species are not required (only the peak height needs to be free of interference) even higher concentration ratios could be tolerated. In actual practice, the titration of lead for the determination of thallium, for example, does not need to be very accurate because a range of actual values will lead to the observation of the correct peak height for thallium. (Also see Figures 2 and 3 and Table I.) Furthermore, if too much lead is “accidentally” used, this solution containing excess lead is readily diluted with blank zinc sulfate and the titration continued in the reverse direction. Finally, a knowledge of the amount of lead required to give a completely resolved thallium wave in fact provides a good estimate of the lead concentration actually present and the second titration is rapidly completed. If the peak potentials are closer than those for thallium and lead in zinc sulfate, the only additional complication is that stricter control over the titration of the interfering species is required to obtain the correct peak height (see data in Table I). In the limiting case where both species had identical peak potentials an exact null would be required. Unless the two electrode processes had substantially different wave shapes, e.g., different n values, such a situation could not be adequately handled by the procedure demonstrated in this work because the cognitive criteria for successful resolution (peak potential and/or wave shape compared with standards) would be meaningless. In summary, the virtually complete generality, the extreme simplicity, and the experimental basis on which the method is founded combine to make the problem of determination of two species arising from overlapping waves tractable for routine analysis via the procedure reported in this work. The instrumental features required for the method are digital data acquisition, data storage, and the ability to manipulate data via very simple arithmetic procedures, all of which are readily available in a minicomputer or microprocessor controlled polarographs. Now that this kind of instrumentation is commercially available, many restrictions previously applying to polarographic analysis should be drastically minimized. To emphasise the value of the digital method it is interesting to note that Martin and Shain (18)have used conventional analog instrumentation to accomplish essentially the same goals as described in this paper. However, their approach requires
two hanging mercury drop electrodes and work has to be performed in a dual cell format. In our experience, the difficulty of obtaining two equivalent electrodes and cells and even worse, maintaining them in equivalent condition, particularly for anodic stripping voltammetry, renders the analog approach unacceptable for routine analysis. The power of computer-assisted polarographic analysis in simplifying procedures is therefore clearly demonstrated by the present example.
ACKNOWLEDGMENT The authors gratefully acknowledge Tecnico Electronics (Northcote, Victoria, Australia) and Princeton Applied Research Corporation (Princeton, N.J.) for providing the instrumentation used in this work. LITERATURE CITED (1) I. M. Kolthoff and J. J. Lingane, “Polarography”, Interscience, New York, N.Y., 1952. (2) H. Schmidt and M. von Stackelberg, “Modern Polarographic Methods”, Academic Press, New York, N.Y., 1963. (3) I. Ruzic, J. Electroanal. Chem., 22, 422 (1969); 25, 144 (1970); 29, 440 (1971). (4) D. E. Smith, “Computers in Chemistry and Instrumentation”, 2, 369 (1972). (5) H. E. Keller and R. A. Osteryoung, Anal. Chem., 43, 342 (1971). (6) J. Ogle, P. Tomsons, I. Tutani, and J. Stradins, Zavd. Lab., 36, 1180 (1970). (7) R. 0 . Clem and W. W. Goldsworthy, Anal. Chem., 45, 918 (1971). (8) L. Kryger, D. Jagner, and H. J. Skov, Anal. Chim. Acta, 78, 241 (1975). (9) L. Kryger and D.Jagner, Anal. Chlm. Acta, 78, 251 (1975). (10) S. P. Perone and L. B. Sybrandt, Anal. Chem., 43, 382 (1971). (1 1) S.P. Perone and W. F. Gutknecht, Anal. Chem., 42, 906 (1970). (12) A. M. Bond, B. W. Kelly, and G. J. Maloney, Anal. Chim. Acta, 81,31 (1976), and references cited therein. (13) L. Meites. “Polarographic Techniques”, 2nd ed, Interscience,New York, N.Y., 1965, pp 623-670. (14) E. S. Pilkington, C. H. Weeks, and A. M. Bond, Anal. Chem., Submitted for publication. (15) 0 . E. Bratt, Electrochem. Techno/., 2, 323 (1964), and references cited therein. (16) G. E. Batley and T. M. Florence, J. Nectroanal. Chem., 55, 23 (1974). (17) J. H. Christleand R. A. Osteryoung, J. Nectroanal. Chem., 49, 301 (1974). (18) K. J. Martin and I. Shain, Anal. Chem., 30, 1808 (1958).
RECEIVEDfor review December 30, 1975. Accepted March 22, 1976. B.S.G. also expresses his appreciation to the University of Melbourne for financial assistance in the form of a Post-Doctoral Research Fellowship.
Electrochemical Oxidat ion of Tetraethylammonii m Hydroxide in Dimethyl Sulfoxide Donald T. Sawyer,* William H. Doub, Jr., and Paul J. Marsden Department of Chemistry, University of California, Riverside, Calif. 92502
Tetraethylarnmonlum hydroxlde in the presence of a tetraalkylammonlum ion supporting electrolyte in dimethyl sulfoxide is oxidized at 4-0.85 V vs. SCE by a diffusion controlled oneelectron process. The products of the electrolysis reactlon include acetaldehyde (which condenses to aldol), triethylammonlum ion, diethylammoniumIon, diethylacetamide, and polymeric materlal. On the basis of the cyclic voltammetry, controlled potential coulometry, and product analyses, a self-consistent mechanlsm for the oxidation process Is outlined.
Although there have been two recent reports concerningthe electrochemicaloxidation of hydroxide ion in aprotic solvents 1628
*
( I , 2),the results are in conflict and the reaction mechanism and products have not been established. On the basis of chronopotentiometric and coulometric data, the first study ( I ) concluded that hydroxide ion, in 0.1 M tetraethylammonium perchlorate-dimethyl sulfoxide (TEAP-DMSO) solutions, is oxidized by a one-electron process to produce molecular oxygen and water as the primary products. The production of oxygen was postulated because cathodic chronopotentiometry, after controlled potential oxidation of tetraethylammonium hydroxide at +0.8 V vs. SCE, yields a wave with an Er/4of -0.8 V. Separate experiments have established that oxygen dissolved in TEAP-DMSO yields a chronopotentiometric reduction wave with a value for ET/4of -0.75 V, which is in agreement with earlier studies ( 3 ) .An earlier in-
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
1
' I tl.0
I
!
-1.0
0
I
I
IO
I 00
I
-I
I
I
1
0
-2 0
E vs SCE, V
!
-2.C
E vs S.C.E.,V
Figure 2. Cyclic voltammograms of 1 mM TEAOH at a Au electrode in
clic voltammograms in DMSO at a Au electrode (scan rate,
0.1 M TEAP-DMSO (scan rate, 0.2 V/s). The numbers represent the order of the scans
( a )Background for 0.1 M TEAP ( b )3 rnM TEAOH and 0 1 M TEAP; (c) 2 mM TEAOH and 0.1 M LiC104
A shielded gold-foil or platinum-mesh electrode served as the counter electrode. The reference electrode consisted of a Ag/AgCl electrode in aqueous tetraethylammonium chloride solution (0.00 V vs. SCE) that was placed in a tube with a cracked glass-bead junction. A Luggin capillary was used with its tip within 1mm of the working electrode surface. Electrochemical experiments were made by use of a Leeds & Northrup coulometric cell (Model No. 7961) or an all-glass gas-tight cell (10). Product analysis was accomplished by use of a 3-ft Poropak-Q column in a Varian Aerograph Model 1200 gas chromatograph equipped with a flame ionization detector. Infrared spectra were recorded by a Perkin-Elmer Model 621 spectrophotometer which was equipped with BaF2 cells. Chemicals and Supplies. Dimethyl sulfoxide (DMSO) (J.T. Baker Analyzed Reagent Grade) was obtained in pint bottles to minimize water contamination; the water content, as specified by the source, varied from 0.02 to 0.06%. Tetraethylammonium perchlorate (TEAP) was prepared and purified according to the procedure of House et al. (11). Lithium perchlorate and sodium perchlorate (G. F. Smith Chemical Co.) were dried in vacuo and stored in a desiccator. Triethylamine (Et3N), diethylamine (EtzNH), tetramethylammonium hydroxide (pentahydrate) (TMAOH), and tetra-n-butylammonium hydroxide (25% solution in methanol) (TBAOH) were obtained from MC/B. Tetraethylammonium hydroxide (25% solution in methanol) (TEAOH) and TBAOH (10%aqueous solution) were obtained from Eastman, and TEAOH (40% aqueous solution) was obtained from K & K Laboratories, Inc.
Figure
0.2
VIS)
vestigation ( 4 ) also assigned the oxidation process at +0.8 to +0.9 V to the oxidation of hydroxide ion. Another characteristic of these systems is the decrease in hydroxide ion concentrations with time; the loss is due to the Hofmann elimination reaction (5) (CzH5)4N+
+ OH-
---*
H2O
+ (C2H5)3N + CH2=CH2
Recently Simonson and Murray ( 2 )have re-examined the electrolytic oxidation of hydroxide ion, using tetrabutylammonium hydroxide in 0.1 M TEAP-DMSO, and have carefully studied the solutions for products by gas chromatography. Their results provide convincing evidence that oxygen and ethylene are not products, and that the main product species are not volatile. Earlier investigations of the electrochemical oxidation of alkylamines in acetonitrile (6) and DMSO (7),and of the reduction of amine salts in DMSO (8)are relevant to the present study. These investigations indicate that the presence of tetraalkylammonium salts (as supporting electrolytes) and the corresponding trialkylamine (from the Hofmann elimination reaction) may be intimately involved with the oxidation of hydroxide ion. Smith and Mann (9) have outlined a mechanism for the oxidation of trialkylamines in aprotic solvents which is instructive relative to the electrochemical oxidation of tetraethylammonium hydroxide in TEAP-DMSO solutions. Because the anodic peak at +0.8 to $1.0 V and the accompanying cathodic peak a t -0.8 to -1.0 V frequently are observed in DMSO-TEAP-OH- solutions, a fuller understanding of the process and of its products is desirable. The present paper summarizes the results of a re-investigation of this challenging problem. By combining these data with those of the previous investigations (1-4) and the mechanistic interpretations for related amine systems (6-9), a self-consistent oxidation mechanism for the oxidation of tetraethylammonium hydroxide in TEAP-DMSO solutions has been developed.
EXPERIMENTAL Instrumentation. The cyclic voltammetric experiments were performed by use of a combination potentiostat-amperostat constructed from Philbrick operational amplifiers (10).The controlled potential coulometry experiments were accomplished with a Wenking Model 61RH potentiostat; the current-time curves were integrated manually with a K & E Model 62005 compensating polar planimeter. For cyclic voltammetry, either a Beckman platinum inlay electrode or a gold electrode sealed in plastic tubing ( 3 )was used for the working electrode. A gold-foil electrode with a surface area of 20 cm2 was employed as the working electrode in the coulometric experiments.
RESULTS The cyclic voltammetric behavior of tetraethylammonium hydroxide (TEAOH) a t a gold electrode is illustrated by Figure 1. When tetraethylammonium perchlorate (TEAP) or tetra-n-butylammonium perchlorate (TBAP) is used as the supporting electrolyte, curve b results. The cyclic voltammogram is characterized by an anodic wave at +0.85 V and a subsequent cathodic wave a t -1.1 V. Identical behavior is observed when tetra-n-butylammonium hydroxide (TBAOH) is substituted for TEAOH. Both aqueous and methanolic solutions (see Experimental section) of the tetraalkylammonium hydroxides have been used without apparent differences in the cyclic voltammograms. Curve c represents the cyclic voltammogram of TEAOH when LiC104 (or NaC104) is used as the supporting electrolyte. The anodic peak at +0.85 V is absent as is the cathodic peak at -1.1 V. A series of studies has established that the supporting electrolyte must be a t least 80% TEAP or TBAP in order to observe an anodic peak for TEAOH. An anodic peak is not observed for tetramethylammonium hydroxide (TMAOH), regardless of the supporting electrolyte. This behavior implies that both TEAOH (or TBAOH) and TEAP (or TBAP) are necessary for the oxidation process a t $0.85 V to occur. Figure 2 illustrates cyclic voltammograms for 1 mM
ANALYTICAL CHEMISTRY, VOL. 48,
NO. 11, SEPTEMBER 1976
1629
TEAOH at a gold electrode in a TEAP-DMSO solution. The anodic peak at -0.2 V is irreproducible and only appears on the initial scan. It is more pronounced and persistent when a platinum electrode is used, and its peak height increaseswith increasing concentrations of TEAOH. The same effect has been observed by Simonson and Murray ( 2 ) ,who also reported similar anodic processes when acetonitrile is used as the solvent. This peak may be due to a surface oxidation of the electrode. The main oxidation peak at +0.85 V in Figure 2 is due to a diffusion-controlled process; the diffusion coefficient, D, has been determined to have a value of 0.47 X cm2 s-l from chronopotentiometric measurements ( I ) . The height of the reverse cathodic peaks a t -1.2 to -1.4 V increases with the number of scans, and also with increasing scan rates. Similar electrochemicalstudies establish that triethylamine is oxidized at +0.8 V to give a product which is reduced a t -1.25 V. These studies also confirm that Et2NH2+ and Et3”+ are reduced at -1.25 V. Analogous results have been obtained earlier for trimethyl- and tripropylamines (8).The similarity in behavior of the TEAOH-TEAP-DMSO system is noteworthy (curve b , Figure 1). Coulometric oxidation at f1.0 V consumes 0.85 f 0.05 electron/mol of TEAOH and yields a dark bronze-brown solution; this behavior has been observed earlier (1,2).Goolsby and Sawyer ( 1 ) also observed that the concentration of hydroxide ion in TEAOH-TEAP-DMSO solutions spontaneously decreases with time at the rate of 13%per h. This observation has been verified by the present studies. The product peak at -1.2 V that results from the coulometric oxidation of TEAOH is almost non-existent during the first half of the electrolysis. A t the end of the electrolysis its peak height is approximately 2/3 that for the original TEAOH peak. This reduction peak is totally irreversible and has electrochemical characteristics that appear to be identical to those for tertiary or secondary alkylamine salts (8). If it is assumed that the peak is due to triethylammonium ion and the prior diffusion coefficient data are used (D= 0.49 X lob5) ( 8 ) ,the present data as well as those from the previous study ( 1 ) indicate that 0.6 mol of (CH&H2)3NH+ is produced per mol of TEAOH oxidized. In terms of electron stoichiometry this represents 0.8 mol of (CH3CH2)3NH+per faraday of anodic current (on the basis of 0.85 e-/TEAOH for the coulometric oxidation). Electrochemical tests have verified that the product peak from the oxidation of TEAOH is not due to 0 2 or H202, which confirms the observations of Simonson and Murray ( 2 ) . Gas chromatographic analysis of the products from controlled potential electrolysis of TEAOH in a TEAP-DMSO solution has established the absence of oxygen and of any volatile organic molecules. In particular, ethylene, ethane, ethanol, and acetaldehyde are not detected at significant levels and thus are not the final products of the electrolysis. This is in accord with the results of Simonson and Murray ( 2 ) . Combination of CHsCHO, TEAOH, and TEAP in DMSO in an electrolysis cell at concentrations consistent with electrolysis conditions, but without electrolysis, indicates that acetaldehyde is converted to aldol by reaction with TEAOH in a much shorter time than that required for an electrolysis experiment. The formation of aldol also results in a deep yellow-brown color (12) for the solution, which is similar to some of the coloration that develops during the coulometric oxidation of TEAOH. Infrared studies of the product solution from the anodic electrolysis of TEAOH do not indicate any absorption bands beyond those due to TEAP and DMSO. However, if the solution is frozen in a freezer and an infrared spectrum recorded of the remaining liquid fraction, a distinctive carbonyl band at 1650 cm-1 is observed. Addition of acetaldehyde to the 1630
*
solution gives a separate peak at 1715 cm-l before it disappears due to the formation of aldol. Addition of acetaldehyde to a TEAOH-TEAP-DMSO solution also yields a band at 1715 cm-l before it is converted to aldol; the latter does not exhibit any absorption bands in the carbonyl region. Diethylacetamide in a TEAOH-TEAP-DMSO solution exhibits a single carbonyl absorption band at 1650 cm-l that appears identical to that observed for the “mother liquor” from the frozen electrolysis solution. Furthermore, the diethylacetamide solution has a distinctive bronze color. The combination of this color with that for aldol formation in TEAOH-TEAP-DMSO solutions (yellow-brown) yields a solution coloration that appears identical to the bronze color for the product solution from anodic electrolysis of TEAOH. This color has been noted by us ( 1 ) as well as by Simonson and Murray (2).
DISCUSSION AND CONCLUSIONS The anodic peak at +OB5 V for TEAOH in TEAP-DMSO solutions results from a unique combination of tetraalkylammonium hydroxide, tetraalkylammonium ion, and DMSO. Use of tetramethylammonium, sodium, or lithium ions as the supporting electrolyte does not yield a diffusion controlled anodic peak for TEAOH or TBAOH in DMSO, acetonitrile, or dimethyl formamide. The products from the controlled potential oxidation of TEAOH do not include the expected 0 2 and H202, but rather a tertiary alkylamine salt, aldol, and an amide-like species. These products are analogous to those that have been identified by Mann and co-workers (9,13) from the oxidation of tertiary alkylamine in similar solvents. Consideration of the present electrochemical data and product analyses, and the mechanistic studies for the oxidation of tertiary amines (9, 13),provides a rational basis for concluding that Et3NH+, Et2NH+, CHSCHO, aldol, [Et2NCH=CH2In, and Et2NCOCH3 are probable products of the electrolytic oxidation of TEAOH in 0.1 M TEAPDMSO solutions. The primary electron transfer step for TEAOH oxidation must require a unique double-layer structure at the electrode surface, with the OH- ions occupying the inner layer and (CH3CH2)4Nf ions occupying the outer layer. If (CH3)4N+, Na+, or Li+ ions occupy the outer layer, an oxidation peak is not observed. Hence, the primary step involves oxidation of OH- to .OH with the latter in turn, in the double layer, abstracting an H atom from an a-CH2 group of the (CHsCH&N+ ion in the outer layer [Et,N+OH-]
double-
+ H,O + e-
Et36-C-CH3
layer
I B
(1)
1
The latter species, while still in the double layer, either is oxidized directly or more likely reacts with a second electrolytically formed .OH radical. [Et36-C-CH3
I
A
+ OH-] 2
-
L
PH
Et3N-&CH,
double-layer
I
+ e-
(2)
2
H The numbers under the OH- and e- groups represent the cumulative hydroxide ion and electron stoichiometries, respectively, for the oxidation reactions for TEAOH. These primary steps follow closely the arguments of Mann (13)re-
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
garding the electrochemical oxidation of tertiary amines. His experience indicates that the product species of Equation 2 dissociates to yield acetaldehyde and triethylammonium ion
-
OH
+ I
Et3N-Y-CH3
I
+
+
Et3NH
OH
I
2 EtJi-C-CH3
-
(3)
CH3CH0
In the presence of excess base, the dissociation reaction yields the tertiary amine and acetaldehyde; the latter undergoes an aldol condensation reaction
OH
+
I
OH3
-
H
+
Et3N
+
CH-CHO
&
H20
@)
Aldol condensation
On the basis of the studies of Mann (9, 13), the triethylamine that is produced in Equation 3a is oxidized (at +0.9 V) in a series of reactions
+ Et3N Et,&
-
+ OH4
A 2Et2N-C-CH3
I H
-
+ e-
Et3N.
(4)
3
+ H,O
Et2N-C-CH,
I H
-
+ OH-
Et,N-C-CH,
I
+
5
OH
I
+ e-
EbN-C-CH,
I
4
+ Et2N-CHeCH2 /*o
(6)
(6a)
\-
+ CH3CH0
Et2"
p"
(5)
H Et3N
polymer
+ OH-
Et2N-C-CH,
I H
6
-
OH
I
+
Et,N-h-CH,
+ e-
H,O
(7)
5
-
p"
Et2N-C-CH3
I H
+ CH3CH0
Et2"
(7a)
OH
I
+
Etd-C-CH3
OH7
-
I I H
Et,N-C--CH,
+
EtJV-C-CH,
II
(&)
0
I Et," + CH,CHO
H
+ I Et3N-C-CH3
OH
to yield more acetaldehyde, triethyl- and diethylamine (and their protonated salts), some diethylacetamide, and polymer. Any free triethyl- and diethylamine is oxidized according to Equation 4 (8,9,13),but most of these product species should be protonated because of the exhaustion of OH- ions in the vicinity of the electrode. Equations 6a, 7a, and 8a illustrate the formation of acetaldehyde, which in turn is converted to aldol in the presence of hydroxide ion. The ultimate products, in addition to aldol, are diethylammonium ion, diethylamine, triethylammonium ion, triethylamine, 1-diethylamino ethanol, and N,N-diethylacetamide. The presence of Et2NH2+ and Et3NH+ is confirmed by the results from cathodic cyclic voltammetry (Figure 2 ) and the studies of Michlmayr and Sawyer (8).
The product analysis for amine salts from the coulometric oxidation of TEAOH indicates 0.75 mol of (CH3CH2)3NH+ is produced per faraday of electrons ( I ) . Because the controlled potential coulometry for the present study as well as for the two prior studies (1, 2) consistently yields a stoichiometry of 0.85 f 0.05 electron per TEAOH molecule, this means that 0.64 mol of (CH3CH2)3NH+is produced per mol of TEAOH oxidized. Consideration of Equations 1-8 and the cumulative OH- and e- stoichiometries indicates that 7 OHions are consumed per 6 electrons in the overall oxidation of TEAOH to diethylacetamide. This represents a stoichiometry of 0.86 electron per TEAOH molecule, which is in excellent agreement with the coulometric results of 0.85 electron per TEAOH molecule. The inability to form 0 2 or HZOz from the electrolytic oxidation of TEAOH, or to achieve reactions with typical .OH radical substrates ( 2 ) ,confirms that the reactions expressed by Equations 1 and 2 occur in the double-layer of the electrode and that they are favored by high rates and a preferred mechanism. Apparently the formation of 0 2 (possibly via formation of .OH radicals) is possible in moist acetonitrile with NaC104 as the supporting electrolyte at potentials more positive than 1.5 V ( 1 4 ) . The difference between such conditions and the +0.85 V potential for TEAOH oxidation in TEAP-DMSO solutions is a measure of the energetic and mechanistic advantages that are afforded by the TEAOH oxidation mechanism (Equations 1-3 as well as the secondary steps represented by Equations 3a-8a). Although detailed knowledge of the exact product ratios has not been obtained (these ratios depend on the fluxes of OH- and HzO to the electrode surface relative to the current density), an overall mechanism has been outlined which accounts for the observed products, the low coulometric n values, and the bronze-brown color that develops in the electrolysis cell during the course of the electrolytic oxidation of TEAOH in TEAP-DMSO solutions.
OH
I I
Et,N-C-CH,
+ e-
(8)
6
OH
1 Et2N-C-CH3 II
0
+ H20
LITERATURE CITED (1) A. D.Goolsbyand D.T. Sawyer, Anal. Chern., 40, 83 (1968). (2) L. A. Simonson and R . W. Murray, Anal. Chem., 47, 290 (1975). (3) D. T. Sawyer and J. L. Roberts, Jr., J. Elecfroanal. Chem., 12, 90 (1966). (4) E. Jacobsen and D. T. Sawyer, Electroanal. Chern. lnferfacialNecfrochern., 16, 361 (1968).
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
1631
(5) J. March, "Advanced Organic Chemistry: Reactions, Mechanisms, and Structure", McGraw-Hill,New York, 1968, p 758. (6) J. W. Loveland and G. R . Dimeler, Anal. Chem., 33, 1196 (1961). (7) R. F. Dapo and C. K . Mann, Anal. Chem., 35, 677 (1963). (8) M. Michlmayr and D. T. Sawyer, Electroanal. Chem. lnferfacial Electrochem., 23, 375 (1969). (9) P. J. Smith and C. K. Mann, J. Ora. Chem.. 34, 1821 (1969). (10) A. D. Goolsby and D. T. Sawyer, h a / . Chem., 3g, 41 1 (1967). (11) H. 0. House, E. Feug, and N. P. Peet, Jr., J. Org. Chem., 36, 2371 11971) \ .I.
(12) L. C. Gruen and P. T. McTigue, Aust. J. Chem., 17, 953 (1964).
(13) L. C. Portis, V. V. Bhat, and C. K. Mann, J. Org. Chem., 35, 2175 (1970). (14) L. C. Portis, J. C. Roberson, and C. K. Mann, Anal. Chem., 44, 294 (1972).
RECEIVEDfor review May 6,1976. Accepted June 11,1976. This work was supported by the National Science Foundation under Grant No. CHE73-05204.
Generator for Producing Trace Vapor Concentrations of 2,4,6Trinitrotoluene, 2,4-Dinitrotoluene, and Ethylene Glycol Dinitrate for Calibrating Explosives Vapor Detectors Peter A. Pella Analytical Chemistry Division, National Bureau of Standards, Washington, D.C. 20234
A vapor generator was constructed to produce known vapor concentrations of explosives such as 2,4,6-trinitrotoluene, 2,4-dinitrotoluene, 2,6-dinitrotoluene, and ethylene glycol dlnitrate below 1 ppb by volume for calibrating trace explosives vapor detectors. The system Is temperature controlled which permits a wide range of equilibrium vapor concentrations to be generated. These vapor concentrations are diluted by slngle-stage, dynamic, gas blending to obtaln concentrations as low as 0.05 ppb. A quantitativegas chromatographic procedure was developed to evaluate this system by measuring the output vapor concentrations. The systematic error was usually within 15 to 20% of the values expected for TNT, and within 30% for EGDN. The applicability of the system for calibration purposes is demonstrated by performance data obtained with three commercial trace explosives vapor detectors.
Vapors of explosives such as 2,4,6-trinitrotoluene (TNT) and ethylene glycol dinitrate (EGDN) at concentration levels below 1 part per billion (ppb) by volume are required to establish the limits of detection of explosives vapor detectors. These detectors are used in a variety of law enforcement applications and are primarily designed for the detection of vapors of TNT or dynamite in air at trace concentrations. The physicochemical techniques employed for detection are discussed elsewhere ( 1 ) and include electron-capture ( Z ) , electron-capture gas chromatography ( 3 ) ,bioluminescence ( 2 , 4 ) , mass spectrometry (5),and ion-mobility (6). It is important that the generator produce known vapor concentrations from pure explosive materials for testing purposes, because some of these detectors are also sensitive to vapors from other compounds often present in an explosive material, such as dinitrotoluene (DNT) in TNT. Trace vapor generators employing dynamic dilution of vapors of T N T and EGDN have been developed by several workers for testing detector performance. Wall et al. (7) have generated vapors with a system based on the weight loss of the explosive using a thermogravimetric analyzer and measured flow rates. Liebel and Roberts (8) constructed a device for T N T vapor generation, and Dravnieks (9) has used a double-stage system for dilution of EGDN vapors at room temperature by a factor of 106. In this work, a dynamic gasblending system employing a single dilution stage was developed for producing known vapor concentrations of 2,4,61632
TNT, 2,4-DNT, 2,6-DNT, and EGDN. Similar devices have been constructed and evaluated a t the National Bureau of Standards for a number of applications (IO).An equilibrium vapor concentration of either TNT, DNT, or EGDN is generated a t a known temperature by passing nitrogen through a column containing the explosive dispersed on an inert support. As the nitrogen leaves the column, it is saturated with the explosive vapor. The equilibrium vapor is diluted with air by gas blending to provide the vapor of the desired final concentration. This vapor generator was evaluated by measuring the diluted vapor concentrations of TNT, DNT, and EGDN by a gas chromatographic method. Measurements of the equilibrium vapor concentrations of each of these materials were also made as a function of temperature. Performance data were obtained with three explosives vapor detectors of different manufacture to determine if the range of vapor concentrations generated by this system was suitable for the purpose of calibration.
EXPERIMENTAL Description of Generator. A diagram of the system is presented in Figure 1. Dry nitrogen passes through a charcoal filter and a molecular sieve (13x-40/60 mesh) trap before entering the column containing the explosive material. The column is immersed in a temperature regulated bath and was controlled to f0.05 OC. The accuracy of the controller was checked with an NBS platinum resistance thermometer and was found to be accurate to better than h0.2 "C. The equilibrium vapor from the column enters an electronic metering valve (AMs Co., Box 873, Lake Elmo, Minn. 55042) which precisely dispenses a predetermined flow of the vapor to the mixing manifold. The explosives vapor is diluted with air in the mixing manifold, and then passes to the sampling manifold where it exits the system through two ports. The flow rates of the equilibrium explosives vapor and diluent air are regulated by means of up- and downstream differential flow controllers (UFC and DFC) (Moore Products CO., Spring House, Pa.). The flow rates are measured by the calibrated flowmeters R1 and Rz. To minimize the adsorption and condensation of explosives vapors on the glass surfaces, the glass tubing a t the column exit is heated to 80-90 "C. The glass surfaces were also pretreated with dimethyl dichlorosilane to minimize adsorption of the vapors. The metering valve, mixing manifold, and sampling manifold are maintained a t 40-50 "C. Electronic Metering Valve, The flow rate of the equilibrium vapor ranged from 30 to 50 ml/min. The diluent air flow rate ranged from 1000 to 7000 ml/min, permitting a dilution of up to about 200fold. However, dilutions greater than 1000-fold are necessary to produce output EGDN concentrations of less than 0.5 ppb. This was
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
