Tong, H.Y.; Monson, S.J.; Gross, M. L.; Bopp, R. F.; Simpson, H.J.; Deck, B. L.; M o w , F. C. Chemosphere 1990, 20, 1497-1502.
Richard F. Bopp
Department of Earth and Environmental Sciences Rensselaer Polytechnic Institute Troy, New York 12180-3590
Comment on “Formic and Acetic Acids in Coastal North Carolina Rainwater” SIR: The article by Avery et al. (I) presents interesting data on the concentrations of formic and acetic acids in precipitation samples. The paper contains calculations showing the contributions of the two acids to the acidity of the samples. These calculations are erroneous, and the firstsentence in the abstract is not supported by the data. As pointed out by Stumm and Morgan (2),parameters such as acidity must be defined at a reference level. Acidity measures the concentration of all species containing protons in excess minus the concentration of all species containing protons in deficiency of the proton reference level. The quantities mineral acidity and total acidity may be developed by applying this definition to different reference levels. For a solution containing a mixture of polyprotic acids, the generalized expressions for these quantities are given in eqs 1and 2, where [H-Acyl n 2-1
[H-Acyl = [H+] -
C ( Z- x)[H,A,Y-”] - [OH-] (1)
i=lr=O
[HA,-Acyl = [H+] +
5 ?x[H,A?+’]
i=lx=l
- [OH-] (2)
is the mineral (or strong acid) acidity, [HA,-Acyl is the total acidity, [H+] is the hydrogen ion concentration, [H,A,V-x]is the concentration of weak base i coordinated with x protons with charge y - x , [OH-] is the hydroxide ion concentration, [HJ,Y+”] is weak base i coordinated with 2 protons and charge y x , n is the number of weak acids
+
in solution, z is the number of protons that weak acid i will release in a strong base titration, y is the charge of the fully deprotonated species and x is the number of titratable protons coordinated with the ion. Of course, acidity can also be defined at any intermediate reference level. The authors did not state what type of acidity they were referring to in their paper. It appears the second of two unnumbered equations on page 1877 was used for the calculation of the contribution of formate to acidity % contribution of formate = [formate]/[H+] X 100 (3) and a similar equation was used to calculate the acetate contribution. These calculations require the assumption that mineral acidity is equal to [H+]. Due to the presence of weak acids in the samples, this assumption is probably not valid. Furthermore, eq 1 shows that formate and acetate anions decrease the level of strong acid acidity. Calculations of strong acid and total acidity for the data found in Table I1 of Avery et al. ( I ) are presented here in Table I. The precipitation samples were assumed to be in equilibrium with atmospheric C02,yielding a constant concentration of 10 pequiv/L for carbonic acid. The pK values of 3.75,4.76, and 6.35 were used for formic, acetic, and carbonic acids. The contributions of carbonate as well as the effects of activity coefficients were ignored. Applying eqs 1 and 2 to the Avery et al. samples results in eqs 4 and 5. The three columns under the “acidity” [H-Acy]
[H+]- [CHOO-] - [CH,COO-] - [HCOS-] 2[C032-] - [OH-] (4)
[HAt-Acy] = [H+] + [CHOOH] + [CH,COOH] + 2[H,C03] + [HCOB-] - [OH-] (5) heading indicate acidity calculated by first assuming it to be equal to [H+], using eq 4, and eq 5. The final two columns show the contribution of formic and acetic acids to total acidity. The calculations show first that the assumption that acidity is equal to [H+]is poor for the solutions that were studied. [H+]overestimates mineral acidity and underestimates total acidity. Second, the contributions of formic and acetic acids are significantly less than the 23% suggested by Avery et al. The highest contributions were for the G1 season (2.5 and 5.2%) while the overall average contributions were 2.0 and 4.2%. These calculations are based upon the simplifying assumptions noted above, and it is likely that there were other components to acid-base chemistry of the precipitation samples. The impact of other components on the mineral and total acidity cannot be predicted. The statement that “ammonium ... is fully protonated and hence does not contribute to the hydrogen ion concentration” is also in error. If the species enters the
Table 1. Calculation of Acidity Functions and Contributions of Formic and Acetic Acids“
acidity season
pH
formic acid Ct [HA] [A-]
NG1 NG2 NG3 G1 G2 G3 all
4.72 4.67 4.62 4.41 4.51 4.38 4.49
3.5 3.5 1.5 9.0 7.3 12.8 7.4
a
0.3 0.4 0.2 1.6 1.1
2.4 1.1
3.2 3.1 1.3 7.4 6.2 10.4 6.3
acetic acid Ct [HA] [A-] 2.1 2.0 1.8 4.8 3.5 4.2 3.6
1.1 1.1 1.0
3.3 2.2 3.0 2.3
1.0
0.9 0.8 1.5 1.3 1.2
1.3
~
Ct 10 10
10 10
10 10 10
carbonic acid [H,CO] [HC03] 0.3 9.8 0.2 9.8 0.2 9.8 0.1 9.9 0.2 9.9 0.1 9.9 0.2 9.9
[H+] 19.1 21.4 24.0 38.9 30.9 41.7 32.4
(eq 4) 14.6 17.1 21.7 29.9 23.3 30.0 24.7
(eq 5) 40.3 42.7 45.0 63.7 54.1 67.0 55.7
total acidity contrib, 5% formic acetic 0.8 2.7 0.9 2.6 0.4 2.3 2.5 5.2 2.0 4.1 3.6 4.4 2.0 4.2
All concentrations in microequivalents per liter.
~ ~ ~ 0 3 6 X / 9 2 / 0 9 2 6 - 1 2 5 9 $ 0 3 . 0 0 /0 0 1992 American Chemical Society
Environ. Sci. Technol., Vol. 26,No. 6, 1992
1259
Environ. Sci. Technol. i992,26,1260-1260
atmosphere via the volatilization of NH,, it then undergoes reaction of eq 6. A t the ambient pH levels, this reaction NH3 + HzO
-t
NH4+ + OH-
(6)
may be assumed to proceed completely to the right. Hence every micromolar NH4+corresponds to a reduction of 1 r M protons. In fact, the ammonia would appear as [NH,] in eq 1decreasing mineral acidity and appear as [",+I in eq 2 increasing total acidity and further decreasing the calculated contributions of formic and acetic acids. These authors did not provide enough detail to evaluate the calculation of the background pH to ascertain its accuracy. I t is not clear why the calculation should be an iterative procedure. If the total concentrations of all the strong and weak acid species are known, there should be one equation with an explicit solution. It is likely that formic and acetic acids have significant impact on the pH as well as on the overall acid-base chemistry of atmospheric deposition. Indeed, the presence of these acids results in overestimation of the role of strong (sulfuric and nitric) acids when Gran function analysis of titration data is used to separate strong and weak acids (3). The exact role of these acids can only be quantified after all of the acid-base components have been identified. Much more research is required. Registry No. HC02H, 64-18-6; H3CC02H,64-19-7.
Registry No. HC02H, 64-18-6; HSCC02H, 64-19-7.
Literature Cited (1) Avery, G. B., Jr.; Wiley, J. D.; Wilson, C. A. Environ. Sci. Technol. 1991,25,1875-1880. (2) Stumm, W.;Morgan, J. J. Aquatic Chemistry-An Zntro-
duction Emphasizing Chemical Equilibria i n Natural Waters; John Wiley & Sons: New York, 1981; p 163. (3) Barnard, T.E.; Bisogni, J. J., Jr. Water R e f . 1985, 19, 393-399.
Thomas E. Barnard
Environmental Sciences & Engineering Colorado School of Mines Golden, Colorado 80401-1887 SIR: The term acidity can be used in several ways, which unfortunately has apparently caused some confusion in the interpretation of the rainwater data presented and discussed in Avery et al. (1). The intended use of this word was to indicate the amount or concentration of acid present in a solution (2). This is equivalent to hydrogen ion concentration and is often called acidity or free acidity in rainwater literature (3-7). The term free acidity is not defined in aquatic chemistry reference books (8-10) and so it was not used in the recent paper by Avery et al. (1). The calculation used to compute the percent contribution of organic acids to free acidity appears in the methods section of this paper and was intended to define the meaning of this term for this work. The term acidity is also commonly used to indicate extent of dissociation, as in acidity constants (8, 10). The terms acidity and total acidity can be used to indicate base neutralization capacity (BNC), which is the definition inferred by Barnard and correctly defined in his letter (8, 10). The BNC can be determined by titration; however, for a dilute solution like rainwater this is not a trivial task ( 3 , I I ) . There is no mention of BNC titrations in the Avery et al. (1)paper because none were performed. BNC calculations were also not a part of this study; as Barnards calculations indicate, formic and acetic acids are not a large part of the BNC of rainwater in this area. We agree with Barnard that, by definition, formic and acetic acids do not contribute to mineral (strong) acidity. 1260
The statement about the contribution of ammonium to the hydrogen ion concentration in our work refers to the rainwater at the time of collection and analysis, not tt the process by which the ammonia or ammonium became in. corporated into the rainwater at some prior time. The volume-weighted average concentration of ammonium in rainwater collected over a slightly shorter time period t,hm that reported in the recent paper was 9.2 PM (12). AI the pH of this rainwater (4.47),this would occur as ammonium and would contribute more to the BNC than would the formic and acetic acids under study. The numbers reported in Barnard for the contribution of these organic acids to the total acidity are therefore too high. In tact, Barnard's calculations indicate that formate contributes more to the alkalinity (acid-neutralizing capacity or ANC) of the rain than formic acid does to the BNC. The greatest uncertainty in the calculation of the background pH for rainwater is probably the percentage of strong acids (sulfuricand nitric) that comes from natural versus anthropogenic sources in this geographical region. This estimate was taken from a US. EPA document (13). The iterative calculation performed converged upcin a unique solution that is consistent with estimates of this value for other regions of the world (14, 15). We regret any confusion that has resulted from the use of poorly defined terms and appreciate the additional analysis of our data by Barnard.
Environ. Sci. Technol., Vol. 26, No. 6, 1992
Literature Cited Avery, G. B., Jr.; Willey, J. D.; Wilson, C. A. Environ Sci. Technol. 1991,25, 1875-1880. The V a n Nostrand Chemist's Dictionary; D. Van Nostrmd Inc.: Princeton, NJ, 1953; p 8. Keene, W. C.; Galloway, J. N.; Holden, J. D., Jr. J. Geoptiys. Res. C 1983,138, 5122-5130. Keene, W. C.; Galloway, J. N. Atmos. Enoiron. 1984, 18, 2491-2497. Likens, G. E.; Keene, W. C.; Miller, J. M.; Galloway, J N. J . Geophys. Res. D 1987, 92, 13299-13314. Andreae, M.0.;Talbot, R. W.; Andreae, T. W.; Harriss, R. C. J. Geophys. Res. D. 1988, 93, 1616-1624. Keene, W. C.; Galloway, J. N. Tellus 1988,40B, 322-334. Stumm, W.; Morgan, J. J. Aquatic Chemistry, 2nd td.; Wiley Interscience: New York, 1981; Chapters 3, 4 6. Morel, F. M. M. Principles of Aquatic Chemistry; WilryInterscience: New York, 1983. Pankow, J. F.Aquatic Chemistry Concepts; Lewis Pttblishers: Chelsea, MI, 1991; Part 11. Barnard, T. E.; Bisogni, J. J., Jr. Water Res. 1985, 9, 393-399. Willey, J. D.;Cahoon, L. B. Mar. Chem. 1991,34, 63-"5. Robinson, E. In T h e Acidic Deposition Phenomenon arid Its Effects;Altshuller, A. P., Linthurst, R. A,, Eds.; EP 4600/8-83-016AF; US.Environmental Protection Ageni y, Washington, DC, 1984; Vol. 1, Chapter 2, Section 2. Charlson, R. J.; Rodhe, H. Nature (London) 1982, 255, 683-685. Galloway, J. N.; Likens, G. E.; Keene, W. C.; Miller, J. Pf. J. Geophys. Res. 1982, 87, 8771-8786.
Joan D. Wllley"
Department of Chemistry and Marine Science Program University of North Carolina at Wilmington Wilmington, North Carolina 28403-3297 G. Brooks Avery, Jr.
Curriculum in Marine Science University of North Carolina at Chapel Hill Chapel Hill, North Carolina 27599-3315
0013-936X/92/0926-1260$03.00/0
0 1992 American Chemical Society
