812
Anal. Chem. 1983, 55, 612-615
Table V. DMS Concentrations in Some Natural Waters ng of S
(DMS) sample L-' Elbe River Mouth (8 Oct 1980) 31.4 Dry Tortugas, Gulf of Mexico (8 April 1980) 28.0 North Sea (10 Oct 1980,1050 GMT) 58.0 51"36.9' N, 2'16.5' E Dover Straits'(10 Oct 1980, 1600 GMT) 76.6 51'01.2' N. 1'22.2' E English Channel ( 1 0 Oct 1980,1800 GMT) 116.4 50'49.4' N, 0'58.6' E Open Ocean Surface Seawater (N. Atlantic, 62.5 21 Oct 1980,1000 GMT) 20'20.7' N, 24O45.9' W Seawater Station M56-9 ( 2 3 Oct 1980, 1416 GMT) 11'28.8' N, 28O03.5' W 5 m depth 100.1 35 m 76.1 70 m 38.3 100 m 21.3 130 m 15.5 200 m 5.7 300 m 8.8 500 m 8.1 1000 m 5.4 1500 m 2.5 2000 m 3.8 3000 m 3.8 4000 m 0.8 Open Ocean Surface Seawater (S. Atlantic, 37.1 28 Oct 1980,0100 GMT) 5"50.5'S, 3 3 53.7' w rain, N. Atlantic (18 Oct 1980) 30'58.5 10.4 N, 20'24.4' W analysis has proven impossible. In all cases samples have been analyzed within 36 h. CONCLUSIONS DMS has been determined in severai types of natural waters, using the method described in this paper. Some resulta
are presented in Table V. DMS is present even in samples taken from the deep oceans although there is a rapid decline in concentration below the euphotic zone. High levels of DMS tend to be found in productive areas of the ocean because of the algal production mechanism for DMS. Open Ocean surface values are lower and show a narrow range of values for DMS and productivity. ACKNOWLEDGMENT
We thank M. Dancy for her help with the preparation of the manuscript and graphics. J. M. Ammons is acknowledged for valuable discussions and for the cross calibration of the permeation tube standards. We thank W. Seiler for permission to participate in the cruise of the research vessel Meteor, during which some of the data presented in this paper were collected. Registry No. DMS, 75-18-3, LITERATURE CITED (1) Nguyen, E. C.; Gaudry, A.; Bonsang, E.; Lambert, G. Nature (London) 1978, 275, 637. 12) . . Lovelock, J. E.: Mams. R. J.: Rasmussen. R. A. Nature London) 1972, 237, 452. (3) Maroulis, P. J.; Bandy, A. R. Sclence 1978, 196, 647. (4) Braman, R. S.; Ammons, J. M.; Bricker, J. L. Anal. Chem. 1978, 50, 992. (5) Rasmussen, R. A. Tellus 1974, 26, 254. (6) Challenger, F. Adv. Enzymol. 1951, 12, 429. (7) Sze, N. D.; KO, M. K. Atmos. Mvlron. 1980, 14, 1223. (8) Graedel, T. E. Geophys. Res. Left. 1979, 6 , 329. (9) Cullls, C. F.; Hlrschler, M. M. Atmos. Environ. 1980, 74, 1263. (IO) Andreae, M. 0. Anal. Chem. 1980, 52, 150. (11) Ramstad, T.; Nestrick, T. J.; Peters, T. L. Am, Lab. (Falrfleld, Conn .) 1981, 13, 65.
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RECEIVED for review April 2, 1981. Resubmitted October 12, 1982. Accepted January 14,1983. This work was supported in part by the National Science Foundation Grant No. ATM 80-17574 and by a grant from The Florida State University Foundation.
Simultaneous Determination of Hydrogen Peroxide, Peroxymonosulfuric Acid, and Peroxydisulfuric Acid by Thermometric Titrimetry Phlllppe E. A. Boudevllle Laboratoire Chimie Anaiytique, U.E.R. du Maicament, Facult6 de M6decine et Pharmacle, 2, avenue du Professeur L6on Bernard, 35043 Rennes Cedex, France
An original method of determination by thermometric titrimetry (lT)of the amount of H2S05, H202, and H2S,08 in a single sample within the range of 100-1000 pmoi with Ce4+ and Fez+ is described. This method uses the dlfferences between reaction rates and enthalpy varlations of reactions of these three peroxides with Fez+. For H2S05,H,O,, and H2S208, respectively, the values of the rate constants are fast, 187, and 8.5 L moi-is-i, and those of the enthalpy variatlons are -366, -428, and -328 kJ mol-' at 20.0 OC and with an ionic strength = 0.62 mol L-l. These values have been determined by lT.
Peroxydisulfate ion is often used as an oxidizer or radical 0003-2700/83/0355-0612$01.50/0
generator which is a good polymerization initiator. However, peroxydisulfuric acid is not stable in sulfuric media. It decomposes to give peroxymonosulfuric acid or hydrogen peroxide according tQ experimental conditions. It was interesting to rapidly determine the amount of these three peroxides and numerous papers (1-9) have already been published concerning different titration methods. The spectrophotometric methods (6, 7) allow the determination of concentrations 2 X M with good accuracy (2 to 3%). However they require three different samples: in the first sample, H202is titrated by Ce4+(A = 320 nm) or Ti4+(A = 410 nm). In the second one, H&08 is titrated by Fe2+(A = 304 nm) in the presence of As203and N2 In the third one, the total amount of oxidant is titrated by Fez+ (A = 304 nm). The amount of H2S06is obtained by the difference. In the course of our kinetic study 0 1983 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL 1983 1
2
613
3
Flgure 2. Theoretlcal thermograms (curves 1, 2, and 3): (a) base period, no titrant added: (b) reaction period, titrant added; (c)excess titrant perlod, tltrant added; (d) posttitration period, no titrant added. V TITRANT
lml)
Figure 1. Thermo ram for the titration of a mixture of H2SOS,H202, and H2S20, by Fe2 . EP1, EP2, and EP3 are the respective theoretical
9
equivalent points.
of the reaction of iron(I1) peroxydisulfate by thermometric titration (IO),we have observed that H2SO5, HzO2, and H2SZO8 react with Fe2+with different rates and with different values of AH. The rate determining steps for these three reactions are
HS05- + Fez+ H2OZ+ Fez+ HS&-
fast
HS062-.+ Fe3+
2OH. + Fe3+ + OH-
+ Fe2+-% SO4-. + Fe3+ + HS04-
The values of the rate constants are, respectively, fast, k l = 187 f 5 L mol-l s-l, and kz = 8.5 f 0.5 L mol-l s-l and those of the enthalpy variations for the complete chemical reactions are -366, -428, and -328 kJ mor1 at 200 "C and with an ionic strength = 0.62 mol L-l (these values have been determined by TT). We have investigated the possibility of using these differences to effect a sequential titration of these three products on a single sample. Indeed we can see on the thermogram (Figure 1) of the titration of a mixture of these three peroxides by a solution of Fez+,the three slopes corresponding respectively to HzSO5, HzOz,and H2SzOB.While the first break in the curve gives a good result for the titration of HzS05, this is not the case for the two others: a greater amount of peroxide than foreseen is obtained due to the fact that the reactions are slow. Therefore the direct titration by Fez+ was not considered suitable. Thermometric titrimetry (TT) is a method based on recording the temperature of the reaction mixture in the course of the titration of a reactant A by a titrating agent B (11-14). B is added to A at a regular rate and an excess of B is always added to A (which explains the presence of excess Ce4+in the come of the titration of H202in a mixture) so as to determine the equivalent point (EP)either by a sharp variation of the slope or by the intersection of extrapolations of the slopes before and after the more steeply curved portion. Three kinds of curves are observed (Figure 2): Curve 1 corresponds to a fast and complete reaction; a sharp break represents the EP. Curve 2 corresponds to a fast but uncomplete reaction; we have a curvature and the EP is the point where the "extrapolation of the slopes" cross each other, depending on the K value (15,16).Curve 3 corresponds to a complete but slow reaction; we have a double curvature at the start and at the end and the point where the "extrapolation of the slopes" cross each other is higher than the equivalence (10, 17). EXPERIMENTAL SECTION Instrumentation. The apparatus was described in ref 18. We have deliberately used the simplest possible conditions so as to facilitaterepetitive titrations and eventually their automatization. Thus water was simply passed through an ion exchange resin
without distillation from permanganate and the thermometric titration system was not thermostated (except for measurements of rate constants and AH). All chemicals (Merck p.a., Prolabo R.P., or Carlo Erba RPE) were used as received. Reagents and Standard Solutions. Solutions were prepared by dissolving the reactants in 0.5 M HzSO4 so as to obtain a constant ionic strength ( M = 0.62 mol L-l) and to avoid precipitation of ferri hydroxide in the course of the titration. Peroxydisulfuric acid was prepared immediately before use (to avoid its hydrolysis to HzSO5 which occurs even in 0.5 M HzSO4 (19)) from ammonium peroxydisulfate. Peroxymonosulfuric acid was prepared by heating a solution of HzS208in 2 M HzS04 for 4 h at 70 O C . Under these conditions some HzOzwas formed (19) and about 2% of the initial HzSzO8 remained. The solution was diluted four times and kept away from light to avoid photolysis of HzSO5 to HzOz(20). HzOzwas titrated spectrophotometrically in the presence of excess Ti4+according to the method of Csanyi (7); it was generally 2 to 3%. H2Sz08was titrated according to our method. The total oxidizing capacity of each solution was determined for HzOzsolutions by direct titration with KMn04 and for H#Os and H#z08 solutions by reaction with excess Fez+which was back-titrated with KMn04. RESULTS A N D DISCUSSION Titration of the Products Alone. The titration of H202 at concentrations SW4M in an HzS04(0.5 or 5 M) medium was effected by Ce4+ (fast and complete reaction) with a precision of 1% (Table I). For concentrations lower than lo4 M, Fez+was added first and the excess Fe2+was then titrated by Ce4+. The titration of H2S05was effected by Fe2+in 0.5 M HzSO4 in the presence of Ce3+while keeping the ratio of Ce3+/H2S06 1 4 so as to avoid the decomposition of H2S05induced by Fez+. Under these conditions solutions having concentrations of about 5 X M can be titrated with an accuracy of 2% (Table I). The titration of H2Sz08was effected by adding an excess of Fez+,waiting 10 min, and then titrating the remaining Fez+ by Ce4+. An accuracy of 2.5% was obtained for solutions of about 5 X low5M in 0.5 M H2S04.In 5 M H2S04,HzSz08was hydrolyzed to HzS05but both were titrated (Table I). Mixtures of Hydrogen Peroxide and Peroxydisulfuric Acid. H20z was first titrated by Ce4+,allowing determination of the concentration of Hz02,and then the excess of Ce4+was titrated by Fez+. The addition of Fez+was continued so as to reduce H2SzO8 until there was an excess of Fe2+. We waited 10 min and titrated the excess Fez+by Ce4+(Table I). We were unable to use Mn04- instead of Ce4+because titrations of H202 by Mn04- and of excess Mn04- by Fe2+were tricky by TT and Mn2+reacts slowly with HzSz08. Mixtures of Peroxymonosulfuric Acid and Peroxydisulfuric Acid. Titration was effected by Fe2+in 0.5 M HzS04in the presence of Ce3+keeping the ratio Ce3+/HzS05 2 4. H2SO5 reacts first and the first break in the curve allows determination of its concentration. An excess of Fez+was then added, and after a wait of 10 min the excess Fez+was titrated with Ce4+. This allows determination of the concentration of HzS20B.This titration can be done in 5 M H2S06but the amount of H2S208 hydrolyzed to H2SO5 must be taken into account (Table I).
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL
1983
Table I. Titration of Mixtures of Peroxides amt taken H,O,
H,SO,
X
lo4,M H,S,O,
amt found,a % (std dev)
Ce3+
2 0.4
HZOZ 99.8 (0.8) 98.3 (2.7)
HZSO,
9 0.5
9 1.8 4 2 9
10 10 10 10 10 2 0.2 0.2 8 2.1
10 8
HZSZO,
99.8 (0.8) 97.7 (2.2) 0 1 10 40 80 10 10 10 40 40
80.3 94.7 98.1 100.1 99.8 99.2 (0.8) 98.1 (3) 97.3 (4) 99.1 (0.7) 99.9 (1.1)
101.3 (0.4) 98.8 (0.6)
n
6 4 5 9
3 3 3 2 4 4 4 2 2
WzSO41, M
0.5 5 5 0.5 5 5 5 5 5 5 5 5 0.5 5 5 5 5 5
101.3 (1.2) 99.4 (1.7) 100.5 (1.6) 9 40 100.2 (0.3) 99.1 (0.4) 100.6 (3.1) 9 40 104.1 (1.0) 98.8 (0.1) 100.3 (1.6) 9 40 98.3 (0.2) 96.8 (0.7) 103.5 (6.3) Each experimental result is expressed in % = experimental value X 100/theorical value. The data accuracy is given by the mean of the n values of a set and the precision by the standard deviation if there are more than three measurements or otherwise the deviation with respect to the average. 1.4 8 20 4
Mixtures of Hydrogen Peroxide and Peroxymonosulfuric Acid. There are two possible procedures, either the same as for the mixture HzS05 HzSZO8can be used when the amount of HzOzis greater than that of H2S05 (H202 playing the part of HZSzO8)or the procedure using the first two steps of the titration of the mixture of the three peroxides in the other cases (Table I). Mixtures of Hydrogen Peroxide, Peroxymonosulfuric Acid, and Peroxydisulfuric Acid. In order to avoid the reaction between HzS05and Ce4+(HSOc + Ce4+G HS05. Ce3+),it was necessary to operate in a medium containing 4 to 5 M HzS04 (6) and add Ce3+ so as to obtain a ratio Ce3+/H2S05I 4. H202was first titrated by Ce4+(Figure 3, curve l),and then excess Ce4+and HzS05 were titrated by Fez+(Figure 3, curve 2). An excess of Fez+was then added to reduce HZS208, and we waited 10 min and titrated the excess Fe2+by Ce4+(Figure 3, curve 3). Curve 2 (Figure 3) manifests two breaks (break (b) and (c) corresponding to the titration of Ce4+and H2S05,respectively) which indicate that the reaction of Fez+with Ce4+is faster than with HzS05. However break (b) did not allow determination of the excess Ce4+since, at this point, a certain amount of HzS05had been reduced and an erroneously low result for HzS05would have been obtained. Break (c) allows us to know the sum of Ce4+ H2S05. Excess Ce4+was determined by the difference between the total Ce4+added and the amount of Ce4+that had reacted with HzOz(curve 1,break (a)). Thus the volume of Fe2+which reacted with H2S05was easily determined. We found the amount of Fez+that had reacted with HzSz08by the difference between the total Fez+ added and the Fe2+that had reacted with Ce4++ H2SO5 and the one which had been oxidized by Ce4+(curve 3, break (d)). When the amount of H2SzOsis greater than that of H2SO5, it is necessary to take into account the amount of HzS208 hydrolyzed to HzSO5 (approximatively 2% for 10 min at 20 OC in 5 M HzS04 whatever the initial concentration). The overall titration took 20 min. Calculations are simple when we take the same concentration for the two titrants Ce4+and Fe2+. Other Measurements. For this study we have tried other titrating agents like Mn04-, Ti4+,and SCN- already used by other authors for H20z,Fez+,Ce4+,and HzS05,to see if the determination of the equivalent point by thermometric titrimetry instead of potentiometry or spectrophotometry gave
+
+
+
Figure 3. Thermogram for the titration by 0.19M Ce4+ and 0.25 M Fez+ of a mixture of 2 X lom4M H,O,, 9 X M H2S05,and 20 X M Ce3+. lom4M H2S208in 5 M H2S04and with 40
better results. It was not the case. We have also studied the decomposition of HzS05which is induced by Fez+. Cerous ions inhibit this decomposition and this explains the necessity of always keeping a ratio of Ce8+/H2S051 4 as we can see in Table I: for the same concentration of HzS05if we increase the amount of Ce3+,we obtain a good result when the ratio value is 4. All the results of these studies will be published subsequently.
CONCLUSION The results obtained by our method compare well with those obtained by the spectrophotometric method as to accuracy and limits of detection when the products are alone. In mixtures it is difficult to determine concentrations lower than M for H20zand H2Sz08with good accuracy (spectrophotometry is better, 2 x M). However this method seems the best for the determination of very small amounts of HzS05 M) whatever the concentration of Hz02 or H2S2O8(even when the ratio H2SO5/H2OZor H2Sz08is lower than 10") because HzS05can be determined in the first place whereas with other methods it is last and is obtained by
Anal. Chem. 1983, 55, 615-620
difference after two or three titrations. Other advantages of the TT method are the use of a single sample and the short time of analysis (under 20 min). Last, thermometric titrimetry is a linear method of titration and only requires knowledge of the concentration of the titrating agents which are generally stable for several days. This allows facile automation in cases of repeated measurements. In this respect, note that automatic apparatus for the titration by TT including microprocessor control data analysis are already available. ACKNOWLEDGMENT The author thanks J. L. Burgot for helpful suggestions and numerous informative discussions. Registry No. H202, 7722-84-1; HzS05, 7722-86-3; HzSz0s, 13445-49-3. LITERATURE CITED (1) Kolthoff, I. M.; Belcher, R. “Volumetric Analysls”; Intersclence: New York, 1957; Vol. 111. (2) Pascal, P. “Nouveau trait6 de Chimie MinBrale”; Masson et Cle alteurs: Paris, 1961; Vol. X I I , p 1509. (3) Pungor, E.; Schulek, E.; Trompler, J. Acta Chim. Acad. Sci. Hung. 1954, 4 , 393-456.
615
(4) Csanyl, L. J.; Solymosl, F. Acta Chim. Acad. Sci. Hung. 1958, 13, 257-272. Csanyi, L. J.; Batyal, J. Magy. Kem. Foly. 1983, 69, 103-117. Marlano, M. H. Anal. Chem. 1968, 4 0 , 1662-1667. Csanyl, L. J. Anal. Chem. 1970, 42, 880-1382, Mariano, M. H. Anal. Chem. 1970, 42, 682-684. Schnelder, J.; Nagy, L.; Csanyi, L. J. Acta Chem. Acad. Sci. Hung. 1976. 89, 15-21. ( I O ) Boudevllle, P.; Burgot, J. L.; Chauvel, Y. Thermochim.Acta 1961, 43, 313-324. (11) Tyrell, H. J. V.; Beezer, A. E. “Thermornetrlc Titrlmetry”; Chapman and Hall: London, 1988. (12) Vaughan, 0. A. “Thermometric and Enthalpimetric Titrimetry”; Van Nostrand-Relnhold: London, 1973. (13) Jordan, J. ”New Development In Titrimetry”; Marcel Dekker: New York, 1974 Vol. 2. (14) Barthel, J. “Thermornetrlc Tltratlons”; Wlley: New York, 1975. (15) Tyrell, H. J. V. Talanta 1967, 74, 843. (16) Bernard, M. A.; Burgot, J. L. Talanta 1981, 2 8 , 945-949. (17) Carr, P. W.; Jordan, J. Anal. Chem. 1973, 4 5 , 834. (18) Bernard, A. M.; Burgot. J. L. Talanta 1961, 2 8 , 939-943. (19) Schulek, E.; Punkor, E.; Trompler J. Acta Chim. Acad. Sci. Hung. 1954, 4 , 429-444. (20) Tsao, M. S.; Wllmarth, W. K. J . Phys. Chem. 1953, 63, 346-353.
(5) (6) (7) (8) (9)
RECEIVED for review May 10,1982. Accepted December 13, 1982.
Generation of an Electrochemical Data Base for Pattern Recognition W. Arthur Byers‘ and S. P. Perone*2 Department of Chemistry, Purdue University, West Lafayette, Indiana 47907
Instrumentatlonand methods are presented for determining the Faradalc and capacitive response of an analyte over a wide range of experlmental condltlons. Experlments were carried out by a fractional factorlai deslgn so that the best experimental condltlons or changes in condltlons for analyte identlflcatlon could be determined in a later pattern recognltlon analysis. Percent ethanol, pH, surfactant concentration, number of cycles, scan rate, and mercury drop hang t h e all produced changes In cyclic dlfferential capaclty curves and cycllc stalrcase voltammograms whlch were unlque and reproducible. Capaclllve and Faradalc responses were determined Independently for 19 nitro compounds under most experlmental condltlons.
Electroanalytical techniques have not often been used for elucidation of chemical structure. This is due in part to the fact that normally recorded organic electrochemical responses are usually complex functions of many variables such as electrode material, solution composition, and the time frame of the experiment. Theoretical expressions relating structure to electrochemical activity are valid under very restrictive conditions ( I ) , and they are often difficult to evaluate (2-4). An empirical approach such as computerized pattern recognition would seem to be the best solution for qualitative identification, and it has already been used with some success in electrochemistry (5-8). However, in order to use pattern Present address: Westinghouse Electric CorP., 1310 Beulah Road, Pittsbur h, PA 15235.
z present ad&ess: Chemistry& Materials Science Department, 94550.
Lawrence Livermore National Laboratory, Livermore, CA
0003-2700/83/0355-0615$01 SO10
recognition to assign an unknown to a specific structural class, one must first have a library of data for similar structures, obtained under conditions similar to those for the unknown. Although a vast amount of organic electrochemicaldata have been collected (9),only small subsets have been collected under conditions which are sufficiently similar for a pattern recognition analysis. Bugard and Perone (IO) reported encouraging results for the analysis of 30 organics representing four structural classes, but better results might have been obtained if solution conditions had been varied. Much of the complexity of an electrochemical response is due to the heterogeneous nature of the electron transfer. The interplay of adsorption and diffusion complicate the interpretation of current flowing due to electron transfer. It is common practice to use a solvent system such that adsorption effects are minimized. While this may improve peak shape for more accurate quantitation, one may be throwing away important structural information in the process. The strength and potential dependence of adsorption may indicate the presence of certain functional groups (11, 12). ?r-electron interaction has a characteristic influence on the adsorption behavior of organics (13), and specific interactions between the analyte and some other molecule or ion within the double layer may also be helpful in identification (14,15). For these reasons we believe that structural classificationmight be made more reliably by using solution conditions where adsorption strongly affects electrochemical response. The purpose of this work is to describe new methods and instrumentation which we have developed for producing a large body of electrochemical data and for determining the best measurements to be made for later pattern recognition analysis. The effects of seven different experimental variables on Faradaic and capacitive electrochemical responses have 0 1983 Amerlcan Chemical Society
