Polarographic Determination of Nitroglycerin in Double-Base Powder GERALD C. WHITNACK, MARGARET M. MAYFIELD, and E.
ST.
CLAIR GANTZ
U. S. N a v a l Ordnance Test Station, lnyokern, China Lake, Calif. A rapid and reliable method of analysis for nitroglycerin in double-base powder depends upon an alcoholic extraction of nitroglycerin from the powder and a subsequent polarographic analysis. Nitrocellulose and phthalate esters do not interfere in the analysis. If 2-nitrodiphenylamine, dinitrotoluene, or other nitro compounds are present, a small correction must be applied. The diffusion current is a linear function of the concentration of nitroglycerin. The E112 value of nitroglycerin in 0.5M tetramethyl ammonium chloride (759’0 alcohol), referred to the saturated calomel electrode, is -0.70 volt. The data obtained have a precision, show-n by a standard deviation obtained from five values, of about 9 parts per thousand. The method can be used readily in pilot plant control work and should have wide application in powder analysis.
S
EVERAL methods for the determination of nitroglycerin in
double-base povider (so called because it contains both nitroglycerin and nitrocellulose) have been reported. They give reliable results; but the methods are time-consuming, require considerable attention on the part of the analyst, and often need blonk corrections in order to obtain satisfactory data (4). 1-agoda ( 1 0 ) determined aliphatic nitrate esters bj- a hydrolysis of the ester in 62.5% sulfuric acid and the nitration of m-xylenol by the nitric acid liberated. The nitro xylenol is then readily volatilized by a steam distillation, rvhich permits the application of the method to complex systems. The nitrovylenol reacts with alkalies forming a n intense yellow-colored solution t h a t obeys Beer’s law. This is not a rapid method of anal>-sisand a loss of the nitroxylenol may occur during the steam distillation if certain precautions are not rigidly followed. Other colorimetric methods (3) have been reported for nitroglycerin. These methods depend upon forming a color with the hydroly& products of nitroglycerin and then comparing the color with a series of standards. Examples of reagents used in these methods are ( A ) phenoldisulfonic acid and ( B ) sulfanilic acid solution with ]-naphthylamine hydrochloride solution. Pure I i o t a d u m nitrate is used as a standard in A and a solution of sodium nitrate is used as a standard in B. These methods require considerable time in preparing standards and are subject t o errors from inorganic nitrates, nitrites, and metals that liberate hydrogen in a n acidic medium. Extraction methods will eliminate these interfering contaminxnts. However, such methods only increase the time of analysis and are another source of error. Probably t,he ferrous-titanous chloride method of Becker ( 1 ), who determined nitroglycerin, and nitroglycerin and dinitrotoluene in admixture, is the most widely used method for the determination of nitroglycerin in double-base powder today. I n this method the powder sample isextracted by asuitable Polvent (diethyl ether, methylene chloride, 65 or 70% acetic acid). The estlact is digested in a mixture of acetic acid, hydrochloric acid, and ferrous chloride or ferrous ammonium sulfate, under carbon dioxide, and the resulting ferric ion is titrated with standard titanous chloride-using ammonium thiocyanate as the indicator. Reliable results are obtained regardless of the estroetion method used [a round robin test shovied a standard deviation of 0.27’~ within a lnboratory and 0.3% betLveen laboratories (531. Horn-ever, the total lapsed time for analysis is rather long. Furthermore, one must standardize solutions daily, protect the ferrous and titanous solutions from oxidation, and make frequent blank &terminations on a11 soliutions used.
I n a search for a faster yet precise method for analysis of nitroglycerin in double-base polvder, a reference (6) was found which stated t h a t nitroglycerin was reducible a t the dropping mercury electrode, but gave no data on conditions necessary for reduction. The behavior of nitroglycerin a t the dropping mercury electrode was then studied in this laboratory (9). Based upon this work, a method for extraction and polarographic analysis of nitroglycerin in double-base powder was developed. This report presents and describes the details for the n e v method, vihich is both rapid and precise. APPARATUS AND MATERIALS
Apparatus. Erlenmeyer boiling flasks (125-ml., -$’ 24/40) were connected with Allihn-type condensers (400-mm., -$’ 24/40) for all extraction work. T h e extractions were made on a six-unit Vari-Heat apparatus manufactured by the Precision Scientific Co., Chicago, Ill. Samples n-ere filtered through Whatnian S o . 41 filter paper into 100-ml. volumetric flasks. Polarograins of the samples were obtained in a constant-temperature bath a t 30” C. (h0.2’). Small borosilicate glass beakers (30-ml.) vxre used as polarographic cells. A rubber stopper, to which were attached the dropping mercury electrode, contact electrode, and a glass tube with a fritted glass disk, was placed over the top of the small beakers. Dissolved oxygen was removed from the solution with pure nitrogen just prior t o the polarographic analysis. The nitrogen was passed into a 75% ethyl alcohol solution, and then into the polarographic cell. A Sargent Nodel S X I recording polarograph mas used in all analyses. -2Fisher Elecdropode \vas used to obtain values for Eli2 versus the saturated calomel electrode (SCE). A Sargent Model I11 manual polarograph was found to be applicable for pilot plant control work. Materials. T h e supporting electrolyte stock solution used in the polarographic analysis was 0 . 1 X tetramethyl ammonium chloride (Eastnian Kodak, practical grade) containing 10 ml. of O.lyoalcoholic solution of methyl red per 500 ml. of solution. Ninety five per cent ethyl alcohol was used in extraction and in preparation of solutions. -1 sample of pure nitroglycerin mas obtained from Kieselguhr dynamite b y high-vacuum distillation in a Hickman still and this material analyzed 99.2% nitroglycerin by the ferroustitanous chloride method ( 1 ) . A semimicro Dumas nitrogen analysis showed that the material contained 18.33% nitrogen. This value agrees with the ferrous-titanous chloride analysi The standard graphs used to obtain polarographic data in t h work were prepared from the pure nitroglycerin. The Eli2value, referred to the saturated calomel electrode, for this pure nitroglycerin in 0.5M tetramethyl ammonium chloride (75% ethyl alcohol), was -0.70 volt. T h e cell resistance was less than 1000 ohms as measured with a Wheatstone bridge and the iR drop correction was therefore negligible. Redistilled mercury (c.P. grade) was used as the anode in all beakers for polarographic analysis. il capillary with a drop rate of 6.5 seconds per drop a t - 1.00 volt was used in all work. T h e value for m2’3t”e ( 7 ) was 1.351 mg.2’3 sec.-l’z. The total pressure ( h ) applied to the dropping mercury was 91.5 em. PROCEDURE
Samples of povider are microtomed to 0.002-inch thickne-= or put through a Wiley mill using a 20-mesh screen. Two procedures are then followed: an extraction of the nitroglycerin and a polarographic determination of the amount of nitroglyrerin present. Extraction of Nitroglycerin. A40.5- to 1.0-gram sample of the microtomed or \Tiley-milled poivder is weighed and placed in a 128-ml. Erlenmeyer flask. Fifty milliliters of 95% ethyl alcohol are added and the flask is connected t o a n Allihn condenser. The sample is refluxed a t gentle heat on the Vari-Heat unit for 0.5 hour, and then filtered through a No. 41 R h a t m a n filter paper. T h e flask is carefully washed with 95y0 ethyl alcohol, and the rinsings are added through the filter. The filter paper is rinsed with 95% ethyl alcohol, and the filtrate is made up to volume in a 100-ml. volumetric flask. 899
900
ANALYTICAL CHEMISTRY
Polarographic Determination of Nitroglycerin. An aliquot of the diluted extract is added to 95% ethyl alcohol in the 30-ml. beaker so that the final volume of 95% ethyl alcohol in the beaker is 15 ml. (usually 5-nil. aliquot and 10 ml. of 9553 ethyl alcohol). Five milliliters of the 0.1-44 tetramethyl ammonium chloride solution is then added t o the beaker. The total volume of 20 ml. in the beaker is stirred well, and the beaker is placed in the constant-temperature bath. The stopper containing the dropping mercury electrode assembly is put in place, and the glass tube for introduction of nitrogen is lowered into the solution. After the nitrogen is flushed through the solution for 5 to 10 minutes, the glass tube is raised and nitrogen is allowed t o flow over the solution during analysis. A polarogram of the sample is obtained from 0.0 to - 1.60 volts. Duplicate polarographic waves are produced for the solution, and the average wave height in millimeters is referred t o a standard graph for nitroglycerin. T h e standard graph is obtained from pure nitroglycerin or by extracting nitroglycerin from a sample of “premix” (nitroglycerin and nitrocellulose in the proportions expected in unknown samples of poivder) in the manner described above. The nitroglycerin extract is then made up to volume in a 100-ml. volumetric flask. Aliquots of this solution are added to 5 ml. of the 0.1JI tetramethl 1 ammonium chloride solution in a 30-ml. beaker. The proportion of alcohol to 0.1M tetramethyl ammonium chloride solution should be 3 to 1. Thus, the total volume of solution in the 30-ml. beaker is 20 ml., of which 15 ml. are 95y0 alcohol. The solutions are polarographed according to instructions given in the preceding paragraph. The data should show a linear relationship when a plot of wave height in millimeters versus milligrams of nitroglycerin is made. The concentration of nitroglycerin in the premix is determined by the titanous chloride method. This graph is then used for the calculation of nitroglycerin in the original sample. The percentage of nitroglycerin in the powder is calculated by referring the average wave height in millimeters to the standard graph and obtaining the milligrams of nitroglycerin for 100 ml. of solution. Thus, Per cent nitroglycerin =
mg. of nitroglycerin per 100 ml. X 100 xt. of sample
Table 11.
Recovery of Nitroglycerin from Two Ty-pes of Double-Base Powder Grams 0.3895 0.4001 0.3993 0.4006 0.3903
Type
Nitroglycerin Nitroglycerin Presenta, % Found, %
2.0310 2.0132 2.0438 2.0061 2.0616
b
%
22.4 22.4 22.4 22.4 22 4
22.6 22.1 21.7 22.1 22.2
100.4 98.7 96.9 98.7 99.1
23.4 23.4 23.4 23.4 23.4
23.2 23.4 23.5 23.2 23.6
99.1 100.0 100.4 99.1 100.8
Sitroglycerin recovery Type 1 Average, 2 98.8 Standard deviation, s 1,25 Standard deviation of mean (of 5 ) , sm 0 625 Confidence range (95%) 98.8 f 1.74 a
Recovery,
Type I1 99.9 0.59 0.295 99.9 i. 0 . 8 2
Amount of nitroglycerin present determined b y titanous chloride method. Cordite cannon propellant. A low energy rocket propellant.
nificant figures, the titanous chloride data reported in the tables Ivere rounded-off to this number of figures. The indices of precision used in the statistical analysis data presented in the tables are as follows: Standard deviation (estimate), s = 4 2 (z - z ) 2 / ( n - 1) Standard deviation of a mean of n, sm = s/& Confidence range (fiducial limits) = Z i- ts, where Z = mean of n observations of 2, and t = “Student’s” t ( 8 ) for the significance level desired and n - 1 degrees of freedom. For the 1-in-20 significance level and means of five (4 degrees of freedom), t = 2.78. DISCUSSION
Table I.
Recovery of Nitroglycerin from Known Mixtures Nitroglycerin Present”, Mg. 169 172 170 173 177
Nitroglycerin Found, Mg. 168 170 172 172 176
Sample, Mg. 507.8 515.0 510.1 519.8 531,l Average, Z Standard deviation, s Standard deviation of mean (of 6 ) , srn Confidence range (95%) 0
Recovery,
%
99.4 98.8 101.2 99,4 99.4 99.6% 0.91% 0.41Oj, 99.6 i 1.14%
Sample analyzed 33.3A nitroglycerin by titanous chloride method.
The success of the method depends upon the efficiency of a n extraction with alcohol and the fact that most other materials in double-base powder which may be extracted by the alcohol are either not reducible polarographically or not reducible in a voltage range that interferes with the nitroglycerin wave. The wave for
Table 111. Reproducibility of Duplicate Analysis of Nitroglycerin Nitroglycerin Found, Presents,
% DATA
T o determine the precision and reliability of the method, several experiments were made with a double-base powder sample that had been analyzed previously for nitroglycerin by the titanous chloride method. The results (Table I) indicate that the polarographic analysis of nitroglycerin has a precision, shown by a standard deviation obtained from five values, of about 9 parts per thousand. Sitroglycerin was determined in txvo other types of double-base powder by the polarographic method described. Data presented in Table I1 indicate average recovery values of 98.8 and 99.9%, respectively, with standard deviations of 6 to 12 parts per thousand. On the basis of these results confidence ranges of 98.8 i 1.74 t o 99.9 zk 0.82% may be expected for nitroglycerin recovery. Several samples of the same type of pon-der mere analyzed in duplicate by the method described. The results (Table 111) show good agreement with the ferrous-titanous chloride method, and show a n average deviation of 0.6% between samples in the range of 20 to 40% nitroglycerin. Since polarographic data are meaningful to only three sig-
22.2 22.1 43.2 43.5 33.0 33.1 30.5 30.2 31.7 31.5
%
Difference, 70Abs.
22:4
0.1
0.5
4410
0.3
0.7
33:3 30: 1 3i:2 Differences (average) Standard deviation, s Deviations (average) Standard deviation, 8
...
...
... 0.1 ... 0.3 ... 0.2
Deviatiod,
%
... ...
...
0.3 ,..
1.0
...
0.6
0.2070 abs. 0.10% abs. 0.6% 0.26%
5 Amount of nitroglycerin present determined by titanous chloride method.
nitroglycerin covers a wide voltage range (from about -0.20 t o -1.50 volts). If materials are present which are reduced in this range, they must either be removed or their wave height equivalents accounted for in the total observed wave over this voltage range. For example, if 2-nitrodiphenylamine is present in the powder, the amount is determined by some other method of analysis and the wave height equivalent is then subtracted from
V O L U M E 2 7 , NO. 6, J U N E 1 9 5 5 the total observed wave height for nitroglycerin. Since the amount of 2-nitrodiphenylamine in the ponder is small, there is only a very small correction, and sometimes a t the instrument sensitivity used there will be no interference from this material. Most nitro compounds, such as dinitrotoluene, will be reduced within the voltage range of nitroglycerin; however, o-phthalate esters are reduced a t about -1.90 volts (8). Thus, nitroglycerin and o-phthalate esters can be determined either alone or in admixture by the polarographic method of analysis. No interference from nitrocellulose was observed during this study. T h e amount of nitrocellulose extracted by 95y0 ethyl alcohol appears to be a negligible factor in the analysis. Because oxygen produces a wave in the same voltage range as nitroglycerin, it must be removed Rith pure nitrogen prior to the polarographic analysis. I n the determination of nitroglycerin, the aliquot of the sample should be adjusted so that the wave height for nitroglycerin is a t least half of the full scale division for the instrument. This will give greater precision in measuring wave heights. For samples of double-base powder containing about 30% nitroglycerin a 5-ml. aliquot of the ethyl alcohol-diluted extract will give about a 150-mm. wave height a t a sensitivity of 0.150 Fa. per mm., using the Sargent Model X X I polarograph and a capillary with a value for m2/3t1’60fabout 1.35 mg.2’3se~.-1/2
901 ACKNOW LEDGhlENT
The authors wish to evpress their thanks to the Solid Propellants Branch a t the U.s. Naval Ordnance Test Station, Inyokern, Calif., for samples of powder used in this investigation, and to Rlichael D. Castronova for samples of pure nitroglycerin. This paper is published nith the permission of W.B. RlcLean, technical director of the U.S. S a v a l Ordnance Test Station. LITERATURE CITED
(1) Becker, W.
W., ISD.ESG. CHEM.,AXAL.ED.,5 , 152-4 (1933). ( 2 ) Fisher, R. -I., “Statistical Methods for Research Workers,” 10th ed., Oliver and Boyd, Edinburgh, 1946. (3) Fuller, H. C., “Chemistryand Analysis of Drugs and Medicine,”
pp. 736-8, Wiley, Xew York, 1920. (4) Hirschhorn, I. S.,ANAL.CHnf., 19, 880-2 (1947). (5) J.AKAF Analytical Chemistry Panel for Solid Propellants, private communication. (6) Kolthoff, I. AI., and Lingane, J. J., “Polarography,” Rev. ed., p. 317, Interscience, Ken. York, 1946. (7) Ibid., pp. 60-9. (8) Whitnack, G. C., and Gants, E. S. C., ASAL. CHEM.,25, 553-6 (1953). (9) Whitnack, G. C., and Gants, E. S. C., J . Am. Chem. Soe., 76, 4711-4 (1954). (10) Yagoda, H., IND.ESG. CHEM.,d s a ~ED., . 1 5 , 27-9 (1943). RECEIVEDfor review September 14, 1954. Accepted February 5 , 195%
Polarographic Determination of Traces of Copper, Nickel, Cobalt, Zinc, and Cadmium in Rocks Using Rubeanic Acid and 1-Nitroso-2-naphthol LLOYD E. SMYTHE and BRYAN M. GATEHOUSEl Chemistry Department, University of Tasmania, Hobart, Australia
Trace quantities of copper, nickel, cobalt, zinc, and cadmium may be separated from interfering elements by precipitation as metal rubeanates, followed by separation of cobalt as the nitrosonaphtholate and estimated by subsequent polarographic analysis. The utility of the method is discussed and a comparison is made with colorimetric and spectrographic methods.
T
RACE element studies in rocks using spectrographic and colorimetric methods gave values which differed seriously enough to warrant checking by a suitable polarographic method. The most promising method appeared to be t h a t of Malyuga (3) for the “polarographic determination of copper, cadmium, nickel, zinc, and cobalt in minerals, soils, natural maters, and organisms.” Following the 7%-orkof Rby and ROy (6)rubeanicacidhasbeenused in several analytical methods, mainly for the determination of copper, nickel, and cobalt in the presence of iron, aluminum, and other interfering ions. The more stable, insoluble, metal rubeanates (copper, nickel, cobalt, zinc, and cadmium) are selectively and quantitatively precipitated in the presence of many other ions under carefully controlled conditions. However, Malyuga’s method applied to rock samples and synthetic solutions gave very poor recoveries including ill-defined polarographic waves in the range 5 to 400 p.p.m. of the elements. INVESTIGATION OF METHOD
If Malyuga’s method was to be compared with spectrographic and colorimetric methods, i t became apparent t h a t an investigation of the variables involved was desirable. This investigation
was carried out ( I ) , and in addition an investigation of the formation and properties of metal rubeanates was commenced (6) and will be published later. Maximum recoveries of the rubeanates concerned were obtained a t p H 8 ( I ) . Adjustment of the solution prior to precipitation by adding 4-V ammonium hydroxide until slightly alkaline (3) did not offer sufficient control for reproducibility and maximum recovery. Losses also occurred where normal filtration and ignition procedures were employed (3). Centrifuging of the precipitated rubeanates and digestion with perchloric acid were found preferable to destruction of the rubeanates and filter paper with sulfuric acid and subsequent ignition ( 3 ) . It is probable also that losses on ignition might have resulted during the thermal decomposition of the rubeanates, as some rubeanates produce a red liquid containing part of the metal on destructive distillation ( 5 , 6). KOsignificant differences in recoveries of precipitated rubeanates were observed using times of precipitation of 12 and 36 hours a t 18’ C. Variation of the p H of the final supporting electrolyte used by Malyuga (3) showed that the procedure of adding a few drops of ammonium hydroxide, to make the final supporting electrolyte slightly alkaline, did not give consistent diffusion currents, particularly for copper and nickel. For euample, diffusion currents for a given concentration of copper in the supporting electrolyte, with changing pH, mere: pH 6.0 = 0.10 pa.; pH 8.0 = 0.145 pa.; p H 10.0 = 0.25 pa. Obviously, reproducibility with trace-element estimations could not therefore be obtained without careful control of final supporting electrolyte conditions. A t p H 7 to 8 copper is likely to be 1 Present address, Department of Inorganic Chemistry, N.S.W. University of Technology, Sydney, Australia.
