tical detection limit both for reversible and for irreversible systems is at least l O + M . The results of further investigation will be reported in a subsequent paper. ACKNOWLEDGMENT
The assistance of W.D. Cooke of Corne11 University in evaluating this polarograph is acknowledged with thanks. The polarograph photographed in Figure 3 was fabricated by the ORKL Instrument Department under the direction of G. A. Holt, C. C. Courtney, and D. D. Walker. LITERATURE CITED
(1) Arthur, Paul, Lewis, P. A,, Lloyd, N. A , , ASAL.CHEM.26, 1853 (1954). ( 2 ) Barker, G. C., Cockbaine, D. R.,
Atomic Energy Research Establishment C/R 1404, pp. 3-4, Her Majesty’s Stationery Office, York House, Kingsway, London W.C. 2, 1957.
(3) Barker, G. C., Jenkins, I. L., Analyst 77,685-96 (1952). (4) Booman. Glenn L.. ANAL.CHEM.29. \ - - -
,
(5) Bruss, D. B., DeVries, Thomas, J . Am. Chem. Soc. 78,733 (1956). (6) Ferrett. D. J.. blilner. G. W. C.. Analyst 80, 132-40 (1955).’ (7) Fisher, D. J., “Polarograph, ORXL Model Q-1673, High-Sensitivity, Diode
Filter, Derivative, Recording,” ORNL Master Analytical Manual, TID-7015 (section l ) , Method 1 003042 and 9 003042 (2-13-57), Office of Technical Services, DeDt. of Commerce, Washington 25, D. C: (~, 8 ) Hamm. R. E.. ANAL. CHEM.30. 350 I
(1958). ’ (9) Hume, D. N., Zbid., 30,675 (1958). (10) Ilkovic, D., Semerano, G., Collection Czechoslov. Chem. Communs. 4, 176 (1932). ( 1 1 ) Jackson, W., Jr., Elving, Philip J., ANAL.CHEM.28.378 f 1956). (12) Kelley, $1, T., Fisher, D. J., ZbLd., 28, 1130 (1956). . (13) Zbid., 30, 929 (1958). (14) Kelley, M. T., Miller, H. H., Ibid., 24, 1895 (1952).
(15) Kolthoff, I. M., Lingane, J. J., “Polarography,” 2nd ed., p. 504, Interscience. New York. 1952. (16) Lingane, ’J. J., Williams, R., J . .4m. C‘hem.SOC. 74, 790 (1952). (17) Nicholson, M. N . , A N A L . CHEY. 27, 1364 (1955). (18) Pecsok. R. L.. Farmer. R. W.. Zbid.. 28, 985 (1956). (19) George A. Philbrick Researches, Inc.,
Boston, Mass., “GAP/R Electronic -4nalog Computers,” “Applications Manual” and catalog data sheets. (20) Radin, N., DeVries, Thomas, ASAL. CHEX24,971 (1952). (21) Sawyer, D. T., Pecsok, R. L., Jensen, K. K., Zbid., 30, 481 (1958). ( 2 2 ) Taylor, J. K., Smith, R. E., Cooter, I. L., J . Research Natl. Bur. Standards 42, 387 (1949). (23) Texas Instruments,
Inc., Dallas, Tex., *:?ata sheet, types 650C through 65329.
RECEIVEDfor review June 23, 1958. .%ccepted May 11, 1959. Division of .inalytical Chemistry, 134th Lleeting, .%CS,Chicago, Ill., September 1958.
Pola rogra phic Determination of Pentae rythrito I Trinitrate in the Presence of Nitroglycerin WILLIAM M. AYRES and GUY WILLIAM LEONARD Chemistry Division, Research Department, U . S. Naval Ordnance Test Station, China Lake, Calif.
A rapid polarographic method for the determination of pentaerythritol trinitrate in nitrocellulose propellants was developed for systems that may contain nitroglycerin, nitrocellulose, 2nitrodipheny la mine, and dibutyl p htha Iate as major constituents. A sample of the propellant is dissolved in acetone. One aliquot i s polarographed without further treatment and the wave height obtained is ascribed to total nitrate-i.e., nitroglycerin, pentaerythritol trinitrate, and 2-nitrodiphenylamine. The second aliquot is reacted a t room temperature with sodium hydroxide in ethyl alcohol, the nitroglycerin being decomposed by the sodium hydroxide, and the wave height ascribed to pentaerythritol trinitrate and 2-nitrodiphenylamine. The second wave height is corrected for the 2-nitrodiphenylamine present (determined spectrophotometrically) and the pentaerythritol trinitrate content is calculated or taken from a previously prepared standard curve. The first, or total, wave height is then corrected for both pentaerythritol trinitrate and 2-nitrodiphenylamine and the nitroglycerin content is calculated or taken from a prepared standard curve.
S
the development of a simple one-step synthesis ( 2 ), pentaerythritol trinitrate has been used in various nitrocellulose propellant formulations. The pentaerythritol trinitrate currently manufactured contains the di- a n d tetranitrated molecules as impurities. These impurities are added t o the propellant and determined by these procedures as pentaerythritol trinitrate. Typical constituents present in propellants which could be determined polarographically are dibutyl phthalate, 2-nitrodiphenylamine, nitroglycerin, and pentaerythritol trinitrate. As methods of analysis are available for dibutyl phthalate (4), nitroglycerin (S), and 2-nitrodiphenylamine ( I ) , a rapid method for determining pentaerythritol trinitrate in the presence of nitroglycerin was needed. The polarographic waves for all these constituents (except dibutyl phthalate) overlap and are indistinguishable with techniques now employed. A recent study in these laboratories indicated t h a t nitroglycerin could be destroyed by alcoholic alkaline hydrolysis and that some other organic nitrates (such as the tri- and tetranitrates of pentaerythritol) were only slightly, if a t all, attacked by the base under the conINCE
ditions imposed. After several modifications of the Whitnack et al. procedure for nitroglycerin (S), a technique was developed for polarographically determining the total nitrate in the sample, decomposing the nitroglycerin by alkaline hydrolysis, and polarographically determining the remaining nitrate. iilthough no previous separation of nitrocellulose \Vas made, varying the amount of nitrocellulose present in the sample produced no apparent effect on the polarographic waves for pentaerythritol trinitrate or nitroglycerin ivith either the regular or reacted procedure. The interference due t o the presence of heavy nietal salts is negligible with the amounts normally found in propellant formulations. MATERIALS AND APPARATUS
-4Cary llodel XI recording spectro-
photometer with 1-em. quartz cells a as used for the determination of 2-nitrodiphenylamine. A Sargent hIodel XXI recording polarograph, employing a dropping mercury electrode and a mercury pool anode, was used to obtain and record the polarographic data. All samples were polarographed with no damping in 30-nil. beakers immersed in a constant temperature bath and VOL. 31, NO. 9, SEPTEMBER 1959
1485
were flushed with nitrogen previously scrubbed with 70% ethyl alcohol. The cell resistance was less than 1000 ohms with both procedures.
PROCEDURE
and reacted samples. The wave heights obtained are expressed as microamperes and are corrected for the presence of the propellant. The diffusion current quotient is then calculated for each aliquot and each series is averaged. Standard curves may be prepared from the data and used in the calculations in place of diffusion current quotient.
Weigh a 0.5-gram sample of the propellant, transfer i t t o a 50-ml. glassstoppered volumetric flask, and add Solvents. E t h y l alcohol (9570) approximately 25 ml. of acetone. Place mas obtained from LAC Chemicals, the flask on a mechanical shaker or Culver City, Calif., and t h e acetone shake by hand until the sample diswas obtained from t h e Shell Chemical solves. Dilute the sample t o volume Gorp., Los Angeles, Calif. The pentawith acetone. erythritol trinitrate was originally Total Sample. Pipet a 5-ml. aliquot obtained from Picatinny Arsenal, of the sample into a 25-ml. volumetric Dover, 5 . J., and was used without flask and dilute t o volume with 9570 further purification. The nitroglycethyl alcohol. Pipet a 10-ml. aliquot erin was extracted from dynamite; of this into a 30-ml. polarographic all other chemicals used were of beaker, add 10 ml. of t h e supporting electrolyte (tetramethylammonium standard reagent grade. Reagents. SODIUM HYDROXIDE chioride), stir t h e solution, flush with REAGENT.Transfer 12.5 ml. of a 5 N nitrogen (previously scrubbed with aqueous sodium hydroxide solution 70% ethyl alcohol) for 5 minutes, t o a 50-ml. glass-stoppered volumetric and polarograph. A second polarogram flask and dilute t o t h e mark with should be determined immediately and 95% ethyl alcohol. the wave heights of the two averaged. TETRAMETHYLAMMONIUM CHLORIDE. The wave obtained a t approximately Dissolve 27.65 grams of the solid in 0.82 volt (us. mercury pool) is due to the 175 ml. of distilled water, transfer to a nitroglycerin, 2-nitrodiphenylamine, and pentaerythritol trinitrate in the sample. glass-stoppered 250-ml. volumetric flask, add ?I ml. of a 1 t o 1000 methyl red Reacted Sample. Pipet a 5-ml. solution, and dilute to volume with 95% aliquot of t h e original sample into ethyl alcohol. another 25-ml. volumetric flask and STANDARD SOLUTIONS OF PROPELLAKT dilute almost t o volume with 95% INGREDIENTS. Make up standard soluethyl +alcohol. Pipet a 1-ml. aliquot tions of nitroglycerin, dibutyl phthalate, of t h e sodium hydroxide reagent into pentaeMhrito1 trinitrate, and 2-nitrot h e flask and dilute t o volume with diphenylamine in appropriate concen95% ethyl alcohol. Shake t h e flask trations in 95% ethyl alcohol so t h a t t o ensure homogeneity, take a 10-ml. aliquots of 0 to 5 ml. added t o the 25-m1. aliquot, and treat i t in t h e same reaction flask wiN cover the expected manner as the total sample. Although concentration ranges of the constituents the sample is polarographed immediately after reaction, the time' is not in a regular sample. critical. The authors found no significant change in wave height due to pentaerythritol trinitrate, even after a Table 1. Determination of PETriN 1-hour reaction time. Again the wave heights are averaged. The s a v e height (?To nitroglycerin present) is due to the 2-nitrodiphenylamine and Added, Found, Recovery, the pentaerythritol trinitrate in the Sample hlg. Mg. % sample. 2.60 99.62 1 2.61 Determination of Diffusion Cur2 5 21 5 14 98.85 rent Quotients by Standard Addition 3 7 82 7-91 101.15 Technique. Dissolve a 0.5-gram 10 50 100 77 4 10 42 sample of t h e propellant in acetone as 13 04 100.09 5 13 03 in t h e regular procedure and pipet a Av. recovery (@, yo 100.10 series of 5-ml. aliquots into 25-ml. Std. dev. (est.), o/o 0.90 volumetric flasks. Add appropriate Std. dev. of mean of 5. aliquots (0 t o 5 ml.) of t h e standard % 0.40 solutions of t h e propellant conConfidence range (95Y0), % 1 0 0 . 1 0 ~ 1 . 1 7 stituents t o t h e flasks and treat the solutions as in the procedures for total
Table
1 2 3 4 5
7.58 9.31 5.48 3 73 1.71
3.34 1.44 5.24 9.78 8.86
4.26 5.11 3.41 2.56 1.70
3.33 1.45 5.20 9.73 8.89
4.27 5.15 3.43 2.57 1.72
II.
CALCULATIONS
The diffusion current quotient (I,) is the wave height, in microamperes, due to a particular compound divided by the number of milligrams of that compound in the polarographic solution. The superscripts 1 and 2 designate the factor for the total sample and reacted sample, respectively. Thus, I : (2NDPA) represents the diffusion current quotient for 2-nitrodiphenylamine in the total sample and I: (PETriN) represents the diffusion current quotient for pentaerythritol trinitrate in the reacted sample. The wave height due to the 2-nitrodiphenylamine present in both the total and reacted samples is found by multiplying the milligrams of 2-nitrodiphenylamine (determined spectrophotometrically) by the respective I , found previously in the preparation of standards: ( I : 24'DPA) (mg. PNDPA) = ga.l 2NDPA
and (1; 2S.DP.4) (mg. 2SDP.1) = pa
This calculation is made for both the total and reacted samples and the results are subtracted from the appropriate 5%-areheight. The pentaerythritol trinitrate content of the sample is found by dividing the xave height of the reacted sample (after correcting for 2-nitrodiphenylamine) by I: for pentaerythritol trinitrate: (ga.2)
ANALYTICAL CHEMISTRY
PETriN
Determination of PETriN
99.70 100.59 99.24 99.37 100.34
100.23 100.59 100.59 100.39 101.18
NG Av. recovery (3),yc 99 87 100 60 Std. dev. (est.), 0 20 0 13 Std. dev. of mean of 5, SC 0 OF) 0 06 100 60 + 0 1T 99 87 f 0 25 Confidence range (9570), % 0 2.71 mg. of DBP and 2.47 mg. of 2XDP.4 present in all samples.
1486
- (ga.2 2NDPA) I: PETriN
The pentaerythritol trinitrate content in equivalent C1a.l mag be found by
2.61 5.21 7.82 10.42 13.03
2.91 2.91 2.91 2.91 2.91
2.61 5.21 7.81 10.51 13.06
2.96 2.90 2.89 2.89 2.89
100.00 100.00 99.87 100.86 100.23
101.72 99.66 102.06 99.32 99.31
XG and 2NDPA Present
NG Present PETriX
* 2TDPA
PETriPr' 100 0 0 100
19 36 16 19
* 0 45
NG 100 41 1 36 0 61 100 41 f 1 70
multiplying either the pae2due to pentaerythritol trinitrate by a ratio of the total, and reacted diffusion current quotients for pentaerythritol trinitrate: (pa.*PETriN)
(5)
=
pa.1 PETriN
or by (mg. PETriN) ( I : PETriN) = pa.’
PETriN The pa.1 due to pentaerythritol trinitrate and 2-nitrodiphenylamine are subtracted from the total wave height of the total sample to obtain the nitroglycerin (KG) content: (pa.1) - (pa.’ 2NDPA
+ pa.1PETriN) = pa.1 NG
and
RESULTS AND DISCUSSION
The results obtained by applying this procedure t o synthetic samples are given in Tables I t o 111. First, the analyses were made without reaction with alcoholic sodium hydroxide. Next, the pentaerythritol trinitrate was determined directly from a reacted solution and the nitroglycerin was found by difference between the unreacted solution and the reacted solution. When 2-nitrodiphenylamine is also present i t must be determined by independent means and a correction must be applied to the results for both pentaerythritol trinitrate and nitroglycerin. During the original attempts to determine pentaerythritol trinitrate by the hydrolvtic degradation of nitroglycerin with alcoholic sodium hydroxide, the samples were neutralized after the reaction had progressed for 1 hour. Erratic results were obtained and further investigations indicated that a varying p H was the cause. The wutralization was omitted and the reacted sample was polarographed in the alkaline media. The waves obtained were rather illdefined, but further study showed that a higher concentration of supporting electrolyte made them more definite. A concentration of 0.25121 tetramethylammonium chloride was found t o be optimum. Because of some precipitation in the reacted sample, it was necessary to adjust the n-ater-alcohol ratio of the polaro-
graphic solution. For convenience, the supporting electrolyte solution was used to incorporate modifications, and the resulting polarographic solution for all samples was 0.25144 in tetramethylammonium chloride, containing (by volume) 70% ethyl alcohol, 10% acetone, and 20% distilled water. A sample containing nitroglycerin, when polarographed in the alkaline media (apparent p H 11.6), gave waves for pentaerythritol trinitrate, 2-nitrodiphenylamine, and dibutyl phthalate with half-wave potentials approximately 0.3 volt more positive than those obtained in neutral solution (apparent p H 5.6). Table I11 compares half-wave potentials obtained for each of the constituents in neutral and alkaline media. These values were determined from the undamped wave for the particular compound by graphically solving the i d / 2 intersection with the line drawn tangent to the slope of the wave. Each value represents an average of five determinations with varying concentrations and is presented for comparison rather than as an absolute value. The positive shift of these values in basic solution is due to a shift in potential of the mercury pool (0.037 t o 0.338 volt us. S.C.E.). I n Table 111, the half-wave potential attributed to the nitroglycerin in the sample shifts approximately 0.3 volt to a more negative value and is probably due to a hydrolysis product of nitroglycerin rather than to the nitroglycerin itself. An attempt was made to determine the nitroglycerin in the sample using this wave but, because of its low sensitivity to changes in concentration (1; = 0.4) and the possibility of a s l o ~but continuing reaction of initial products, erratic results were obtained. Although this wave is unsuitable for the precise determination of nitroglycerin, it could be used for a rough estimation of the nitroglycerin content of the sample. For the standard and sample determination, the polarograms were recorded with the damping switch in the off position and a t a sensitivity selected to keep the wave height betxeen 100 and 150 nim. The diffusion current TTas measured from the top of the pen oscillations, and a t least two polarograms were determined for each solution to assure temperature equilibrium. With high nitroglycerin content or high wave height, the nitroglycerin-pentaerythrito1 trinitrate wave tended to maximize.
Table 111. Half-Wave Potentials and Diffusion Current Quotients for Various Propellant Constituents
El/* (CIS. Hg Pool)
Constituent PETriN NG DBP 2NDP.4
Total AnaJysis -0 772 -0.735 -1.74 -0 835
I,, pa./Mg. 1 94 2.59 0.92 1.86
Reacted Analysis -0 496 1.89 SGa -1.02 0 44 DBP -1 41 0 86 -0 518 1 88 2SDP.4 Total Reacted Analysis Analysis PETriS
E ( H ~pool), volt
US.
S.C.E. 0 037 0 338 Capillary constant (1 volt applied potential), mg.2’3sec.-1’2 1 070 1 059 Cell resistance, ohms 400 800 -4pparent pH 56 116 Temperature, C. 30 30 O
n
Hydrolysis product.
I n these cases, the diffusion current line \vas drawn tangent to the level, more negative portion of the trace beyond the maximum. This method is not specific for the pentaerythritol trinitrate molecule but would determine such organic nitrates as are not destroyed by alkaline alcoholic hydrolysis, such as the tetranitrate of pentaerythritol. The Z-nitrodiphenylamine and other nitro derivatives of diphenylamine which exhibit similar polarographic behavior must be determined independently and a correction applied. LITERATURE CITED
(1) Anderson, F. P., Pierson, R. H.,
Gantz, E. St. Clair, NAVORD Rept. 1290, 1951. (2) Camp, A. T., Morans, N. S., Elrick, D. E., Preckel, R. F., J. Am. Chem. SOC. 77,751 (1955). (3) Whitnack, G. C., Mayfield, If. SI., Ganta. E. St. Clair.‘ ANAL. CHEM. 27,899-901 (1955). (4) Whitnack, G. C., Weaver, R. D., Gantz, E. St. Clair, NAVORD Rept. 5022, 1956. RECEIVED for review January 16, 1959. -4ccepted June 15, 1959.
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