608
Anal. Chem. 1983, 55, 608-612
(16) Cesarottl, E.; Gullottl, M.; Paslnl, A,; Ugo, R. J . Chem. SOC.,Dakon Trans. 1977,757-763. (17) Hammerschmidt, R. F.; Broman, R. F. J . Nectroanal. Chem. 1979, 99. 103-110. (18) Hasui, H.; Osuo, H.; Ohmlchl, H.; Fukuzyu, Y.; Tarul, H. Nlppon Kagaku Kaishi 1970,447-455. (19) Hishinuma, Y.; KaJi, R.; Aklmoto, H.; Nakajima, F.; Morl, T.; Kamo, T.;
Akikawa, Y.; Nozawa, S. Bull. Chem. SOC. Jpn. 1979, 52, 2863-2865. (20) Florlani, C.; Caiderazzo, F. J . Chem. SOC.A lQW, 946-953.
RECEIVED for review August 23, 1982. Accepted December 8, 1982.
Determination of Trace Quantities of Dimethyl Sulfide in Aqueous Solutions M. 0. Andreae* and W. R. Barnard Department of Oceanography, Florida Stare University, Tallahassee, Florida 32306
A method Is described for the determination of dimethyl sulfide (DMS) at the nanogram level In aqueous solutions. DMS Is removed from aqueous samples by sparging with a He carrier gas stream. The volatile DMS Is trapped cryogenically with liquid nitrogen on a chromatography column that serves as both the trapping and the separatlon mechanism. After controlled heating to separate DMS from interfering compounds, DMS Is detected by a flame photometrlc detector. The detection iimlt is 0.03 ng of S (DMS), corresponding to a concentration of 0.3 ng L-' for a 100-mL sampie. Precision is 6.2%. Accuracy, sample storage, and strlpping eff iciency are also discussed. This procedure has been used to measure DMS in a variety of natural waters.
Models for the global mass balance of sulfur all indicate the need for a volatile, biogenically derived compound to transfer sulfur from the sea to the atmosphere in order to account for the total atmospheric sulfur content. Efforts to identify this compound have resulted in the measurement of several reduced-sulfur compounds including dimethyl sulfide, carbon disulfide, and carbonyl sulfide, in both the atmosphere and the oceans (1-5). The discovery that living organisms produce volatile, methylated sulfur compounds (6) has led to the investigation of the importance of dimethyl sulfide (DMS) in the transfer of sulfur from the sea to atmosphere (1-3, 7-9). Because of the need for a rapid, precise, and accurate method for the determination of trace amounts of DMS in seawater to help establish its role in the transfer of sulfur from sea to air, we have developed the method described below and tested it on over 200 seawater samples from various areas of the Atlantic Ocean and Gulf of Mexico. DMS has been measured previously. Rasmussen (5) has measured DMS in pond waters by gas-liquid equilibration and GC-FPD determination. Another method involving extraction of DMS via CCll and HgClz from large volumes of seawater (15 L) has also been reported (1). DMS in the atmosphere has been preconcentrated on gold beads and analyzed using a similar detection system (4). Dimethyl sulfoxide (DMSO) has been determined as DMS after borohydride reduction by one of us (IO),but improvements in the detection system have increased the sensitivity of the instrument by 2 orders of magnitude. The procedure detailed below represents the first report of a method for the determination of DMS in water samples that includes the analytical parameters accuracy, precision, detection limit, and a con-
sideration of sample storage problems and interferences. Our method has several advantages over previous methods: it is rapid, accurate, and requires only small sample volumes. The instrument described below has also been used successfully aboard an oceanographic research vessel, eliminating the need for sample treatment to assure stability until samples can be analyzed onshore. EXPERIMENTAL SECTION Apparatus. The instrument configuration is depicted schematically in Figure 1. Known-volume sample loops of Teflon tubing are connected to an Altex series 202 six-way rotary sampling
valve (Rainin Instrument Co., Woburn, MA), The He carrier gas stream passes through a 19-mm o.d., 36 cm long stainless steel pipe packed with a combination of activated charcoal (50-200 mesh, Fisher Scientific, Pittsburgh, PA) and molecular sieve (Union Carbide Type 4A, Fluka Chemical Co., Hauppauge, NY) to remove trace sulfur gases from the He carrier stream. From this scrubber the carrier is routed through the sample loop and the six-way valve and is used to inject the sample into the bubbling chamber through a glass frit at the base of the chamber. From the bubbling chamber, the carrier gas stream passes through a short length of Tygon tubing and into a 15 cm long, 12 mm 0.d. Pyrex drying tube fiied approximatelytwo-thirds full with K2C03 (Mallinckrodt,Inc., Analytical Reagent Grade) to remove moisture from the carrier gas stream. The gas stream then passes into a 6 mm o.d., 30 cm long glass U-tube filled with 15% OV3 on Chromosorb W AW-DMCS 6C-80 mesh. This U-tube serves both as a trap and a separation column. About 2 m of 5 Q m-l Chrome1 wire is wound around the trap and connected to a variable transformer to allow controlled heating of the trap. The trap is connected to a Pyrex burner similar to that described by Braman et al. (4). Gas flow rates for this burner are 125 mL m i d for air and 110 mL min-l for H2and the He carrier gas flow is 100 mL min-'. This burner is enclosed in an aluminum housing which is flanged to a photomultiplier system (98246 PMT and QL30F PMT housing, EM1 GenCom Inc., Plainview,NY). Power for the PMT (700 V) is suiplied by a Model 7101 high-voltage supply and the PMT current is measured by a Model 3A27 electrometer (both manufactured by Pacific Precision Instruments, Concord, CA). A Hewlett-Packard HP 3390A integrator is used to record and integrate the chromatographic peaks. Standards. A gaseous diffusion tube with a known permeation rate was initially used to standardize the instrument. We used a wafer device with a nominal permeation rate of 2 ng of DMS/min (MetronicsASSOC., Inc., Santa Clara, CA). This device was standardized against a gravimetricallycalibrated permeation device by J. M. Ammons (1980, personal communication)and has an actual permeation rate of 2.84 ng of DMS/min at 30 OC. The wafer device was later replaced by liquid DMS standards which were prepared by dissolving liquid DMS (ICN Pharmaceuticals, Inc., Plainview, NY) in degassed ethylene glycol. Standards
0003-2700/83/0355-0608$01,50/00 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL 1983
r7
809
Burner
Bubbllng Chamber
Cowtrap/ Column in liquid N2
Transformer
u
Flgure 1. Sample degassing/flame photometric detection system for DMS determination.
prepared in this manner have been shown to have a number of desirable qualities for volatile organics (11). Primary standards were prepared by adding liquid DMS to a known weight of ethylene glycol and then weighing the total solution to determine the weight concentration of the standard. After the addition of DMS to the glycol the flask was immediately sealed and shaken for 15-20 min to ensure complete mixing. A working standard was prepared by injecting a few microliters of the primary standard into a weighed amount of degassed glycol. Working standards were prepared so that the concentration was in the range of 1-2 ng of S/pL of solution. Analysis Procedure. Samples were loaded into the sample loop (5-37 mL depending on the concentration of the sample) via suction through the six-way valve. Once loaded, the six-way valve was switched to the inject position and the sample was forced out of the sample loop by the He carrier gas into the bottom of the bubbling chamber, through the fritted glass diffuser, and into the chamber itself. Immediately prior to injection of the sample the cold trap was immersed in liquid nitrogen. Once the sample is in the bubbling chamber, it is sparged by continuous purging with the He carrier gas for 20 min to remove the volatile DMS. After this the liquid nitrogen is removed and the cold trap is heated by switching on the variable transformer. The packed trap then serves as a very simple temperature-programmed gas chromatograph and separates the different sulfur compounds collected at the head of the packing. The DMS elutes in a sharp peak after 53 s, when a voltage of 19.5 V at a current of 6.5 A is applied to the heating coil. Following the analysis, the suction vacuum is applied to the drain outlet and the sample pumped out. Trap heating is continued until all water originally visible in the trap/column has evaporated. RESULTS AND DISCUSSION Analytical Parameters. Accuracy. One of the funda-
mental problems in the determination of many trace substances in natural matrices is the absence of certified standards, which makes it difficult to evaluate the accuracy of an analytical method. In order to minimize systematic errors, we base our calibration on gravimetric standards and the addition of standards to the natural matrix. We have not observed any differences between the results obtained by adding DMS standards to seawater samples which contain natural levels of DMS (method of standard additions) and those obtained by adding DMS standards to seawater from which DMS has been removed by gas stripping. For convenience reasons we normally follow the latter approach. For calibration, we insert a glass T with an injection port between the six-way valve and the bubbler and inject microliter amounts of a solution of DMS in ethylene glycol into a volume of degassed seawater as it is being pushed from the sample loop to the bubbler. The calibration curve (log of peak
area vs. log of the amount of S (DMS)) is linear from the detection limit of 0.03 ng of S (DMS) up to about 10 ng of S (DMS). It has a slope of about 1.8, somewhat dependent on the H2-to-airratio in the flame. Slopes somewhat below the theoretical value of 2.0 for the S2emission are typically observed with flame photometric detectors. The accuracy of this calibration method is limited by that of the gravimetric preparation of the standard and by the accuracy of the microliter syringe used. Due to the inherent high accuracy of gravimetric procedures, the most important limitation to accuracy resulta from the syringe: typically f 2 % when half of the maximum volume of the syringe is used. The calibration in terms of amount of DMS per analysis can therefore be performed with an accuracy of about f2%. The sample volume measurement by the loop on the six-way valve is highly accurate; it can be calibrated gravimetrically by weighing the liquid delivered by a large number of injections. In order to further assess the accuracy of our calibration, we compared the results of the method described above with the calibrations by injections of standards of DMS in glycol into a dry system, thereby eliminating the effects of matrix composition and stripping efficiency, and with a calibration of the detector alone, using a permeation tube standard. Five replicate injections of 10 p L of DMS-in-glycolstandard into the heated injection port without the simultaneous introduction of a water sample gave a result of 0.94 f 0.06 ng of DMS. Injection of the same standard into a water sample followed by 20 min of stripping yielded a value of 0.93 f 0.06 ng of DMS (n = 4). This difference is not statistically significant; the stripping process is therefore quantitative within the limits of analytical precision. The accuracy of the glycol standards was checked against a permeation device standard (Metronics Assoc., Inc., Santa Clara, CA) with a known permeation rate of 2.84 ng/min at 30 "C. At long trapping times the agreement between the two standards was within the precision of the method. At shorter trapping times, discrepancies were found between the two standards. This discrepancy is due to the addition of a constant amount of DMS from the permeation device during the 1 min peak elution time. When this amount is considered, the agreement between the two standards is better than 0.5%. Precision. Table I represents the results of 12 duplicate sample determinations. These data were normalized by dividing the value of each determination by the mean for each pair of values. The precision (defined as the relative standard deviation of the normalized data) is 6.2%. Detection Limit. When the detection limit is defined as the amount of analyte that produces a peak twice the height
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610
ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL 1983
Table I. Results of Duplicate Determinations
sample no. 9.10.80 14302 10.10.80 12202 11.10.80 09002 11.10.80 10002 11.10.80 11002 11.10.80 15002 18.10.80 SK-1 18.10.80 SK-2 18.10.80 SK-3 24.10.80 lOOOZ M56-9-1 27.10.80 21002
Table 11. Retention Times (min)
amt of S found, ng L-' 1st deter- 2nd determination mination 38.2 46.3 31.0 35.5 72.4 80.0 32.1 16.8 28.8 30.5 58.7 16.1
30.8 41.3 31.4 32.1 70.3 79.4 27.1 13.9 33.1 26.7 59.1 14.8
rnL rnin-1
Flgure 2. Instrument sensklvky response from 2.4 ng of S (DMS) as a function of air flow rate.
of the instrument noise, a detection limit of 0.03 ng of S (DMS) is obtained. This represents an improvement in detection limit of 2 orders of magnitude over our previous detection system (IO). The major reasons for this improvement are modification of the burner housing and the installation of a head-on bialkali PMT rather than a side-on multialkali PMT. The bialkali PMT is better matched to the S2emission with higher sensitivity around 400 nm and lower sensitivity in the yellow and red regions than the multialkali PMT. The head-on geometry made it possible to move the active area of the tube closer to the flame and thereby increase the light yield. The burner housing has been modified so that an adjustable metal curtain can be moved up or down in front of the quartz burner, allowing a large fraction of the flame noise to be effectively screened out. This curtain results in an improvement by a factor of about 10 in the signal-to-noise ratio over our previous geometry (IO). The sensitivity of the instrument is also affected by variations in the burner gas flow rates. Our studies indicate that the sensitivity is greatly affected by variations in the air flow rate and less affected by variation in the H, flow. The change in response due to variations in the air flow rate is clearly seen in Figure 2. The change in sensitivity is dramatic below 100 mL min-l. Changes in the H2 flow rate with the air flow rate constant at 125 mL produced a similar shaped curve but the change in sensitivity was less pronounced. During actual analysis the air flow was not operated at the maximum sensitivity because of a persistent flame-out problem caused by water vapor condensation at the exit vent of the quartz burner and subsequent extinction of the flame by water droplets. Analyses were made with the air flow rate set slightly above the response maximum shown in Figure 2. This created a hotter flame which minimized condensation and only slightly decreased the instrument sensitivity. Drying of t h e Gas Stream. Since the carrier gas passes through aqueous samples, a significant amount of moisture is carried along in the stream. It is necessary to remove most of the moisture from the gas stream to avoid clogging of the liquid nitrogen cold trap. This was initially accomplishedby using a U-tube in a cold bath at -35 "C; however, for shipboard
air
cos
MeSH
3 s DMDS so2
ppQS/
OV3a
TCPb CarbopakC PPQSd
9.9 V
9.9 V
11 V
17.8 V
17.8 V
0.37 0.56 0.94 1.18 1.22 1.99
0.35 0.52 2.27 0.95 1.17 2.64
0.64 1.06 1.29 1.94 1.88 3.09
0.85 1.33 1.90 2.46 2.40 3.84 5.95
0.89 1.46 1.95 2.53 2.44 3.84
QFle
15%OV3 on Chromosorb W A W DMCS 60-80 mesh. 10%Tricresyl phosphate on Chromosorb W A W 80-100 mesh. Carbopak B/1.5%XE60/1.0%H,PO,. Porapak QS 80-100 mesh. e 5%DC-QF1 on Porapak QS, 80-100 a
mesh, analyses we wanted to avoid using the bulky cryogenic unit necessary to cool the methanol in the cold bath. Several drying agents had been tried previously (IO) but we eventually found that KzC09 was effective and did not remove detectable amounts of DMS even after several hours of analysis. In order to examine the possibility of a loss of DMS to the K2C03 drying tube, we injected DMS into the system between the six-way valve and the bubbler, which initially was kept dry. Then a fresh K2C03tube was added before the cold trap/ column and several more aliquots of DMS were injected. For four replicate injections of 0.9 ng of S (DMS), the recovery was 97.7%,which is not significantly different from 100% at the stated precision of 6.2% relative standard deviation. Water was added into the bubbler and the system operated for 2 h to load the KzCOswith water. Then, three replicate injections of 0.45 ng of S (DMS) were made. Recovery now was found to be 100.3%. From these results we conclude that no significant loss of DMS to the K2C03drying tubes occurs under the conditions used in our work. Furthermore, as we normally calibrate our system with aqueous standards, and with a K&03 tube in place, any such adsorption loss would be compensated for. The K2C03tubes must be replaced once they become saturated with water vapor since they can otherwise impede the carrier gas flow. This can usually be observed visually before it becomes a problem. Interferences. Two types of potential interferences were considered in this study. One was a negative interference from adsorption of DMS by the glass container and tubing surfaces and the other was positive interference due to the elution of other sulfur compounds at the same time as DMS. Adsorption of DMS by the glass and tubing surfaces was checked by injecting known amounts of DMS vapor (-5 ng) into the gas stream at various points and comparing the response with that obtained by injection directly before the cold trap. In all cases recovery was quantitative. All glass surfaces were silanized (SYLON-CT,Supelco, Inc.); this pretreatment has been found to minimize losses by deactivation of all surfaces that contact DMS vapor (IO). Without surface deactivation the recoveries were substantially lower and highly variable. All connectionsbetween glass apparatus were made with short sleeves of Teflon tubing to minimize adsorptive losses (IO). Other reduced sulfur compounds were analyzed to determine whether their elution times coincided with DMS, resulting in a positive interference. We have investigated the retention characteristics of several short packed columns of the geometry described above. All columns were used both as cold trap and separating column. The results are summarized in Table 11. OV3 gave the sharpest peaks and therefore the best detection limits. Figure 3 shows the separation on OV3 for the reduced sulfur compounds HZS,
ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL 1983
611
Table 111. Results of Degassing of DMS via Carrier Gas Stream as a Function of Timea
h
/IB
time, min
concn of DMS (as ng of S (DMS) L-')
5 10 15 20
34.8 45.2 47.4 47.7
He flow rate: 100 mL min-', room temperature. Table IV. Effects of Different Treatments on Sample Stability during Storage DMS concn [ng of S initial concn (DMS) L-'1 (0 h) unfiltered
484.5 484.2 454.6 431.9
filtered Flgure 3. Peak separation of possible Interfering reduced sulfur compounds: (a) wlth GC column conditionlng at 100 OC in GC oven overnlght; (b) after treatment of column with sllanlzlng reagent.
CH,SH, CSz, (CH3)& and (CH3)zSz.When the column was treated only by conditioning at 100 OC in a GC oven overnight, CS2,DMS, and DMDS were well separated (Figure 3a). When we tried to remove the slight tailing of the DMS peak by treatment of the column with a silanizing reagent (Silyl-8, Pierce Chemical Co., Rockford, IL) at 150 "C in a GC oven, tailing of the DMS peak was indeed improved, but the CS2 and DMS peaks were not separated any more (Figure 3b). In addition, the retention times were slightly reduced. HzS, CH3SH, and (DMS CS2)still showed excellent separation under these conditions. Thus, in the absence of CS2, some improvement of peak shape can be attained by silane treatment. Ifthe presence of CSz is suspected, however, exhaustive silanization of the packing used in this study must be avoided. The tricresyl phosphate column gives good separation of DMS and CS2,but due to the broader peak shape the detection limit for DMS is about a factor of 4 lower on this column than it is using the OV3 column. Commercial grades of helium contain small amounb of H2S and DMS, their concentrations depending on the supplier and grade of the gas. This will result in a positive gas blank interference. This gas blank was eliminated by passing the carrier gas stream through a gas scrubber consisting of activated charcoal and molecular sieve. When samples containing algal cells (e.g., unfiltered seawater) are introduced into the system, some algal material becomes trapped in the glass frit a t the base of the purging unit. This material will continue to release DMS in significant amounts for long times (hours to days) and produce a system blank. This problem can be eliminated by including an in-line filter unit (2.5 cm diameter, Nuclepore Corp., Pleasanton, CA) with a glass fiber filter (GF/C, Whatman, Clifton, NJ) in the suction line through which the sample is introduced into the system. This filter has to be replaced after a few samples (depending on algal concentrations) to avoid cross-contamination. Stripping Efficiency. We have also investigated the stripping rate of DMS from the sample by the carrier gas stream. The results of this study are shown in Table 111. After 15-20 rnin the increase in measured DMS concentration is less than the instrument error. Immediately following these 15-20 rnin runs these same samples were stripped again for an identical time period. No detectable DMS was found in these second purges. We have fitted an exponential removal equation to these data and found that 99% of the DMS is removed from the solution after 19 min of purging. Consequently, all samples were stripped for 20 min to ensure com-
DMS concn [ng of S (DMS) L-' ] sample treatment UFUA UFUA Ra UFA UFA Ra
+
FUA FUA Ra FA FA Ra a
12 h
24 h
48 h
371.6 432.7 431.4 518.3 306.4 480.6 527.9 503.7 382.0 383.9 502.6 616.4 396.0 404.0 492.7 459.4
246.0 321.1 500.9 448.0 383.7 488.9 508.3 566.3 341.5 305.1 493.7 476.1 440.4 419.5 481.4 464.2
94.8 19.2 498.1 511.6 453.9 464.8 513.8 464.2 57.3 24.1 441.5 495.8 343.5 356.6 423.0 450.8
SamDle refrigerated at 4 "Cuntil analvsis.
plete recovery of dissolved DMS. Sample Storage. A large volume of seawater was collected from a coastal location to study various methods of storing samples. Replicate 250-mL splits of this volume were stored in polyethylene bottles filled to the top to eliminate any beadspace in an effort to minimize partitioning into the gas phase. These splits were then subjected to the following treatments: UFUA, unfiltered unacidified stored at ambient temperature; UFA, unfiltered acidified stored at ambient temperature; FUA, filtered unacidified stored at ambient temperature; FA, filtered acidified stored at ambient temperature. All filtered samples were filtered with a 2.5 cm diameter glass fiber filter (GF/C, Whatman, Clifton, NJ) connected in-line to a gravity feed line to avoid sample degassing. Samples were acidified with 2 mL of concentrated HC1 (Mallinckrodt,Inc., Paris, KY). Additionally,duplicates of the above treatments were also stored in a refrigerator at 4 "C. The results of this experiment are presented in Table IV. These results show no clear advantage to either filtered or acidifying samples. In fact, filtration tends to lower the concentration no matter how much care is used to avoid sample degassing. Refrigeration of samples, whether filtered or unfiltered, acidified or unacidified, tends to be the best way to maintain sample integrity at least for periods up to 48 h. We have adopted this sample storage strategy when immediate
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 35 m 70 m 100 m 130 m 200 m 300 m 500 m 1000 m 1500 m 2000 m 3000 m 4000 m
100.1 76.1 38.3 21.3 15.5 5.7 8.8
Open Ocean Surface Seawater (S. Atlantic, 28 Oct 1980,0100 GMT) 5"50.5'S,
8.1 5.4 2.5 3.8 3.8 0.8 37.1
3 3 53.7' w
rain, N. Atlantic (18 Oct 1980) 30'58.5 N, 20'24.4' W
10.4
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.
--
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 polymerizationinitiator. However, peroxydisulfuric acid is not stable in sulfuric media. It decomposes to give peroxymonosulfuric acid or hydrogen peroxide according tQ experimentalconditions. 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
