during the EO-min stop interval. As observed with the packed column, the number of stop intervals has little effect on either N or R. Five significant observations can be concluded about interrupted elution gas chromatography from these studies: 1) Regardless of the particular type of column used, i e . , either packed or SCOT? it is desirable to use the simpler nonvented system because venting serves no useful purpose. 2 ) SCOT, i.e., open tubular columns, retain a higher number of theoretical plates and resolution during extended stop periods while packed columns degrade rapidly. 3) Although pressure surges may produce deleterious effects with TC detectors, these effects are minimized
with a matched two-column vented system; and 4) the number of stop-start intervals has little effect on the separation ability of a particular GC column. 5) The deleterious effects of the pressure surges resulting from the stopstart action on the number of theoretical plates and resolution are much smaller with open tubular columns than with packed columns. Received for review March 26, 1973. Accepted July 2, 1973. This research was conducted under the McDonnell Douglas Independent Research and Development Program.
Differential Kinetic Analysis of Nitric Oxide-Nitrogen Dioxide Mixtures by Reaction with Iron(l1) in Sulfolane as Solvent J. F. Coetzee,' D. R. Balya, and P. K. Chattopadhyay D e p a r t m e n t of Chemistry, University of Pittsburgh, Pittsburgh, Pa. 75273
The determination of nitrogen(I1) and nitrogen(1V) oxides in ambient air and a t emission sources is of major current concern. These gases are highly toxic and play a significant role in the formation of photochemical smog. Both are emitted in hazardous amounts from sources such as incineration, combustion of fossil fuels, and operation of internal combustion engines. In the immediate vicinity of such sources, NO, levels up to several parts per thousand may be produced. Several current methods for the determination of nitrogen(I1) and nitrogen(1V) oxides have been reviewed recently ( 1 ) . Methods fall into two broad categories (2): (a) indirect colorimetric procedures for total NO, in which nitrite or nitrate is actually determined and preoxidation of NO to NO2 (actually N204) is required. and ( b ) direct instrumental methods, such as gas chromatography ( I ) and nondispersive infrared (3) and chemiluminescence ( 4 ) spectrometry for the determination of nitric oxide. Gas chromatographic methods are not yet sensitive enough for direct measuremea a t atmospheric concentrations, although future developments in column design may minimize this limitation. It appears that chemiluminescence methods in particular hold much promise. However, a t this time no single method is the method of choice in all applications because of varying relative importance of features such as cost and complexity, accuracy, sensitivity, selectivity, speed, and the ability to determine N O and NO2 individually. We report here a differential kinetic method which has certain useful features. It is based on the formation of the iron(I1) nitrosyl ("brown-ring") complex by both NO and KO2. which react a t different rates. This reaction has 1Please address all correspondence to this author. (1) P. K . Mueller, E. L. Kothny, L. E. Pierce, T. Belsky, M. Imada. and H. Moore, Anal. Chem., 4 3 ( 5 ) . 1 R (1971). (2) D. R. Baiya, M. S.Thesis, University of Pittsburgh, 1972. (3) C. J. Halstead, G. H. Nation, and L. Turner, Analyst (London). 9 7 , 64 (1972). (4) A. Fontijn, A. J. Sabadeli, and R. J. Ronco, Anal. Chem., 42, 575 (1970).
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been used in aqueous solution for the colorimetric determination of total NO, in concentrations above 100 ppm (5, 6). The stoichiometry of the reactions was shown to be Fez+ 3Fe2'
+
NO,
+
+
2H'
NO Z Fe(N0)" f
Fe(N0)"
+
2Fe3-
+
H,O (2)
The method is relatively free of interferences, including sulfur dioxide at concentrations in air below 6000 ppm and hydrogen sulfide below 400 ppm (6); higher concentrations of the latter cause some reduction in color. We have studied reactions 1 and 2 in sulfolane containing (for practical reasons, discussed below) 1.6 vol YC of water. Sulfolane appears to offer certain advantages for air pollution studies, particularly for the analysis of "grab" samples collected a t emission sites. For example, NO and particularly NO2 have much higher solubilities in sulfolane than in water, and sulfolane is highly resistant toward oxidation and a wide variety of other forms of chemical attack. Since it also has a very low vapor pressure, the possibility exists that it also may be a useful medium for the collection of air samples by aspiration, but collection efficiencies may be unduly low.
EXPERIMENTAL Apparatus. Spectra of the iron(I1) nitrosyl complex in both water and sulfolane were taken with a Perkin-Elmer Model 124 double beam recording spectrophotometer equipped with temperature control to zt0.5 "C. Kinetic d a t a were obtained with a Durrum Instrument Company (Palo Alto, Calif.) Model D-110 stopped-flow spectrophotometer equipped with a Kel-F flow system and temperature control to f O . l "C. Reagents. Sulfolane was purified as described before (7), even though such elaborate purification probably was not essential. For example, sulfolane suitable for acid-base work has been ob(5) E. L. Kothny and P. K. Mueller. 7th Conference on Methods in Air Pollution Studies, Calif. State Department of Public Health, Jan. 1965. (6) G. Norwitz, Analyst (London), 91, 553 (1966). (7) J. F. Coetzee, J. M. Simon, and R. J. Bertozzi, Ana/. Chem., 41, 766 (1969).
ANALYTICAL CHEMISTRY, VOL. 45, N O . 13, NOVEMBER 1973
tained by simple treatment of the commercial solvent with activated alumina (8). Iron(I1) perchlorate hexahydrate was obtained from G. Frederick S m i t h Company, a n d nitric oxide a n d nitrogen dioxide lecture bottles from Matheson Gas Products. Other chemicals used were all reagent quality. Procedure. Because of explosion hazards associated with the use of anhydrous perchlorates and also t o circumvent possible problems associated with the absorption of moisture by t h e anhydrous solvent. iron(I1) perchlorate was introduced a s t h e hexahydrate and additional water was added when necessary so t h a t the medium always contained 1.6 vol 70of water (hereafter referred t o simply a s “sulfolane”). T h e same absorption spectrum was obtained for the product o f t h e reaction of iron(I1) perchlorate with NO. NOz, and N a N 0 2 in water and with N O a n d NO2 in sulfolane, Subsequently, all measurements were made a t 450 n m , the absorption maximum. A calibration curve was constructed using 10- 3 to 10-4M NaNOz in aqueous solution. Beer’s law was obeyed, with c = 450 I. c m - l mol-1. Solutions of N O a n d NO2 in sulfolane were prepared by injection through a septum into a bottle containing sulfolane under a nitrogen atmosphere, followed by stirring for 20 min. T h e concentrations of these solutions were determined, after adding aliquots t o excess water, by reaction with iron(I1) and using the nitrite calibration curve. Solutions of NO were checked for t h e presence of NO2 or nitrite with deaerated Saltzman reagent (9). Solutions of NO2 were checked for the presence of nitrite by removing NO? in a nitrogen current and then determining nitrite with Saltzman reagent. T h e concentration of NO2 found in the NO solutions was ea. 2 mol 70 of t h a t of NO ( t h e manufacturer specifies 1.5% NO2 in t h e NO gas used). and t h a t of nitrite in the NO2 solutions was less t h a n 1%. Kinetic measurements were carried out on solutions of NO, of S O 2 , a n d of mixtures of N O and NO2. a t a temperature of 30.0 f 0.5 “C, and with at least a 50-fold excess of iron(I1) perchlorate and perchloric acid in order to maintain pseudo-first-order conditions. T h e faster reactions were studied in t h e stopped-flow spectrophotometer, while the slower reactions were also followed in the recording spectrophotometer, when allowance was made for the dead time of cu. 10 sec.
RESULTS AND DISCUSSION Pseudo-first-order rate constants, h = 0.693/t1,2, for N O and KO2 are listed in Table I. For NO, the plot of h us. Fe(I1) concentration is (a) linear, showing that only one Fe(I1) species reacts (probably F e ( H ~ 0 ) 6 ~ see + ; below) and only one complex is formed, and ( b ) passes through the origin. showing that the reaction is essentially complete. Furthermore, h is virtually independent of the initial concentration of NO (1-3 x lO-4M) and also of the concentration of HC104. However, for NOz, h varies with the cube of the Fe(I1) concentration and with the square of the HC104 concentration (3-15 x 10-2M). These results indicate that in the rate-determining steps of the two reactions in sulfolane, the stoichiometries are the same as those established for the overall reactions in water (Equations 1 and 2). For NO, the second-order rate constant is 6 x 10IAkl sec-l, which is 4 powers of ten smaller than the value in water ( I O ) . Sulfolane is a very weak ligand for the majority of metal ions ( I I ) , and it is likely that in our solutions, which contained between 6 and 30 mol of water/mol of Fe(I1). the latter was always present as the fully hydrated species. We believe that dissociation of the hexahydrate in sulfolane is slow and constitutes the rate-determining step. From the above, it follows that for maximum differentiation between NO and NO2 the concentrations of both Fe(I1) and HC104 should be kept a t a minimum consistent with maintaining reasonable first-order conditions, avoiding undesirable solvolysis of the iron species present, (8) D. H . Morman and G . A. Harlow. Ana/. Chern., 39, 1869 (1967). (9) A . C. Stern. Ed., “Air Pollution,” Vol. 2, Academic Press, New York, N.Y., 1968. p80. (10) K . Kustin. I . A. Taub, and E. Weinstock, Inorg. Chern., 5 , 1097 ( 1966). (11 ) J. F . Coetzee and J. M . Simon, Anal. Chern., 44, 1129 (1972).
100
I \
0
Time Figure 1. Typical stopped-flow oscillograph for a solution in SUIfolane containing ca. 1 X 1 0 - 4 M NO, 4 X 1 0 - 4 M NOz, 0 . 0 5 M %total water F e ( C 1 0 4 ) z 6 H z 0 ,0 . 1 M HCI04, and 1 . 6 ~ 0 1 Time base values are: a, 2; b, I O : and c, 50 sec/full scale. Reaction of NO is virtually complete after 1 sec, and that of NOz after 300 sec
Table I . Dependence of Pseudo-First-Order Rate Constants for NO and NO2 on Concentration of F e ( l l ) a [Fe(ll)],M kNO, sec-’ k N o s , sec-’ kNci/kNo2 0.145 0.118 0.089 0.059 0.030 a
9.4 7.5 5.8 3.84 1.97
0.75 0.338 0.196 0.0157 0.00480
Conditions: initial concentrations of NO and NOz ca. 2 temperature = 30 “C.
12.5
22.2 30 245 41 1 X 1 0 - 4 M and
of HCi04 0.10M:
~~
Table I I . Differential Kinetic Analysis of NO-NOz Mixturesa [NOlo/[NOzlo
Total concn. M
Actualb
Method AC
Method Bd
1.00 x 10-3
0.275
8.6 X 6.3 x 10-4 3.40 10-4 1.30 x 10-4
0.420
0.263 0.408 0.486 0.365 0.173
0.259 0.398 0.478 0.360 0.170
x
0.500 0.375 0.180
nConditions: concentration of Fe(ll) 0.050M and of HCiOd 0.10M: temperature = 30 “C. Prepared by mixing separately standardized solutions. [NO10 determined by the stopped-flow method. [NO210 obtained by difference [NO2Io determined by the logarithmic extrapolation method, [ N o l o obtained by difference.
and completing the determination of NO2 in a reasonable length of time. For the analysis of mixtures, we used Fe(I1) and HC104 concentrations of 0.050 and 0.10M, respectively, giving a ratio of pseudo-first-order rate constants of ca. 270. A typical stopped-flow oscillograph is shown in Figure 1. From the relationships
ANALYTICAL CHEMISTRY, VOL. 45, NO. 13, NOVEMBER 1 9 7 3
2267
1.21
0.4
1
I
Typical results obtained in the stopped-flow determination of NO in NO-NO2 mixtures are listed in Table 11; concentrations of NO2 were obtained by difference from the limiting absorbance which was closely approached a t t = 4-5 min. The same mixtures were also analyzed by the "logarithmic extrapolation" method (12), using a recording spectrophotometer. In this case, the concentration of NO2 was determined and that of NO was obtained by difference. At a time t, after virtually all NO has reacted. it is readily shown that in the presence of a large excess of Fe(I1) -ln(A, - A , ) = h\,?t - In A , (5)
I
I
t I
0
I
1
i
I
20
40
60
80
I
100
I
120
Time, sec. Figure 2. Logarithmic extrapolation plot for the same mixture as in Figure 1 Optical path length is different from that in Figure 1
ln~[NOl,/[NOl,~= InR,,
=
hsdt -
to)
(3)
so that a plot of -In (A, - A t ) us. t should be linear with a slope = kN02 and an intercept = -In A , , where A , refers to NO2 alone. A typical plot is given in Figure 2 and the results are also entered in Table 11. Agreement with the stopped-flow results is satisfactory. The main limitation of the methods described here is that their sensitivity is only moderate (near lO-SM, depending on the sensitivity of the photometer used). In their present form, these methods appear to have potentialities for the analysis of "grab" samples collected a t NO, emission sources. Further work would be required to test sampling procedures for ambient air.
and lnIIWlo/[NO~ltl = 1nRNO2 = kNo2(t - t o )
-
-
(4)
it is found that a t t = 1 sec, R N O 20 and R N ~1.01, ~ while a t t = 2 sec, corresponding values are 400 and 1.02, respectively. Consequently, if for an equimolar mixture of NO and NO2 the absorbance a t t = 1 sec is taken as a measure of [Nolo, an error of -4% would result, and a t t = 2 sec, the error would be +2%. For [NO]o/[NO2], ratios other than unity, the errors naturally would be different.
Received for review February 22, 1973. Accepted May 4, 1973. We thank the National Science Foundation for financial support under Grant GP-16342 as well as Grant GU-3184 under the University Science Development program.
(12) H. 8. Mark and G. A. Rechnitz, "Kinetics in Analytical Chemistry," Interscience, New York, N.Y., 1968, p 80.
Fast Analytical Procedure for the Separation and Determination of the Polythionates Found in Wackenroder's Solution by High Speed Liquid Chromatography J. N. Chapman and H. R. Beard Salt Lake City Metallurgy Research Center, Bureau of Mines, U.S. Department of the Interior, Salt Lake City, Utah 847 72
Since the 1920's, there has been interest in determining polythionates in solution. The reaction of H2S with SO2 in solution yields Wackenroder's solution, which contains S Z O ~ ~S30G2-, -, S 4 0 e 2 - , and S&2but no other polythionates or S2O& ( 1 ) . Many methods have been developed for determining polythionates contained in this mixture, but there is no satisfactory way of separating and quantifying them that is not lengthy and time-consuming (2). Because the time involved in these separations is long, the stability of the polythionates from the beginning to the end of analysis is in (1) J. H. Karchmer, Ed., "The Analyticai Chemistry of Sulfur and Its Compounds," Wiley Interscience, New York, N.Y., 1970, Part I , p 238. (2) G. Nickless, Ed., "Inorganic Sulfur Chemistry." Elsevier Book Co., New York, N.Y., 1968, Chap. 6.
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ANALYTICAL CHEMISTRY, VOL. 45,
doubt. Karchmer (3) writes, "There appear to be no satisfactory methods for the determination of individual polythionates in the presence of each other." The method described in this paper will separate and quantify the above named polythionates in 15 minutes. The method depends on the adsorption and elution from activated carbon by high-speed liquid chromatography. The factors governing the separation are described.
EXPERIMENTAL Apparatus. All development a n d analytical work was performed o n t h e Waters ALC-100 A n a l y t i c a l Liquid Chromato(3) J. H. Karchmer, Ed., "The Analytical Chemistry of Sulfur and Its Compounds," Wiley Interscience, New York, N.Y.. 1970, Part I , p 241.
NO. 13, NOVEMBER 1973
