Determination of Trinitrotoluene in Warhead Exudates by Linear Sweep Polarography GERALD C. WHITNACK Chemistry Division, Research Department, U. S. Naval Ordnance Test Station, China lake, Calif.
b A reliable, linear sweep polarographic method of analysis has been developed for the rapid determination of trinitrotoluene (TNT) in milligram samples of warhead exudates. The method is based on the presence of only microgram amounts of TNT in a 2570 acetone and 75% 0.1M lithium chloride solution, and depends upon the measurement of the wave produced b y the TNT a t -0.66 volt vs. a Hg pool in this solution. 2,4-Dinitrocyclotetramethylenetetratoluene, nitramine (HMX), and cyclotrimethylenetrinitramine (RDX) in concentrations up to a 50-50 mixture with 2-, 4-, 6-TNT do not interfere with the method, while the isomers 2-, 3-, 4- and 2-, 4-, 5-TNT, in concentrations in the 10-6 to grams per ml. range used in the method do not appear to affect the measurement of the 2-, 4-, 6TNT wave at -0.66 volt. The use of an X-Y recorder with a linear sweep polarograph allows the determination of TNT to b e made within a few minutes.
A
and reliable method of analysis was needed for thp amount of trinitrotoluene (TNT) in milligram samples of warhead exudates and explosive mixtures which might range from a few tenths of a per cent of T N T to as much as 50%. Infrared spectrophotometric methods have been used successfully to analyze T N T isomeric mixtures (7-9) : however, interference from many substances found in the exudates of warheads prohibits the use of these methods. Titrimetric, colorimetric, and solvent extraction techniques (8, 4) are time consuming, require large samples, and the solutions have to be treated very carefully. Polarographic methods have been successfully applied to the rapid analysis of small samples of explosive compositions containing T N T and cyclotrimethylenetrinitramine (RDX) ( I , 3, 4-6), and since explosives such as RDX, cyclotetramethylenenitramine (HMX), and dinitrotoluene (DNT) might be present in exudates from warheads, a linear sweep polarographic method of analysis for T N T in such mixtures was investigated. The purpose of this report is to 970
RAPID
ANALYTICAL CHEMISTRY
present and discuss the polarographic method of analysis that was finally developed, and its application to the analysis of some warhead exudates. EXPERIMENTAL
Apparatus and Materials. A linearsweep cathode ray Polarotrace, manufactured by Southern Instruments, Ltd., in England, was used throughout this work. Acetone solutions of trinitrotoluene, 2,4Ainitrotoluene, RDX, and H M X were prepared in the 10-6 gram per ml. range and aliquots of these standard solutions were added to a supporting electrolyte and reduced. The supporting electrolyte used in this work was aqueous 0.1M LiC1, with enough acetone added to make the final solution 25% acetone and 75% O.1M LiCl by volume. Thus, 2.5 ml. of acetone were added to a 10-ml. volumetric flask and diluted to volume with the 0.1M LiCl solution. The dropping mercury electrode had a drop time of 7 seconds in distilled water and m = 7.7 mg. per drop. Borosilicate glass polarographic cells, furnished with the Polarotrace, and provided with side arms for the anodic connection and for bubbling with nitrogen, were used in all work. About 2 ml. of the acetone-lithium chloride solution were placed in the cell, the solution was then purged with pure nitrogen for about 3 minutes and reduced with nitrogen flowing over the solution. The pure nitrogen was passed through a 75% water-25% acetone solution before entering the polarographic cell solution. All polarographic measurements were made a t 25' =t 0.10' C. Redistilled Hg, c.P., was used as the anode and all current peak potentials and heights are referred to an Hg pool. A Moseley Model 2-A X-Y recorder was used to record some of the data. This method of plotting the polarograms produced greater accuracy in measurement, a gain in sensitivity and resolution, and an improved method of obtaining a permanent record of data then either plotting by eye or taking a picture of the cathode-ray polarograms. The X and Y ranges of the recorder were set to record a polarogram that measured twice the number of divisions seen on the scope. A standard8I/2- X 11-inch graph paper of 10 X 10 to the l/z inch was used for recording the data. This size paper was then easy t o file in standard loose-leaf notebooks.
Procedure. A 5- to 10-mg. sample of the exudate is added to 2.5 ml. of C.P. acetone in a 10-ml. volumetric flask. The exudate is dissolved in the acetone with gentle shaking of the flask. The contents of the flask are then diluted to volume with the O.1M LiCl solution and a 2-ml. aliquot of the mixed solution is placed in a borosilicate glass polarographic cell. After flushing nitrogen through the solution for 3 minutes, the polarogram is recorded with the start potential dial of the Polarotrace set a t -0.40 volt and the scale factor dial of the instrument adjusted correctly for the concentration on T N T in the solution. A scale factor setting on the Polarotrace of 2.5 on direct current is best for solutions of T K T that are about 5 X 10-6 gram per ml. The concentration of T N T is obtained by a standard addition technique, using a gram per ml. of T N T solution as the standard solution. After the wave height on the original sample is obtained, a known concentration of the T N T standard solution is added to the solution in the cell and the wave height for T N T is again recorded. The calculation of T N T in the original sample is then determined from the two wave heights. Additions of 0.01 t o 0.10 ml. of the T N T standard solution are used to minimize volume effects. RESULTS AND DISCUSSION
The data in all figures and tables were obtained in a 25% acetone-75% 0.1M LiCl solution. The supporting electrolyte used by Lewis (0.05M NazN03-0.05iM borax) was tried in early experiments, but abandoned in favor of the neutral solution of lithium chloride. The first peak current (i,) for T N T in the LiCl solution occurred a t -0.66 volt, and was sharper than the i, of -0.55 volt that was found for T N T in the buffer solution of Lewis. In addition, the first T N T wave in the buffer solution slowly decreased in height, while in the LiCl solution the first T K T wave height (ih) remained the same for several hours. Acetate buffers and HC1 solutions were also tried but gave no better T N T waves. Table I shows the i, and i h values for solutions of T K T and DNT. The values for T N T observed a t -0.83 and - 1.03 volts, respectively, are obtained as a double wave. Single waves are produced at -0.66 volt
and -1.47 volts. The wave at -0.66 volt is approximately 2.5 times larger than the waves at -0.83 volt and 1.47 volts, respectively, a:id 5 times larger than the wave a t -1.03 volts. The wave a t -1.03 volca is '/z that of either the wave a t -0.83 volt or that at -1.47 volts. A linear relationship of i h values us. concentration of T N T waa found for the wave at -0.66 volt over a tenfold range in T N T gram per concentration (10-6 GO ml.). gram per ml. solution A 5.08 X of 2,4-DNT produced two waves with i, values of -0.86 volt and -1.33 volts, respectively. The first wave height is about 2 times larger than the second wave. The first wave f 8 x T N T (-0.66 volt) is not affected. by 2 , P D N T in concentrations up to a 50-50 mixture. It is also very likely that larger amounts of D N T will not affect the T N T results, as good separation of D N T and T N T i, values is obtrtined in the LiC1acetone solution. Since R D X is found in some exudates, this explosive was examined by the linear sweep technique. The results are given in Table :[I. None of the i, values for R D X are near the -0.66volt wave of TNT, a d with up to 50-50 mixtures of R D X and T N T there was no effect of R D X on l;he i h value of the first T N T wave. I-: should also be possible to measure the most negative R D X wave (-1.83 or -1.60 volts), as neither T N T nor D N T give waves a t such negative voltlzge. The derivative current of the Polarotrace appeared best for measurement of this R D X wave (Table 11). R D X sometimes contains small amounts of H M X as an impurity, and if R D X is used in the warhead explosive composition i t is possible that very small amounts of H M X may also be present in the warhead exudates. H M X solutions were studied polarographically and the results are given in Table 11. Thus, no interference from H M X on the T N T wave at -0.66 volt was otrserved. A few mixtures of H M X and R D X were studied and the data are presented in Table 111. The R D X wave about -0.90 volt might be used to analyze for R D X in the presence of H M X and the H M X content then calculated by subtracting the R D X wave height equivalent from the total wave (HMX RDX) a t -1.1 volts. In this work, the m8:thod of standard addition was used exclusively in analysis of the sample warhead exudates for TNT as these exudates may contain other constituents that alone or combined might affect the i h value of the T N T wave. The use of standard addition makes the effect of the constituents on the T N T wave negligible.
+
Table 1.
Trinitrotoluene and Dinitrotoluene in 25% Acetone-75yo Lithium Chloride
(0.1M ) (TNT = 4.44 X low6gram per ml.; DNT = 5.08 X 10-6 gram per rnl.) Scale Start potential, Explosive i,, Volts i h , Divisions factor volts TNT -0.66 27.0 10 --0.40
TNT TNT
-0.83 -1.03 -1.47 -0.86 -1.33
TNT DNT DNT
Table II.
(RDX
10
11.0 6.0 11.0 21.0 13.0
RDX and HMX in 25% Acetone-75% Lithium Chloride (0.1M) 5.00 X gram per rnl.; HMX = 5.08 x 10-5 gram per ml.)
=
Divisions
Scale factor
Start potential, volts
45.0 10.0 24.0 19.0 10.5 18.5 28.0 13.0
1.5 1.5 1.5 10.0 0.10 2.5 4.0 0.04
-0.70 -0.70 -1.10 -1.45 -1.35 -0.80 -1 .45 -1.45
ih,
Explosive RDX RDX RDX RDX RDX HMX HMX HMX a
-0.65 -0.65 -1.20 -0 60 -1 10
10 10 10 10
i,, Volts -0.90 -1.15 -1.43 -1.83 -1.6OU -1.06 -1.80 -1.63
Derivative current.
A standard graph for T N T could be prepared, and used satisfactorily if one knew the general composition of a particular warhead exudate or explosive mixture would not change. Although two measurements per sample are needed in the standard addition technique of the linear sweep polarographic method, the results are obtained very rapidly. Five different samples of warhead exudates were analyzed for T N T content by the proposed method. The results (Table IV) showed the T N T amount in this particular type of exudate varied from about 0.10% to as high as 37%. Since the viscous nature of the samples gives some doubt of their homogeneity, only tests of reproducibility of the analysis on duplicate samples from three runs were made. The data indicated the method was precise to +5y0of the amount of T K T present. RDX, 2,4DNT, and H M X in concentrations up to the amount of T N T present appeared to have little or no effect on the T N T determination, and by using the standard addition technique the effects of other constituents found in exudates from most warheads are negligible. If constituents are present that produce a wave within +IO0 mv. of -0.66 volt, they may have to be removed from solution before the TST wave can be accurately measured. However, the Polarotrace will resolve some traces whose i, values are within +50 mv. of each other with the derivative circuit. Some tj-pica1 camera and X-Y plots
Table 111. RDX and HMX in AdmixLithium ture in 25% Acetone-75% Chloride (0.1M ) (RDX = 2.5 X 10-6 gram per ml.; HMX = 2.6 X 10-6gram per ml.) th,
i,, Volts
Divisions
Scale factor
-0.92 -1.07 -0.84 0.98
20.0 9.0 32.0 10.0
2.5 2.5 0.025 0.025
-
Table
IV.
Current Direct Direct Derivative Derivative
Analysis of Some Head Exudates
War m\-m
ll\ 1
Sample No. Grams 4 8 10 12 16
Wave height found, Divi- Scale per sions factor cent
5.0 X 13.0 2.5 14.4 3.3 X loT3 21.0 40.0 37.3 3.0 X 23.0 2.5 3.0 5.2 X 36.0 10.0 9.0 1.2 X 10.0 0.25 0.1
of TNT, DNT, RDX, and H M X soiutions are shown in Figures 1-3. In Figure 1, the four waves obtained with T N T solutions are shown. Figure 2 illustrates the first wave obtained for TNT (-0.66 volt) in a sample of exudate and shows the increase of peak current a t this voltage upon addition of a standard TKT solution. Figure 3 shows the X-Y recordings for the same solution. The divisions of current obtained with the X-Y traces are twice those found with the camera VOL. 35, NO. 8, JULY 1963
971
Figure . 3.. . X-Y recorder traces of TNT
972
ANALYTICAL CHEMISTRY
LiCl solution of T N T appears to be very stable. The height of the R D X derivative wave at -1.60 volts also appeared to be unaffected in solutions containing these four explosives. In developing the method, the alpha 2,4,6 was expected to be the predominant form of T:XT found in the exudates and only this form was used in the standard TN'I' solution and in mixtures with the other explosives. The melting point cf the 2,4,6-TNT used in this study was 81 O to 82" C. Since small amount;; of the beta 2,3,4 and gamma 2,4,5 forms of T K T might also be present in the exudates, solutions of these explosives were prepared in the LiC1-acetone solution and reduced. In the l o + to 10" gram per ml. concentration range, no waves were observed for either the beta or gamma form of TNT a t -0.66 volt. Both of these forms did produce peak currents a t more positive potentials and more negative potentials than the -0.66-volt peak used to determine
the T N T in this method. The melting points of the beta and gamma T N T used in this investigation were 103" to 104" C. and 109" to 111" C., respectively. The observed differences in wave heights and peak potentials of these isomers of T N T may allow one to determine them in admixture by a polarographic procedure. Further study of mixtures of the T N T isomers is in progress. In conventional polarography, maxima suppressors are sometimes necessary in quantitative work. However, with the linear sweep polarographic method maxima suppressors are seldom needed and were not necessary in this work.
exudates and explosives used in these studies.
ACKNOWLEDGMENT
Technical R e p t . Xo. 2546 (1958),Zbid. ( 9 ) Pristera, Frank, Halik, Michael, Technzcal R e p t . No. 2221 (1955), Zbid.
The author expresses his thanks to Charles W. Falterman of the Propulsion Development Dept. for the samples of
LITERATURE CITED
( 1 ) Dennis, S. F., Powell, A. S., Astle, M. J., J. Am. Chem. SOC.71, 1484 il94Si. ( 2 ) Halfter, G., Winkler, H., Die Chemie 57, 17/18,124 (1944). ( 3 ) Hetman, J., "Trace Techniques," usme: the K-1000 Cathode Rav Polaroera&. Vol. 1. P. 43. Southerh Instru\ - - - - I -
heits' Ltd., Cambeiley, Surrey, England, (1949). (4) Lewis, D. T., Analyst 79, 644-8 I 1- Q.54) , - - ,. ( 5 ) Page, J. E., Quart. Rev. (London) 6 , 262 (1952). ( 6 ) Pearson, J., Trans. Faraday SOC.44, I
683 (1948).
( 7 )Pr(ste;a,' Frank, Fredericks, W. E., Technical R e p t . No. 2485 (1958),
Samuel Feltman Ammunition Laboratories, Picatinny Arsenal, Dover, N. J. (8) Pristera, Frank, Fredericks, W. E.,
RECEIVEDfor review October 8, 1962. Accepted April 23, 1963.
Potentiomietric Titration of Acids in an N,N-Dlimethyl Fatty Amide CHARLES A. REYNOLDS, JAMES LITTLE, and MERLE PATTENGILL Department o f Chemi:;try, University of Kansas, Lawrence, Kan.
b Potentiometric titrations of a variety of acids and acid mixtures ranging in strength from mineral acids to phenols have been performed using a cornmercially available mixture of N,Ndimethyl fatty amides as a solvent. A conventional glass indicating electrode was used with a saturated calomel reference electrode modified b y replacing the saiurated potassium chloride solution with a saturated aqueous solution of lithium chloride. The solvent gave reproducible titration curves which, iri all but a very few cases, had flat buffer regions. This fact, coupled with its large potential range, makes the solvent especially suitable for differentiating titrations.
I
years, :t wide variety of substances have been used as solvents for the titration of acids. Thus far, however, each solvent tried has presented a t least one definite drawback, either in scope and applicability, or in mechanics. A brief survey of some of the more popular solvents illustrates this point. Basic soIvents such as ethylenediamine (4, 9) and n-butylamine (6) have been used in the determination of weak
and very weak acids for several years. However, the leveling effect these solvents exert makes differentiation between mineral and carboxylic acids im-
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possible. I n addition, these solvents possess very small potential ranges (11) which severely limit the size of the potentiometric break which can be obtained, and they are very prone to carbon dioxide absorption. Ethylenediamine is so strong in this latter respect that it has been recommended as a quantitative carbon dioxide absorbent
2
4
6
8
1
0
VOLUME TBAOH (rnl.)
Figure 1. Titration of mineral and sulfonic acids A. Perchloric acid 6. p-Toluene sulfonic acid C. Hydrochloric acid D. Nitric acid E. Sulfuric acid
Acetone (8)and acetonitrile ( l a ) have been disqualified for the titration of mineral acids and mineral acid mixtures because of strong acid-solvent reactions (3). Further, the titration curves of carboxylic acids and phenols in these solvents usually have steep buffer regions, making differentiating titrations difficult. dlthough pyridine (6) can be used for the titration of mineral, as well as weak acidq, steep buffer regions are obtained with many carboxylic acids and phenols. In addition, its highly offensive odor makes provisions for the removal of its vapor necessary. Dimethylformamide (DMF) (4) and methyl isobutyl ketone ( I ) have been reported to be of only limited use for the titration of mineral acids or their mixtures because of strong acid-solvent reaction (3). DMF's instability in excess base (4, its high volatility, and its high VOL. 35, NO. 8, JULY 1963
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