740
Anal. Chem. 1985, 57,740-743
of F n between 9:OO-1O:OO and 12:OO-13:OO ignoring NO, removal rate in the neighborhood of downtown Tokyo in summer. This is considered to be comparable to nitrate formation rates evaluated in the plume chemistry by Meagher et al. (41, Richars et al. (5),and Forrest et al. (6). In this case, the volatility of "*NO3 (21-23) and the reaction of sulfuric acid with nitrate (24),which have been discussed as artifact nitrate terms, might result in the decrease of particulate nitrate measurement on a Teflon filter and the increase of nitric acid measurement on a polyamide filter. However, shorter time interval sampling would suppress the artifact effects to the greater extent because it would decrease the contact time of sampled nitrate with ambient sulfuric acid and increase the possibility of keeping the equilibrium with respect to ",NO3 through the sampling time. The 1-h interval sampling in this study is thought to help avoid artifact production although 2 h to 1 days inverval samplings have been made so far in the low volume filer sampling (25-28). Registry No. NO3-, 14797-55-8.
LITERATURE CITED (1) Spicer, C. W. Atmos. Environ. 1977, 7 7 , 1089-1097. (2) Orel, A. E.; Seinfeld, J. H. Environ. Sci. Techno/. 1977, 7 7 , 1000- 1007. (3) Appel, B. E.; Kothny, E. L.: Hoffer, E. M.; Hidy, G. M.; Wesolowski, J. J. Envlron. Sci. Techno/. 1978, 72,418-425. (4) Meagher, J. F.; Stockburger, L., 111; Bonanno, R. J.; Bailey, E. M.; Luria, M. Afmos. Envlron. 1981, 75,749-762. (5) Richards, L. W.; Andersen, J. A,; Blumenthal, D. L.; Brandt, A. A. Atmos. Environ. 1981, 75,2111-2134. (6) Forrest, J.; Garber, R.; Newman, L. Atmos. Environ. 1981, 75, 2273-2282. (7) Spicer, C. W.; Howes, J. E., Jr.; Bishop, T. A.; Arnold, L. H.; Stevens, R. K. Atmos. Environ. 1982, 16, 1487-1500.
(8) Small, H.; Stevens, T. S.:Bauman, W. C. Anal. Chem. 1975, 4 7 , 1801-1809. (9) Mulik, J.; Puckett, D.; Williams, D.; Sawickl, W. E. Anal. Lett. 1978, 9 , 653-663. (10) Morris, A. W.; Riiry, J. P. Anal. Chim. Acta 1963, 29, 272-283. (11) Sandberg, J. S.;Levaggi, D. A.; Demandel, R. E.; Slu, W. J . Air folluf. Control Assoc. 1976, 26,559-564. (12) Moskowitz, A. H. "Particle Size Distribution of Nitrate Aerosols in the Los Angeles Air Basin"; EPA/600/3-77/053, 1977. (13) Spicer, C. W.; Schumaker, P. M.; Kouyoumijian, J. A.; Joseph, D. W. "Sampling Methodology for Atmospheric Particulate Nitrates"; EPA/ 60012-781367, 1978. (14) Cox, R. D. Anal. Chem. 1980, 52,332-335. (15) Bayliss, N. S.;Watts, D. W. Aust. J . Chem. 1963, 76,943-946. (16) Ingold, C. K.; Millen, D. J.; Poole, H. G. J . Chem. SOC. 1959, 2576-2587. (17) Bayliss, N. S.;Watts, D. W. Ausf. J . Chem. 1958, 9 , 319-329. (18) Spicer, C. W. Reaction of NOx in Smog Chambers and Urban Atmospheres, Formation and Fate of Atmospheric Nitrates, Workshop Proceedings; Barnes, H. M., Ed.; EPA-600/9-81-025, 1981. (19) Spicer, C. W.; Joseph, D. W.; Ward, G. F. "Studies of NOx reactions and O3 Transport in Southern California-Fall, 1976"; PB 83-200 360, 1983. (20) Grosjean, D.; Friedlander, S. K. J . Air folluf. Control Assoc. 1975, 25, 1038-1044. (21) Kadowaki, S.Atmos. Environ. 1977, 77, 671-675. (22) Stelson. A. W.; Friedlander, S. K.; Seinfeld, J. H. Atmos. Envlron. 1879, 73,369-371. (23) Yoshizuml, K.; Okita, T. J . Air follut. Control Assoc. 1983, 33, 224-226. (24) Harker, A. 6.; Richards, L. W.; Clark, W. E. Atmos. Environ. 1977, 77, 87-91. (25) Okita, T.; Morimoto, S.: Izawa, M. Atmos. Environ. 1976, 10, 1085-1089. (26) Forrest, J.; Tanner, R. L.; Spandau, D.; D'Ottavio, T.; Newman, L. Atmos. Environ. 1980, 74, 137-144. (27) Grennfelt, P. Afmos. Environ. 1980, 1 4 , 311-316. (28) Cadle, S.H.; Countess, R. J . ; Kelly, N. A. Atmos. Environ. 1982, 76, 2501-2506.
RECEIVED for review September 26,1984. Accepted November 19, 1984.
Estimation of Acidity in Rainfall by Electrical Conductivity Robert A. Stairs* and Jaleh Semmler' Department of Chemistry, Trent Uniuersity, Peterborough, Ontario, Canada K9J 7B8
Measurement of electrical conductivity of a sample of rain, or other natural freshwater, and of the same sample after passage through a bed of ion exchange resin in H+ form, allows the calculation of acidity (or alkalinity) and salinity. Results for 64 samples of rain, compared wlth those based on titration, are as follows: for acidity C(K)= -0.0070 -k 1.0809C(t), f0.0135, from -0.05 (alkaline) to 0.160 mequiv L-'; for salinity, C(K)= 0.0017 4- 0.9996C(t), f0.0191, from zero to 0.330 mequlv L-'. The method is suitable for automation.
The method described here was conceived in the context of an acid precipitation study. It is, however, adaptable with caution to the proximate analysis of freshwaters generally. The principle is simple. All ions contribute to the conductivity of a solution in direct proportion to their concentrations in equivalents per unit volume and their equivalent conductances. A solution containing a single ionic solute, such as hydrochloric acid, requires only a single conductance measurement for analysis. Two solutes, for instance HCl plus Present address: D e p a r t m e n t of Chemistry, U n i v e r s i t y of Waterloo, Waterloo, Ontario, Canada N2L 3G1.
NaC1, require two measurements. The second measurement is performed after the solution has been allowed to pass through a column of a strongly acidic, cation exchange resin, which will replace the sodium ions with hydrogen ions. The two conductances should conform to eq la,b where K~ K~ = hac,+ h,C, (la) K~
=
hac,4- hac,
Ob)
and x2 are the measured conductances, A, and A, the equivalent conductances of HC1 and NaCl, and C, and C, the corresponding concentrations. The values of A, and A, should be appropriate to the conditions (temperature and concentration). If the solutions are sufficiently dilute, as rainwater may be expected to be, the limiting values Ao may be used without serious error. They are obtained from the single-ion values ioi, representative examples of which are listed in Table I ( I ) . If correction is necessary, a brief iteration may be used, with A a suitable function of the ionic strength. Rainwater usually contains appreciable amounts of an assortment of ions: Ca2+,Mg2+, K+, Na+, NO3-, C1-, and H+ or HC03- (not both), as well as COz. The value of Xo for H+ is uniquely large. Those for the other cations cluster around 0.062 (&O.Oll) S cmm2mequiv-l, and the anions (except OH- and HC03-) are similarly clustered, around 0.076 (f0.005). This leads to the expectation that treatment of
0003-2700/85/0357-0740$01.50/0 0 1985 American Chemical Society
",+,
ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
741
Table I. Ions Present in “Representative Rain”:O Concentrations and Limiting Equivalent Conductances Ci, mequiv L-’
ion
H+
0.067 f 0.005 0.004 f 0.001 0.001 f 0.001 0.023 f 0.002 0.010 f 0.001 0.003 f 0.001
Nat K+ NH4+ Ca2+ Mg2+
”mean cationwc
Xoi,b S cm-2mequiv-I
hoi,* S ern+ mequiv-’
Ci, mequiv L-’
ion
OH-
0.3499 0.0502 0.0735 0.0734 0.0595 0.0531
HCOc
0.1976 0.0800 0.0714 0.0764 0.0445
“mean anion’lC
0.0774
so2-
0.067 f 0.005 0.030 f 0.003 0.005 f 0.001
Nos-
c1-
0.0663 f 0.0030
* 0.0062
OEastern Ontario, 1981. From the Acid Precipitation in Ontario Survey (3). bConventionalvalues ( I ) divided by 1000. Temperature 25 “C. c“Mean cation” and “mean anion” values of Xo are means weighted according to the relative concentrations of the individual ions. rainwater in the manner described above for NaC1-HC1 solutions should yield conductances that could be used to obtain estimates of the acidity C, and the salinity C, (defined as the total concentration of all neutral salts, in mequiv L-l) through equations (la,b) with suitable mean values of A, and A,. The value of A, is calculated from Xo for H+, combined with a “mean anion” value obtained by weighting the individual Xo values according to their expected relative concentrations. A, is similarly obtained from the “mean cation” and “mean anion” values. The large value of Xo for H+ ensures that A, and A, will differ by much more than their uncertainties. Rainfall in the Peterborough area is usually slightly acidic (Table I). Occasionally, however, it is alkaline. The method described above would yield, in these cases, a false indication of acidity. The required equations are then different. Assume a salinity C, as before, and an alkalinity C b due to bicarbonate. Treatment with the acid resin will convert the salt to acid as before, and the bicarbonate will be converted to COz, which does not contribute to the conductance. The equations are now K1 = A,c, + d b C b (24 K2
= Aac,
(2b)
where the new constant Ab is the sum of Xo for HC03- and the “mean cation” value, as above. In the laboratory, the choice of which pair of equations to use is easily made on the basis of a single pH measurement, but the intention is to use this method as the basis of an automatic device. It should record rain acidity (or alkalinity) and salinity, and either transmit the data in real time to a readout device or store the data for later retrieval. The choice may be made on the basis of a third conductance, measured after passage of a separate sample over a strongly basic anion exchange resin in bicarbonate form. In an acidic sample, acid will be removed as COz and salt converted to bicarbonate. In an alkaline sample, both salts and bicarbonate appear as bicarbonate. The resulting conductances will be (acidic case) (basic case)
K3 K3
=
= hbCs
Ab(Cs
+ cb)
(34
(3b)
where A b is as defined above. Discrimination is then accomplished through the function defined by (4) From eq 1b and 3a, appropriate to the acidic case, it is seen that 4=-
C, 5.6) gave no straight line portion in the graph of antilog (4- pH) vs. volume. The value of u1 was taken to be zero. Carbon dioxide was measured by up The first end point in the second titration, us, was taken as representing salts of strong acids, and u4 - uz - u3 as the carbon dioxide arising from bicarbonate originally present, i.e., the bicarbonate alkalinity. (Rain samples, being exposed to atmospheric COz,can have no carbonate or hydroxide alkalinity.) Multiplication of these volumes by CbIU,, where Cb is the concentration of the titrant NaOH in millimoles per liter and u, the sample volume, yields acidity (or alkalinity) and salinity in milliequivalents per liter and carbon dioxide concentration in millimoles per liter (because it was only titrated to bicarbonate at the second end point). It is not possible to express the acidity or salinity in moles, owing to the mixed valences of the ionic species present. The conductance measurements were carried out by using small flow-through cells with bright platinum electrodes, using a bridge assembled from a Leeds and Northrup students’ potentiometer and a decade resistance box, with a square-waveoscillator at ca. 1 kHz as source and an oscilloscope as null-point detector. The measurements were done at 25.0 “C. The experimental arrangement is shown schematically in Figure 1. The rain sample, about 200 mL, was poured into the reservoir R. From there, portions of it passed slowly (at about 5 mL min-’) through the two paths: (a) through the cell C1, the acid resin column A, the cell Czand to waste, and (b) through the basic resin column B, the cell C3,and to waste. The conductances K ~ K, Z , and K~ were measured repeatedly during the slow passage of solution, and the results averaged. Calibration was effected by turning the two taps T1 and Tz, so that the resin columns were bypassed, and passing 0.0100 M KC1 solution through all the cells. Chemicals used were analytical reagent grade. Resins used were Dowex-50W(Hf) (strongly acidic) and Rexyn 201 (OH) (strongly basic). Water was purified by reverse osmosis, and polished by means of the Gelman “Water I”. RESULTS Rainfall Analyses. During the period of 28 May 1982 to 8 September 1983,64 samples of rain have been analyzed using both methods. The pH titration method using Gran’s plot, or a version of it, is becoming standard in acid precipitation analysis, so the
a
b S=
range
acidityb
salinity
-0.0070 f 0.0025 1.0809 f 0.0380 0.0135 -0.0500 to 0.1600
0.0017 & 0.0040 0.9996 & 0.0361 0.0191 0.0050 to 0.3300
”Fit of eq 8 for acidity and for salinity for 64 rain samples. All concentrations in meauiv L-l. *Negativeaciditv = alkalinitv (bicarbonate). cStandard error of-estimate, H = [ci(Ci(i()C ; ( K ) ) 2 / ( n- 2)1’/2.
results from this method were taken as “true”. Portions of the same samples were also analyzed as described above, i.e., by conductance before and after treatment with strongly acidic resin. Minor rearrangement of equations (1) yields
= A , ( c , c,) K 2 - K 1 = (A, - h,)C, (7) Accordingly K~ was plotted against C, + C, (from the titration results) and K~ - K~ vs. C,. The slopes yielded estimates of the Kz
effective values Aa and AB,which were compared with values calculated for “representative rain”. The composition of this “rain” is shown in Table I, and represents analyses during 1981 of rain from four sites surrounding Peterborough within 200 km, roughly averaged (3). Table I11 shows the predicted and empirical values of A, and A,, with the predicted value of Ab. Too few examples of alkaline rain were encountered, and too few determinations of K~ have been made, to allow a reliable estimate of Ab to be made from the data. The same data were fitted to eq 8, where C ( K )is the conC(K) = U + bC(t) (8) centration of acid or salt calculated from the conductance data and C ( t ) is the corresponding concentration obtained by titration. Table IV shows the values of a and b obtained from the fit of 64 points. The “predicted” values of A,, Ab, and A, were used. The standard error of estimate (defined in a note in the table) and the standard errors of the constants are also shown, with the ranges of acidity (alkalinity) and salinity covered by the measurements. No obvious bias was noted within the precision of the measurements. The precision, as measured by the standard error of estimate, was not particularly good, being in the case of each parameter about 15% of a typical value. As the basis for the design of a low-cost, automatic device, it should nevertheless provide a means both of monitoring rainfall in one locality for acidity and of creating a network of data-collecting stations that need not be visited after every rainfall. The present need is not for highly precise local data, but for a finer array of points a t which time-resolved data are obtained. With such an array, it should be possible to make more reliable analyses of the growth and movement of storms that carry unusual acid loads.
ACKNOWLEDGMENT The assistance of Luna Ho, Jeffrey Hudson, Ligia Sanchez and Lorie Windrem is acknowledged. The manuscript was typed by Dorothy Sharpe.
743
Anal. Chem. 1985, 57,743-740
(3) Galloway, J. N.; Crosby, B. J., Jr.; Likens, G. E. Limnol. Oceanogr. 1979, 24 (6), 1161-1165.
Registry No. Water, 7732-18-5. LITERATURE CITED
RECEIVED for review September 4, 1984. Accepted November 1%1984. This research was supported by the Air Resources Branch of the Ontario Ministry of the Environment.
(1) Parsons, R. "Handbook of Electrochemical Constants"; Butterworth: London, and Academic Press: New York, 1959; p 85. (2) "Acid Precipitation in Ontario Study", ARB-11-62 ARSP; Air Resources Branch, Ontario Ministry of the Environment: Toronto, 1982.
Hydration of Nitric Acid and Its Collection in the Atmosphere by Diffusion Denuders Delbert J. Eatough,* Vernon F. White, and Lee D. Hansen Thermochemical Institute and Department of Chemistry, Brigham Young University, Provo, Utah 84602 Norman L. Eatough Department of Chemistry, California State Polytechnical University, S a n Luis Obispo, California 93407 Elizabeth C. Ellis Southern California Edison, Rosemead, California 91770
The collectlon and determination of gas-phase nltrlc acld by use of denuder tubes are dependent either on the dlrect measurement of nltric acid collected In a denuder or on the anaiysls of total Inorganic nitrate with and without prlor removal of gas-phase nitrate by a denuder. I n both cases, the amount of nitric acld predicted to be collected by the denuder Is dependent on the diffuslon coefflcient used to describe the collection of nitric acid by the denuder. I t has been assumed in past studies that the specles present In the atmosphere and collected by the denuder Is HNO,(g). Both field and laboratory studies with tungstlc acid and nylon denuders show that the species present and collected is actually HNO,.xH,O( g), where the value of x Is dependent on the atmospherlc water concentratlon. Thus, the diffusion coefflclent is also dependent on the atmospheric water concentratlon. Gas-phase ammonia Is also shown to exist as the monohydrate at atmospheric water concentrations. I n additlon, the collection efflclency of tungstlc acid dlffuslon denuders for hydrated nltrlc acid Is affected by the atmospherlc water concentratlon. Both the hydration of nltrlc acid and the apparent hydration of the denuder surface must be taken Into account If a tungstlc acid denuder Is used to collect gas phase nitric acid.
The determination of gas-phase nitric acid in the atmosphere is important to the understanding of the contribution of NO, emissions to rain or fog chemistry and to visibility. It is known from the results of both laboratory and ambient studies that the collection of gas-phase nitric acid is complicated by shifts in the equilibria of volatile nitrate compounds as well as by the release of HN03(g) from aerosols acidified during sampling. Shifts in the particulate nitrateHN03(g) equilibrium can occur if either the concentrations of gas-phase nitric acid, ammonia, and/or water are altered or the temperature or pressure of the airstream is changed during sampling. The equilibrium between HN03(g) and NH4N03(s)can result in ambient concentrations of HN03(g) of several parts per billion (1). Removal of either NH3(g) or HN03(g) by a denuder results in volatilization of both NH3(g) 0003-2700/85/0357-0743$01.50/0
and HN03(g) from NH4NO3(s) until equilibrium is again achieved. In addition, the collection of aerosols on a filter can result in particle-particle interactions which lead to the release of HN03 from aerosols during sampling by reactions such as NaNO,(s)
+ H,SO,(l)
= NaHS04(s) + HNOdg)
(1)
Two techniques have been used to monitor gas-phase nitric acid in the atmosphere on a real-time basis. These are the specific detection of HN03(g) using Fourier transform infrared spectrometry (2, 3) and the measurement of HN03(g) by difference as NO(g) using chemiluminescence after reduction of N02(g)and HN03(g)with and without removal of HN03(g) (2,4). The chemiluminescent technique requires removal of particulate matter from the airstream prior to the measurement of HN03(g) and the IR method requires heating and/or reduction of the pressure of the sampled airstream containing both gases and particles in the measurement chamber. Thus, each of these previously deployed real-time measurement schemes has the problem of artifact formation of HN03(g) during the measurement. There are two general approaches which have been used to sample gas-phase nitric acid in filter sampling trains. Both methods allow for the collection of total inorganic gaseous and aerosol nitrate. However, both techniques have sampling artifact problems caused by the separation of gaseous and aerosol nitrate during sampling. The first approach measures gas-phase nitric acid by first removing particulate matter on a filter which has been shown not to collect HN03(g)and then collecting nitric acid gas on a suitable stack of scrubbing filters. Filters such as Teflon membrane (5) or acid washed quartz (6) do not collect gasphase nitric acid or nitrogen oxides. Nylon (7, 8) or NaCl impregnated filters (9) are effective for the collection of nitric acid without interference from the absorption of nitrogen oxides. However, Forrest et al. (6, 9) have shown that loss of HN03(g)from particles can occur due to reactions similar to that in eq 1 and, in addition, they have demonstrated that volatilization of NH4N03(s)can occur during high volume sampling. Both of these effects are also present during low volume collection of nitrate (10,11). Similar loss of HN03(g) from NH4N03(s)has been demonstrated in laboratory ex0 1965 American Chemical Soclety
