ANALYTICAL CHEMISTRY
292 of tests were performed. As a test material an aqueous 0.15M solution of ferric nitrate was used. Complete absorption line derivatives were obtained with the null-balance system and the conventional system. Plots of the rate of change of magnetic absorption with frequency versus T-f oscillator frequency were obtained. A detailed comparison by superposition showed the identical shape of the two lines. This test indicated that the “electronic” standard functions in accord with the simple theory and does not introduce errors in line shape. Since the primary function of the spectrometer amplifiers is to provide sufficient error voltage to operate the pen and helical potentiometer motor in the null-balance system, tests were conducted to determine the effect of gain variations on the system. Deliberate changes in spectrometer audio amplifier gain were introduced and the peak-to-peak amplitude of the recorded derivative was measured for the various gain conditions. A maximum variation of f o.43yO in the average derivative was rccorded for a gain variation of & 18% from normal. This is approximately the limit of precision that can be expected of the prcsent equipment. A more realistic approach for checking the effects of circuit instability on the result8 obtained with the null-balance system was obtained by disabling the voltage stabilization circuit of thc spectrometer narrow band audio amplifier. Figure 3 shows the variations in peak-to-peak amplitude of the recorded line for the null-balance system compared with results for the conventional system, both operating under the above conditions. Readings were taken on a single specimen that remained untouched beginning approximately 10 minutes after the equipment v a s turned on from a cold start and extending over a period of 30 hours. Equipment wae alternated between conventional operation and null-balance operation during this test period with a minimum of disturbance and under controllable conditions. The standard
deviation over a period of 30 hours was 0.3% for the nullbalance system as compared with & 4.4y0for the conventional system. A source of difficulty encountered with the spectrometer operated conventionally is the long time required for the equipment to come to temperature equilibrium. With the conventional operation several hours warm-up time is required before the instrument readings become stable, even with stabilized amplifiers. Twenty minutes after start with the null-balance system, readings were consistent within z!= 0.3%. For the conventional system after a 1-hour warm-up the average amplitude had yet to reach a stable level. I n addition to quantitative moisture measurements, the nullbalance system a t present is being used to facilitate relaxation time measurements, or situations where a change in spectrometer radio-frequency energy level in the specimen coil is desired while holding the system gain constant. On the basis of performance to date, the system appears to provide performance of the type essential for precision amplitude measurements. It permits amplitude measurements utilizing amplifiers that do not have to be stabilized to a high degree, provides a continuous check of system gain with the specimen in place and in its normal environment, and warm-up time is conPiderably reduced where time is an important element in operation. LITERATURE CITED
(1) Pound, R. V., and Knight, W. D., Rev. Sei. Instr., 21,219 (1950). (2) Shaw, T.M., and Elsken, R. H., J . C h a . Phys., 18,1113 (1950).
(3) Ibid., 21, 565 (1953). (4) Watkins, G. D., “An
7.-f. Spectrometer with Applications to Studies of Nuclear Magnetic Resonance Absorption in Solids,” thesis, Harvard University, 1952. ( 5 ) Watkins, G. D., and Pound, R. V., Phys. Rep., 82,343 (1951).
RECEIVED for review August 10, 1954.
Accepted October 2, 1964
Nonaqueous Titration Method for Determination of the Purity of Hexahydro-l,3,5=trinitro-~-triazine And Its Content in Wax and Polyisobutylene-Motor Oil Compositions SEYMOUR M. KAYE Pieetinny Arsenal, Dover,
N. J.
A rapid, reliable method was needed for the determination of the purity of hexahydro-1,3,5-trinitro-s-triazine (RDX) and its content in explosive compositions. Its purity and amount present in composition with wax and polyisobutylenemotor oil were determined by titration in a dirnethylformamide medium using a solution of sodium methoxide in benzene-methanol as titrant. The end point of the titration is obtained visually, using azo violet as indicator.
T
HE determination of the purit! of hexahydro-1,3,5-trinitros-triazine (RDX, hexogen) has long been troublesome to analysts of explosives. Several attempts have been made through the years to establish a satiefactoiy method. I n 1921 Rathsburg (6) first used titnnous chloride to determine the “nitration product of hexamethylenetetramine>” but he neither balancrd the equation nor identified the reaction ploduct He suggested thr equation: CsHsNa(N0z)s 12 Tic13 + CsHeNs
+
Desvergnes ( 2 )attempted to determine it by breaking it down to nitrogen in a nitrometer, but conceded failure because less than five sixths of its total nitrogen was liberated in elemental form. A later attempt to reduce hexahydro-1,3,5-trinitro-s-triasine with titanous chloride solution (6) succeeded only to the extent of about 6070, even on prolonged boiling. The use of ferrous chloride effected a negligible reduction. When both reducing agents were added to the same sample, st,rangely enough, the reduction was over 90% complete. By use of a 300% excess of titanous chloride, 20 ml. of 0.7N ferrous chloride, and a 30minute boiling period, the reduction was made to proceed to 98 to 99% of the theoretical. This method, however, was completely empirical, with even the slightest deviation from the above conditions resulting in errat,ic results. A characteristic color reaction with sodium nitroferricyanide has been utilized for a spect,rophotometric method of anal)-& ( 1 0 ) . A maximum absorbance for the color system is obtained a t a wave length of 625 mg. Alt,hough the system does not conform to Beer’s law, a linear relationship exists between transmittance and oonccntration in the range of 100 to 200 p.p.m. This method, too, is
V O L U M E 27, NO. 2, F E B R U A R Y 1 9 5 5
293
highly empirical, and requires very critical experimental conditions. Identifying physical properties such as spectrophotometric data (4, 7 ) and x-ray diffraction patterns (8), as well as related chemical work ( 1 , g), has been reported. Frits and Lisicki ( 3 ) have demonstrated that many types of organic compounds can be determined by titration as acids in suitable organic solvents, and have recommended both the use of sodium methoxide in benzene-methanol as titrant and dimethylformaniide as solvelit. Dimethylformamide ( D M F ) was chosen as the solvent because of the ready solubility of hexahydro-1,3,5-trinitro-s-triazineand the fact that it is conimercislly available at a reasonable p i k e and requires no further purification prior to use The nonaqueous titration method described in this report has been applied to samples of the pririficd product and synthetic samples of composition A-3 (91% RDX, 9% wax dcsensit.izer) and composition C-4 (91 % RDX, 9% polyisobutylene-motor oil bindrr).
Table I. Sample No.
Sample
KO.
R D X Found, %
1 2 3 4 5
.4verage
Deviation from Average Found, %
Error, c/o 0.07 0.20 0.12 0.04 0.16 0.18 0.10 0.12
91.07 90.80 91.12 90.96 90.84 91.18 90.90 90 98
Added, % Found average, c70 Standard deviation, & ' Error, %
0.09 0.18 0.14 0.02 0.14 0.20 0.08 0.12
91.00 Y0.98 0.15 0.12
Table 111. Analysis of Synthetic Samples of Composition c-4 Sample No.
R D X Found,
rL
Deviation from Aveiage Found, L'/c
Error, %
Analysis of Samples of Purified RDX
R D X Found,
Average
Table 11. Analysis of Synthetic Samples of Composition A-3
99.88
Added, % Found average, 70 Standard deviations, Error. % '
7%
X
Error, %
Deviation from Average Found, %
0.14
Average
90 97
0 00
0 14
0.10
sodium methoxide solution to a green end point which persist8 for 30 seconds. Calculate to purity or percentage of hexahydro1,3,5-trinitro-s-triazinein composition A 4 as follows:
100.00 99.88 0.13 0.14
5 Obtained by dividing the summation of the squares of the individual deviations by a number equal to one less than the number of determinations and taking the square root of the quotient.
EXPERIMENTAL PROCEDURE
Preparation of Reagents. Azo Violet Indicator. Prepare a saturated solution of 1,4-nitrobenzeneazoresorcinolin benzene. Sodium methoxide solution, O.1N. Add 100 ml. of absolute methanol to a 2-liter borosilicate glass storage bottle. Wash approximately 5 grams of freshly cut sodium metal with absolute methanol and add piecemeal to the 2-liter bottle. The reaction of sodium with the methanol can be controlled by occasional immersion of the reaction vessel in ice water. When the reaction is complete, add 150 nil. of absolute methanol and 1500 ml. of benzene, C.P. grade, and store protected from carbon dioxide and moisture. Standardize against benzoic acid, Sational Buresu of Standards grade, with t h e aid of azo violet indicat ir as folloms : Transfer 50 ml. of dimethylformamide, technical grade, to a 250-ml. beaker and add 4 to 5 drops of azo violet indicator solution. Cover t h e beaker with a glass cover containing a hole in t h e center to admit a buret tip, and titrate to a clear blue color with 0.LV sodium methoxide solution, employing a magnetic stirrer. Transfer an accurately weighed portion of approximately 0.5 gram of t h e benzoic acid to the 250-ml. beaker. Titrate with 0.1N sodium methoxide to the clear blue color. Calculate the normality of the sodium methoxide solution as follows:
W
Sormality of sodium methoxide solution = ___ 0.1221v where SV = weight of benzoic acid and V = volume of 0.liV sodium methoxide solution used in the titration after the dimethylformamide has been neutralized. iilthough the reagent is reasonably stable, it should be restandardized every few days. Determination of Purity and Content of Composition A-3. Transfer 50 nil of dimethylformamide, technical grade, to a 250-ml. beaker and add 4 to 5 drops of azo violet indicator solution. Cover the beaker with a glass cover containing a hole in the center t o admit a buret tip, and titrate to a clear blue color with 0.1N sodium niethoxide solution, employing a magnetic stirrer. Transfer an accurately weighed portion of approximatelv 0.3 gram of dry sample to t h e beaker. Titrate with 0 1.4'
VN Purity or percentage of R D X in sample, % = 7.40 -~
W
where V = volume of 0.1-Vsodium methoxide solution used in titration after dimethylformaniide has been neutralized N = normality of 0.1-V sodium niethoxide solution W = weight of sample 011 a moisture-free basis Determination of Content of Composition C-4. Transfer an accurately weighed portion of approxiniately 1.5 grams of dried sample t o a 150-ml. beaker. 9 d d 25 ml. of carbon tetrachloride, transfer beaker and contents to a hot plate, and swirl until all of the binder is dissolved as evidenced by the complete separation of hexahydro-1,3,5-trinitro-a-triazinecrystals. Remove the beaker and contents from the hot plate and cool to room temperature. Use a fine porosity filter stick and suction to remove all of the carbon tetrachloride. Add 50 ml. of diniethylformamide, technical grade, to t h e beaker and stir the mixture using t h e filter stick until all of t h e R D X is dissolved. Transfer the solution quantitatively t o a 100-ml. volumetric flask, and make u p to volume with dimethylformamide. Prepare a blank using dimethylformamide so t h a t the blank is handled in exactly t h e same manner as the sample. Withdraw a, 20-ml. aliquot by pipet and transfer to a 100-ml. tall-form beaker containing 5 drops of azo violet indicator solution and a magnetic stirrer. Cover the beaker with a glass cover containing a hole in t h e center to admit a buret tip, and titrate with 0.1N sodium methoxide to a green end point which persists for 30 seconds. Employ the magnetic stirrer in t h e course of the titration. Repeat the titration procedure for t h e blank. Calculate to percentage of R D X in composition C-4 as follows: RDX,
%
=
(A
- R ) (7.40) ( N ) W
where A = volume of 0.1N sodium methoxide used for sample B = volume of O.IN sodium niethoxide used for blank N = normality of O.1N sodium W = weight of sample on a moisture-free basis represented by aliquot taken DISCUSSION
The purified product employed in the methods described above was prepared by recrystallizing U. S. Government specification
ANALYTICAL CHEMISTRY
294
grade Type A material [no octahydro-1,3,5,7-tetranitro-s-tetrazine (HMX) present] from acetone until a corrected melting point of 201” C. was obtained. I n the original survey of the problem of titrating composition ‘2-4, several approaches were attempted with little success. One method involved the direct titration of composition C-4 in dimethylformamide. This resulted in low, erratic results characterized by drifting, indefinite end points. These were undoubtedly due to the occlusion of hexahydro-1,3,5-trinitro-s-triazine crystals by the polyisobutylene binder, which prevented complete solution in dimethylformamide. .4 second approach involved the extraction of the binder with carbon tetrachloride, removal of the carbon tetrachloride with the aid of a filter stick, and direct titration of the hexahydro-1,3,5-trinitro-s-triazineafter addition of dimethylformamide. Of necessity, the sample weight for this procedure was in the order of 0.3 gram. While sharp end points were ebtained, the precision of this procedure was far from satisfactory when applied to plant samples. Synthetic samples, on the other hand, gave good precision. It soon became apparent that the hexahydro-l,3.5-trinitrc-s-triazine content of individual 0.3-gram samples varied by as much as 59$ This error, probably due to nonuniformity of the sample was overcome by increasing the sample Reight to 1.5 grams, and aliquoting. Using this procedure the results obtained on plant samples of composition C-4 gave good precision. RESULTS
The nonaqueous titration method has been applied t o samples of purified hexahydro-1,3,5-trinitro-s-triazine containing no
octahydro-1,3,5,7-tetranitro.s-tetrazine(HAIS) and synthetic samples of compositions A-3 and C-4 (Tables I, 11, and 111). I n view of the average standard deviation of 0.13% and the average error of 0.13% obtained, as well as t,he simplicity and rapidity of the developed method, it is recommended t h a t the method be incorporated in applicable government and industrial specifications for the determination of the purity of hexahydrc1,3,5-trinitro-s-triaxine and its content in explosive compositions. ACKNOWLEDGMENT
The author wishes to express his appreciation to E. F. Reese, A. J. Clear, C. J. Bain, Robert Frye, and J. D. Armitage of Picatinny -4rsenal for help rendered in the preparation and publication of this report. Appreciation is further expressed to the Ordnance Corps for permission to publish this paper. LITERATURE CITED
(1) Bellini, Ann. chim. appl., 31,125-9 (1941).
(2) Desvergnes, Chimie & industrie, 28, 103844 (1932). (3) Fritz and Lisicki, AKAL.C m x , 23, 589 (1951). (4) Jones and Thorne, Can. J . Research, 27B,828-60 (1949). (5) Kouba, Kickelighter, and Becker, A N ~ LCHEM., . 20, 948 (1948). (6) Rathsburg, Ber., 54B,B183-4 (1921). (7) Schroeder, Wilcox, Trueblood, and Dekker, ANAL.CHEM.,23, 1740-7 (1951). (8) Soldate and Noyes, Ihid., 19, 442-4 (1947). (9) Ternazra, Chimica e industria ( M t l a n ) , 17, 686-7 (1935). (10) Wright, Holston Defense Carp., Control KO. 20-T-17, Series A, 6 July 1953, Kingsport, Tenn. RECEIVED May
1 0 , 1954.
Accepted September 30, 1954.
Estimation of Metallic Mercury on the Surface of Tinned Copper G E O R G E T. KERR, SYLVESTER S. MACUT’, and C A R L C. NEELY2 A i r c r a f t - M a r i n e Products Inc., Harrisburg, Pa.
An evaporation method has been developed whereby 0.3y0 of mercury can be estimated on tinned-copper samples weighing approximately O.? gram. Accuracy was ascertained by using specially prepared samples containing known quantities of mercury. For samples containing in the order of 2 mg. of mercury, the average difference between the known and estimated content was 10.21 mg., or an error of approximately &lo%. Application to quality control is promising.
T
INNED-copper bodies are produced in quantity for use as various types of contacts in electrical industries. It has been found that “tumbling” these bodies with mercury forms a tin amalgam giving a much improved electrical contact. As a n excess of mercury results in the eventual amalgamation and breakdown of copper, it is important that the quantity of mercury deposited on each piece be controlled. Approximately 0.3 to 0.5% of mercury per sample, depending upon the shape of the body, gives excellent results electrically. However, each sample weighs approximately 0.7 gram. Thus, determining t h e quantity of mercury per sample for a large number of samples utilizing conventional methods would have been tedious. The amalgamation method (1,8 ) for determining mercury was considered. This consists in chemically reducing mercury compounds t o metallic mercury, vaporizing the metal, and condensing the vapor on B cooled, tared strip of silver. The mercury is Present address, Medical School, Temple University, Philadelphia, Pa. Present address, College of Chemistry and Physics, The Pennsylvania State University, State College, Pa. 1 2
thus weighed directly. In the present problem, chemical reduction would not be necessary, as the mercury is present as the metal. This method was not promising for a large number of analyses, but it led to the development of a similar procedure. EXPERIMENTAL
One hundred tin-plated copper samples, each weighing approximately 0.7 grams, were numbered u i t h a metal punch for identification, and were individually weighed to the nearest 0.1 mg. using a conventional analytical balance. The weighings were made by a technician with no previous weighing experience. The pieces were placed in a 250-ml. glass-stoppered Erlenmeyer flask and “tumbled” for 1 hour. Six samples mere chosen at random and reweighed. The average weight loss was less than 0.1 mg.; the maximum weight loss amounted to 0.2 mg. To the 100 samples was added 0.2000 gram of mercury. The samples were tumbled with the mercury for 1 hour and again weighed to the nearest 0.1 mg. The data showing weight of mercury deposited on each sample are listed in Table I. The total weight of mercury detected amounted t o 0.1863 gram as compared Kith 0.2000 gram, the amount actually used. Thus the average error is -0.14 mg. per sample, probably attributable to weighing errors. Apparatus. -4250-ml. round-bottomed flask is fitted to any laboratory-type vacuum pump capable of producing 1 mm. of pressure. The pump is protected with an ice trap. I t is preferable that the flask be fitted with either spherical or standardtaper lass joints. The flask is heated in an oil bath containing Dow-&orning silicone oil S O . i 1 0 . Test Procedure. The 100 specially prepared mercury-coated samples were placed in the flask and the system was evacuated to 1 mm. of pressure, Meanwhile, the oil bath was heated on an electric hot plate to 190’ C. This temperature should be sufficient to effect complete mercury removal a t 1 mm., since the vapor pressure of mercury a t 190” is approximately 12 mm. The flask and contents were immersed in the bath t o a depth
