Anal. Chem. 1992, 84, 3004-3006
9004
Development of a Gradient Ion-Pair Chromatographic Procedure for the Simultaneous Detection of Nitrogen-Sulfur Oxides Produced during the Reaction of SO, and NO,, Species in Aqueous Solution Mar& Geissler and Rudi van Eldik' Institute for Inorganic Chemistry, University of WittenlHerdecke, Stockumer Strasse 10,5810 Witten, Germany
An -air chromatographic procedure urlng an acetonltrlk gradient was devfor the dmuitaneow detection of a series of nltrogen-eUnur oxides produced during the reactkn of nitrogen and sulfur oxldes. These oxides Include nitrllotrlsulfonate (NTS),hrldodku#onate (IDS), hydroxylamlnedl-(-),-wm(NHw, hydroxylmlneaunonate (HAMS), and aminowlfonate (SA). Detection Umits that could be reached were In the ppb range. The standard Ions NO2-, Na-, and SO4*- were ako Included inthe~ratkn~olt~potentlalpr~lnmbrtu of nitrogen and sulfur oxides. The developed method allows anlons with charges from -3 to -1 to be separated on the same column in one analytical run. The appllcaMllty of thb technlque k demonstrated by a systematlc study of the hydrolyde of NTS.
Table I. Sulfur-Nitrouen - ComDounds
HON(SO&" hydroxylaminedisulfate HADS HONH(SO3)- hydroxylaminesulfonate HAMS N(SOs)a" nitrilotrisulfonate NTS HN(S03)z2imidodisulfonate IDS H2N(SO3)aminosulfonate SA -NO(NO)SOs- N-nitrosohydroxylamie-N-sulfonate NHAS
ee
\
I The autoxidation reactions of sulfur and nitrogen oxides play an important role in atmospheric chemistry in terms of acid rain formation.' Such reactions are generally accepted to be catalyzed by transition-metal ions and ~omplexes.~-~ It is for this reason that we are particularly interested in the mechanism of the metal-catalyzed autoxidation of SO, and NO, species in aqueous In particular the interaction of SO, and NO, species in the presence of metal ions and complexes and the role of such reactions in atmospheric oxidation processes are of current interest. Analysis of the final oxidation products (Sod2-,S208, NzO, etc.) do not reveal much information on possible short-lived intermediates, generally referred to as N-S oxides (Table I).' The aim of the present work was therefore to develop an analytical procedure for the detection of species such as N(SOS)~~-, HN(S03)z2-, HzNSOs-, and other N-S oxides (Scheme I).7 For this purpose we chose ion-pair chromatography as an analytical technique7p8and have developed a very sensitive (ppb range) method for the analysis of such species. This method now enables us to perform systematic kinetic studies on the stability and redox behavior of various N-S oxides. It was for instance reportedQthat nitrilotrisulfonate (NTS) undergoes rapid hydrolysis in water even at room ~
~
(1) Chang, S.-G.; Tmsi, R.; Novakov, T. Atmos. Enuiron. 1981, 15, 1287. (2) Weschler, C. J.; Mandlich, M. L.; Graedel, T. E. J. Geophys. Res. 1986, 91, 5189, 5205. (3)Jacob, D. J.; Gottlieb, E. W.; Prather, M. J. J. Geophys. Res. 1989, 94,12975. (4) DellertRitter, M.;vanEldik,R.J. Chem. Soc.,DaltonTram. 1992, 1037,1045. (5) Zang, V.; Kotoweki, M.; van Eldik, R. Znorg. Chem. 1988,27,3279. (6) Zang, V.; van Eldik, R. Znorg. Chem. 1990,29,4462. (7) Littlejohn, D.; Chaug, S.-G. Anal. Chem. 1986, 58, 158. (8)Dionex Corp. Application report 11/87/2. (9) Sisler, H.; Audrieth, L. F. J. Am. Chem. SOC.1938, 60, 1947. 0003-2700/92/0364-3004$03.00/0
NnzOH
+
IDS
I
HSOa-
NH4'
++I H+
HSOI-
temperature. We have therefore investigated such hydrolysis reactions as a function of pH and include our first results in this article. The developed analytical procedure w i l l enable us in future investigations to investigate the effect of metal ions and complexes on such processes and their possible role in atmospheric oxidation processes.
EXPERIMENTAL SECTION Materials. All reagenta used were of analyticalreagent grade. Eluents for the ion chromatographic work were prepared from sodium carbonate, tetrabutylammonium hydroxide (TBAOH), acetonitrile, and doubly deionized water. The potassium salta of NTS, IDS,and SA were prepared by a modified version of the procedure reported in the literature.B Reagent grade SA was also obtained from Merck. The potassium salta of HADS,HAMS, and NHAS were prepared as reported in the literature.lOJ1 Instrumentation. A Sykam ion chromatograph, equipped with a S loo0 pump and inert PEEK pump head, a Rheodyne injector, a gradient mixer and a S 3110 conductivity detector, (10)Rollefson, G. K.;Oldershaw, C. F. J. J . Am. Chem. SOC.1932,54, 977. (11) Nyholm, S.; Rannitt, L. Znorg. Synth. 1957, 5, 117. (12) Chang, S.-G.;Littlejohn,D.;Lm,N. H. ACSSymp. Ser. 1982,188, 127.
0 la92 American Chemkal Soclety
ANALYTICAL CHEMISTRY, VOL. 64, NO. 23, DECEMBER 1, 1992
SO05
NHAS /'
8.74
I
1
8.27
1'1 I
40L
14.32
I
I
I
21.78
~
l
l
28.51
32.21
c / (mol I-' x10-4)
Flguro 2. Cailbratlon curves for HAMS, SA, HADS, NHAS, IDS, and NTS. /
i
4
-
8
1
1
2
1
1
8
1
2
[
1
1
2
4
1
2
8
1
3
2
1
3
8
~
Figure1. Typical chromatogramfor the analysisof S-N oxldes obtained with the CH&N gradient procedwe described in the text.
was used for the analysis. Control of the pump and gradient mixer, as well as data acquisition and handling, was performed with the aid of an on-line computer system and an AXXI-Chrom 727 software package. A RP column with a poly(styro1divinylbenzene)copolymer (10pm) was used for the separations. A cation-exchange column was used as the suppressor column. The eluent consisted of 1.5 mM NazCOs and 2.5 mM TBAOH. Acetonitrile was added in a step gradient,starting with an initial concentration of 17% and increasing linearly up to 22 % (t = 0 to t = 13 min), to 32% (t = 13 to t = 19 min) and kept at this level from t = 19 to t = 28 min. The acetonitrile concentration was subsequently reduced linearly to 17% (from t = 28 to t = 31 min) and held at this level for ca. 15min in order to equilibrate the column before the next analysis. The flow rate was kept constant at 1 mL/min, and 20 p L of the alkaline samples were injected for each analysis. Measurements. Test solutionswere thermostated in a water bath at 25.0 i 0.2 OC. Samples were withdrawn as a function of time and quenched by running them into an alkaline (pH = 13) solution. In some experimentsbuffers were used to stabilizethe pH of the test solutions. These were selected in such a way as to limit their effect on the analytical procedure. Standards of the compounds to be analyzed were run before the analysis of samples with unknown concentrations.
RESULTS AND DISCUSSION Analytical Procaduw. Figure 1showsthe chromatogram of a test sample obtained by the procedure described above. NTS, IDS, NHAS, HADS, S042-, NOa-, and NO2- were separated. The first peak in the chromatogram resulta from the presence of both HAMS and SA. Due to their similar structure and the same charge, it is not possible to separate them with this method. The high retention time for IDS can be related to the alkalinity of the eluent (pH = ll), since in this pH range IDS is in the form N ( s 0 ~ ) 2 ~i.e. - , a -3 charged species. Its retention time is therefore similar to the retention time for the other -3 charged species, NTS. The linearity of the signal response was tested and confirmed for the concentration range from 5 X 10-6to 5 X M for all the compounds studied. A qualitative analysis was performed by using an integration software in which deviations from linearity due to peak broadening for higher concentrations, when the concentrations were calculated by measuring the peak heights as reported in the literature,' could be avoided. Typical examples of the various calibration curves are given in Figure 2. Detection limits calculated from the calibration curves using the 3a definition are given in Table 11. Kinetic Investigations. When NTS is dissolved in water at 25 "C,rapid hydrolysis accompanied by the release of H+ ions occurs. We found with our method that the first produda are IDS and Sod2-, followed by a second, much slower step
Table 11. Detection Limits (dtl), Response Factors (rf), and Linear Dynamic Ranges (ldr) of the 8-N Compounds Obtained with the Chromatographic Procedure Described in the Text compd dtl/mol L-l rf/(area X 10') ldr/mol L-1 2.006 5 X 10-6-6 X 10-8 NTS 2.9 X lo4 2.834 5 X 10-6-6 X 10-9 IDS 2.5 X 6.607 5 X 10-6-5 X 1W8 HADS 1.1 x 10+ NHAS 2.3 X 10-6 4.245 5 X 10-6-5 X lo-*
SA HAMS
3.750 1.998
1.6 X 6.3 X 106
in which IDS hydrolyzes to SA and
N(S0,);HN(S0,);-
+ H,O
-
5 X 10-6-5X 10-* 5 X 10-6-5 X 10-8 sod2-.
H+
+ H,O
HN(S0,);-
H+
H,N(S03)-
+ HSOL + HSO,
With an initial NTS concentration of 2 X M, the concentration maximum of IDS was found at t = 8min; after 4.5h the concentration of IDS waa about half this maximum value. The pH of the solution dropped to 2.9 within 40 min of the reaction. The production of H+ ions will cause the hydrolysis rate to increase significantly, since such reactions are in general acid catalyzed. Similar observations have been reported in the literature,B although no exact reaction times were quoted, only that the first hydrolysis step waa very rapid. We found NTS to be absolutely stable in alkaline solutions (pH > 12)and observed only a very slow hydrolysis reaction in weakly alkaline solutions (pH = 8.5). Under such conditions the first traces of IDS were only observed after 3 days. For this reason all samples were quenched in alkaline (pH = 13)solution before performing the analytical detection. IDS and SA were found to be stable in neutral solution, such that the subsequent hydrolysis must be due to the significant decrease in pH, i.e. increase in acidity, which results in acid-catalyzedhydrolysis. In all subsequent work buffers were introduced to control the pH of the test solution and so prevent the catalytic effect of the produced H+ ions. The pH range investigated was from 2.0 to 5.8. Above pH = 5.8 the reaction was too slow to be investigated on a reasonable time scale. A phosphate buffer (0.05 M)was used to control the pH at 5.8. The resulta in Figure 3 demonstrate a relatively slow hydrolysis of NTS accompanied by the formation of IDS. The first-order rate constant for the hydrolysis of NTS at pH = 5.8 was calculated from the data in Figure 3 to be 5 X 10-68-1. Similar experiments were performed in a phthalate buffer (0.05 M) in the pH range 2.0-5.3. The observed rate constants indicate an acid-catalyzed hydrolysis process (see
3006
ANALYTICAL CHEMISTRY, VOL. 64, NO. 23, DECEMBER 1, 1992
20000 16000 -
12000; \
x
4
80001
Y
..
-.
OP
,
0
,
800
400
NTS
40001
t
1
or -4000'
1200
t/min
Figure 3. Hydrolysisof NTS and formation of IDS as a functlon of tlme at pH = 5.8. Experimental conditions: [NTS] = 1.0 X M; [phosphate buffer] = 0.05 M; temp = 25.0 OC; [NTS] (D); [IDS] (0).
7 1
2 0-
.
O O - ? ,
1
1
1
'
l
,
I
,
1
0 0 0 0 0 0 0 2 0 0 0 4 0006 0 0 0 8 0010
CH+l / mol I-' Figure 4. kobsas a functlon of [H+] for the hydrolysis of NTS at 25.0 OC.
Figure 4) in which kob reaches a limiting value at high [H+l. For the hydrolysis of NTS the following mechanism is proposed: NTS
K + H+ e
'
1000 ' 2000 ' 3000 ' 4000 ' 5000 ' 6000 l/CH+I / (mol I-' ~ 1 0 ' ) versus [H+]-' for the data reported in Figwe
4.
In addition, the value of K suggests a pK, value for NTS*H+ of 2.6. The second hydrolysis step leading to the formation of SA is much slower. At pH = 2.2 no evidence for the formation of SA was found after 45 h. The ratio of IDSISA dropped to 0.5 at t = 6 days. An important result of this investigation concerns the catalytic property of the H+ ions produced during the hydrolysis of NTS, IDS,and SA. Under atmospherically relevant conditions the concentration of these species may be significantly lower than that used in this study, such that the buffer capacity of cloud and fog water may be adequate to control the pH and suppress the acid-catalyzed reactions. When this is not the case, hydrolysis of these compounds will result in a rapid formation of the products. With the analytical procedure now available it will also be possible to investigate the effect of trace metal impurities, metal ions, and complexes on the formation and hydrolysis reactions of thenitrogen-sulfur oxides discussed in this report.
ACKNOWLEDGMENT We gratefully acknowledge financial support from the Bundesministerium fiir Forachung und Technologie and the Commission of the European Community (STEP Program).
NTS.H+
lk +
Flgwe 5. plot of
0
IDS
= (kK[H+])-'
RECEIVED for review April 8, 1992. Accepted September
From the double reciprocal plot in Figure 5 the values k and 409 M-l, and K were calculated as 7.8 X respectively. It follows that the protonated NTS.H+ species hydrolyzes ca. 2 orders of magnitude faster than NTS itself.
1, 1992. %&try NO. N(OH)(SOsH)2, 14450-91-0; NH(OH)SOaH, 13698-45-3; N(SO&I)s,19082-431); NH(SO&I)2,13766-30-8;NHaS03H,5329-146;N(OH)(NO)SOsH,26640-26-6; NO2,14797-650; SO,, 1480879-8; NOS,14797-55-8;HzO, 7732-1&5.
for which kobs = kK[H+]/(l K[H+])and
+ k-l.
