Vol umetric Dete rminatio n of Perc hIorates in Pyrotechnics BERNARD J. ALLEY and HIRAM W. H. DYKES Propulsion laboratory, U. S. Army Missile Command, Redsfone Arsenal, Ala.
b A rapid, direct procedure for determining the concentration of ammonium and potassium perchlorates in pyrotechnics was developed. In the procedure, the perchlorate ion is quantitatively reduced by titanium hydride in 1 :3 sulfuric acid; the reduction is followed by potentiometric titration of total chloride with 0.1N silver nitrate reagent. Duplicate analyses can be completed by a single operator within 45 minutes. The estimated relative standard deviation for an individual analysis of reagent grade potassium perchlorate is 0.1 2%, based on analyses of 10 samples. The mean error for the same analyses was zero.
r
A PYROTECHNIC MIXTURE, the degree of correlation between the oxid'zer concentration and the ballistic performance variables is generally high. Accordingly, the purity of the oxidizer raw material and its concentration in t h o nlixture must be rigidly controlled to a'wre optimum ballistic performance. Among the oxidizers most extensively uccd in pyrotechnic mixtures are ammonium and potassium perchlorates. LTcthods in current use for analyzing perchlorates have bc?m reviewed and summarized by Crump and Johnson ( 9 ) ; Kurz, Kober, and Berl (3); Nabar and Ramachandran (4): and Burns and Muraca ( I ) . RibeudQ (5)has waluated and compared Paar-bomb ignition, titanous reduction, and tetraphenylphosphonium perchlorate precipitation procedures for determining perchlorates in polysulfide propellants. The volumetric methods described in the literature fall into two main classes: wet chemical reduction of the perchlorate ion, followpd by back-titration of the excess reducing agent; and thermal or wet chemical reduction of the perchlorate ion with subsequent titration of the chloride formed. Titanous compounds have been the preferred reducing agents for the wet chemical techniques because the titanous ion reduces the perchlorate ion quantitatively and more rapidly than many other reducing agents thst poqsess even greater reducing potential. The combination of titanous reducing agents and the first class of techniques is highly satisfactory for general perN
1124
ANALYTICAL CHEMISTRY
chlorate determinations, but presents diffculties when applied to pyrotechnic mixtures. Methods based on backtitration of the titanous ion are subject to interferences from both the reducing and oxidizing agents in these mixtures, and the titanous reagent must be constantly maintained in a nonoxidizing atmosphere. The second class of procedures has proved more accurate in determining perchlorates in pyrotechnic mixtures. Those methods employing thermal reduction, however, are generally time-consuming and require special apparatus and safety precautions. The method described here is a wet chemical reduction type which eliminates some of the disadvantages associated with existing methods by utilizing titanium hydride as the reducing agent for the perchlorate ion. Reduction is completed rapidly, and no special storage or handling of the titanium hydride is necessary. The method is sufficiently rapid, precise, and accurate to b. used for both the assay of perchlorates and the routine control determination of perchlorates in finished pyrotechnic mixtures. It has been successfully applied to polysulfide propellants and igniter mixtures produced a t this laboratory. EXPERIMENTAL
Apparatus. A Sargent-Malmstadt spectroelectrometric titrator (Sargent KO.5-29700) was used for the titration of chloride. The electrode combination consisted of a shielded platinum reference electrode immersed in the silver nitrate reagent and a platinum ring indicator electrode surrounding the glass stirrer. The titrant flow rate was maintained a t 6 ml. per minute by a 50-ml. controlled delivery buret (Sargent No. 5-11087). The grid bias on the first thyratron tube (2D21) in the control unit was set a t the factory recommended value of -4.0 volts d.c. with respect to ground, and the voltage was checked periodically. Mzterids. Titanium hydride powder, J-1715 AR, was purchased from Metal Hydrides, Inc., Beverly, Mass., and used as received. The water was distilled and essentially free of chlorides. n'ith the exception of the perchlorates, the raw materials which went into the synthetic mixtures were of the same pyrotechnic grade as those used in production mixtures. All
other materials used in the procedure were reagent grade. Procedure. Transfer a sample containing 0.20 to 0.25 gram of ammonium perchlorate, or an equivalent weight of potassium perchlorate (weighed to *0.1 mg.), to a 250-ml. wide-mouthed Erlenmeyer flask that has a ground-glass joint. Add, in order, 0.5 gram of titanium hydride, 30 ml. of water, and 10 ml. of concentrated sulfuric acid. Connect the flask immediately to a water-cooled Liebig condenser that has a 30-cm. jacket, and reflux the solution a t a moderate rate on an electric heater for 15 minutes. Remove the condenser and flask from the heater, rinse the inside of the condenser and the flask neck with water, and then immediately place the flask with its contents in an ice bath. Cool the solution to 25" C. and filter it through a 40-ml. Gooch crucible, fitted with an asbestos mat, into a 200-ml. tall-form Griffin beaker. Just prior to titration, add 2.0 grams of ammonium persulfate and stir the solution on the titrator until the ammonium persulfate completely dissolves and decolorization occurs. Titrate the chloride potentiometrically with standard 0.1N silver nitrate reagent. Calculate the percentage of perchlorate in the sample in the conventional manner. The procedure as described is a general one, applicable to several types of pyrotechnic mixtures. For best results, however, minor modifications are introduced for certain applications. These modifications are noted below. RESULTS
The optimum weight range of the perchlorate to use to obtain precision and accuracy was established by analyzing dried reagent grade potassium perchlorate. The results of analyzing ten successive samples over a period of several days are shown in Table I. The small estimated standard deviation and the mean error of zero show that the method is adequate for the assay of potassium perchlorate. Compczrable results were obtained with dried reagent grade ammonium perchlorate. For assay, corrections must be made for such impurities as free chlorides and chlorates, which would be included in the perchlorate determination. The concentrations of these impurities are generally small and are regularly established as a part of the initial perchlorate acceptance testing. For routine control,
Table 1.
Assay of Reagent Grade Potassium Perchlorate
Sample 1 2 3 4 5 6
7 8 9 10
Taken, gram
Found, wt. %
0.2603 0.2651 0.2857 0.2838 0.2508 n 3054 0.3197 0.2700 0.2855 0,2794
99.96 99.89 99.89 100.14 100.04 99.84 99. S i 99.93 100.14 100.14 Av. 100.0 Std. Dev. 0.12
it is sufficient to analyze the perchlorate both before and after its incorporation in the pyrotechnic mixture, without correction for theqe inipurities. The optimum weight of potassium perchlorate to ensure quantitative reduction of the perch orate ion is 0.25 to 0.30 gram. Larger weights have been quantitatively reduced (sample 7, Table I ) , but results are nconsistent even though the amount of titanium hydride used is conqiderably greater than the stoichiometric amount. All the samples listed in Table I were refluxed for 15 minuies in accordance with the general procedure. Actually, if ammonium and potassium perchlorates are analyzed in the absence of other materials, quan1,itative reduction of the perchlorate ion is completed in 10 minutes. The suitability of the procedure for determining potassium perchlorate in igniter mixtures was established by analyzing five synthei ic mixtures repreqenting all the relnti ve concentrations of each ingredient that would be found in production mixtures. All ingredients for each mixture were weighed directly into the 260-ml. Erknmeyer reaction fla-k t o eliminate thr sampling error. The initial mixture compositions and the recovery of potassium perchlorate are shown in Table 11. The results demonstrate that the procedure can be accurately applied t o the iqniter mixtures con4dered. One modification was made t o the general procedure: enough extra ammonium pcrsulfate was added, in increments, to oxidize the additional titanous ions generated by the titanium in the mintureq. Tablc 111 Yhows the results of testing duplicate sqmples to estRb1i.h reproducibility of the method when applied to finished igniter mixtures. Each mixture was a 20-gram batch which had been su3jected t o the standard processing operations of weighing, mixing, drying, and grinding. The experimental error in this series comprises the sampling error, plus errors
Table
II.
Recovery of Reagent Grade Potassium Perchlorate from Synthetic Igniter Mixtures
Mixture Constituent, wt. % weight, Polyiso- Potassium Recovery Nylon butylene perchlorate % gram Aluminum Titanium Boron 0.5039 0.5477 0.5407 0.5356 0.4857
23.70 9.55 19.11
... ...
13.45 21.54 21.10 12.90 20.28
...
...
24:09 14.57
... ...
...
4.27 8.20
4.45 2.51 4.90
58.40 66.40 54.89 58.74 56.95
... ...
99.93 100.05 100.03 100.19 100.00
Av. 100.0 Std. Dev. 0.10
associated with the procedure. Kevertheless, there is no experimentally determined difference between the data variances in Tables I and 111. The recovery of reagent grade ammonium perchlorate from synthetic polysulfide propellants prepared in the same manner as the igniter mixtures is shown in Table IV. The general procedure was modified for these analyses by adding the ammonium persulfate before the filtration step. Similar results were obtained for finished polysulfide propellants in both the cured and the uncured states. Prior to analysis, the cured propellants were cut into strips inch thick to facilitate the degradation of the polymer by the 1:3 sulfuric acid. Special care must be exercised to prevent loss of ammonium perchlorate during cutting. Mention should be made of two conditions which render the procedure either unsuitable or less accurate than presented here. First, the procedure can not be applied when the perchlorate in the pyrotechnic is imbedded in a matrix which is impervious to attack by 1:3 sulfuric acid. Second, cellulose nitrate interferes with the malysis. When this substance is present a t a concentration of about 15 to 25% by weight, the experimental perchlorate recovery may be lower than the theoretical by 1 to 3% relative. DISCUSSION
A few trials may be required to establish a suitable reflux rate during reduction of the perchlorate ion. If the initial rate is too rapid, the solution foams excessively and hydrogen chloride might escape through the condenserparticularly if the flask and condenser walls are dry. If the rate is very slow, the perchlorate ion will not be completely reduced within the 15 minutes. I n this laboratory, the Erlenmeyer flask is placed on an electric heater which has a variable transformer, and the graduated transformer dial is set to give an equilibrium reflux rate of about 20 drops per minute. Since the solution is already hot (concentrated sulfuric
Table 111. Reproducibility of Determination of Potassium Perchlorate in Finished Igniter Mixtures
Mixture 1 2 3 4 5
Sample weight, gram 0.4857 0.4493 0.5225 0.5665 0.4811 0.4053 0.5761 0.4763 0.5348 0.5123
Dupli-
cates, wt. yo 61.95 62.03 58.31 58.02 65.74 65.91 65.32 65.17 56.97 56.89
Difference 0.08
0.29 0.17 0.16 0.08
Pooled Std. Dev. 0 . 1 2 Table IV. Recovery of Reagent Grade Ammonium Perchlorate from Synthetic Polysulfide Propellant Mixtures
KHaClOa
taken, gram
0 0 0 0 0
2462 2906 2474 2069 3554
"4ClO4 Recovery, taken, Recovery, gram % % lor) 99 99 99 100
00
97 96 9.5 00
0 2277 0 2538 0 2111
100 99 100 Av. 100 Std. dev. 0
04 72 19 0 13
acid was added to the water), it is heated for 15 minutes after the dial is set. The heater is turned off immediately after each reduction. Ammonium persulfate oxidizes chloride under the conditions of this procedure. However, the oxidation occurs so slowly that no error is detected in the analysis if titrations are conducted a t any time within 15 minutes after adding the ammonium persulfate. The length of time that elapses between the filtering of the solution and the addition of the ammonium persulfate likewise has no effect on the results. The purpose of filtering the refluxed solution is to remove unreacted material and thus decrease electronic noise and prevent possible fouling of the electrodes. After duplicate determinations, the indicator electrode is cleaned VOL. 36, NO. 6, MAY 1964
1 125
with dilute ammonium hydroxide and a paste of abrasive cleanser to prevent formation of a silver chloride film which would cause erratic termination of the titrant. All instrumental operating conditions are adjusted to provide a maximum potential change a t the end point with a minimum of electronic noise. The magnitude of the potential change at the end point of the chloride titration depends primarily on the concentrations of chloride and titrant, the titrant flow rate, and the rate of stirring, and these factors should have the maximum
values possible. The 6-ml.-per-minute titrant flow rate used in this procedure is the maximum recommended by the titrator manufacturer. Because of the high flow rate, calculations were made of the linear regression of milliliters of silver nitrate on grams of sodium chloride primary standard. The data were statistically evaluated, and the null hypothesis that the intercept (blank) was equal to zero was tested and accepted a t the 95% confidence level. This finding is consistent wit,h the mean errors of the data presented in Tables I, 11, and IV.
LITERATURE CITED
Burns, E. A., Muraca, R. F., ANAL. CHEN.32,1316 (1960). (2) Crump, N. L., Johnson, N.C., Ibid., (1)
27, 1007 (1955). (3) Kurz, E., Kober, G., Berl, M., Zbid., 30,1983 (1958). (4) Sabar, G. M.. Ramschandran. C. R.. ' Zbid., 31; 263, (1959);( ( 5 ) Ribaudo, Charles, Methods of Ans-
lynng Polysulfide-Perchlorate Propellants," Tech. Rept. 2334, Samuel Feltman Ammunition Laboratories, Picatinny Arsenal, Dover, N. J., September 1956.
RECEIVED for review December 20. 1963. Accepted February 19, 1964.
Mu1tidi mensional Chromatography of Arenes Produced during Cornbusti on MlTSUGl MUKAI, BERNARD D. TEBBENS, and J. F. THOMAS University of California, Berkeley, Calif.
b The polynuclear aromatic hydrocarbons synthesized in a flame during incomplete combustion of various fuels are sufficiently similar to suggest that the processof their formationis independent of the fuel used. To test this hypothesis a laboratory investigation was carried out with burner enclosed in a system which allowed controlled several dissimilar combustion of gaseous fuels and quantitative recovery of combustion products of interest. Quantitative determination of six arenes showed a constant proportional production of anthracene, pyrene, fluoranthene, bcnzo(a)pyrene, perylene, and benzo(e)pyrene. Analytical procedures included paper chromatography, paper-to-paper transfer, and new solvent systems which separate isomers such as pyrene and fluoranthene. The spectrofluorometer was used for quantitation of arenes after elution from the chromatogram. Types and relative quantities of arenes produced appear to depend on the temperatures of flame zones and on complexity of combustion products.
P
REVIOUS WORK
(11-14) performed
by this laboratory has indicated that the polynuclear aromatic hydrocarbons synthesized in a flame during the incomplete combustion of various fuels are produced through the same reaction mechanism and their formation is independent of the fuel used. Interest in this phenomenon received additional stimulation from observations reported by Sawicki et al. (8). The analyses of many atmospheric samples obtained over cities in the United States indicated 1126
ANALYTICAL CHEMISTRY
a correspondence in the relative quantities of seven arenes. It was suggested that the analysis of any one arene in such a sample might be used as an index of the quantity of the others in the same sample. To test this hypothesis, a laboratory investigation was carried out. Various fuels were burned under different conditions of combustion. Analytical techniques other than those used for atmospheric samples (8-10) were developed for the quantitative resolution of typical combustion products. B comparison of overall yields and the yield of individual combustion products is made. The conclusions based on the laboratory investigation are compared with those obtained when the same analytical techniques were applied to atmospheric samples and samples obtained from automobile exhaust. 9 comparison is also made with data obtained from the literature (1, 2, 4-8). EXPERIMENTAL
Combustion Apparatus and Techniques. A schematic drawing of the
combustion apparatus is shown in Figure 1. The burner is of a type which allows mixing of primary air and fuel directly a t the flame. Secondary air is admitted through a diffuser plate a t the base of the furnace and is metered, as are the primary air and fuel, from individual tank sources. The burner enclosure and sampling train are all glass with a manometer and ignition port adjacent to the burner. A sintered-glass thimble is used to collect the particulate material. The aqueous phase is condensed and trapped in a condenser system maintained a t
approximately 2' C. The gaseous phase is passed through a dry iceacetone trap which condenses a portion of the organics. Nitrogen, carbon dioxide, and the noncondcnsable volatile organics are exhausted through the suction pump to the atmosphere. The fuels used include methane and propane, which are classed as cleanburning fuels over a wide range of the combustion spectrum, and isobutylene which emits large quantities of particulate material except when burned in a narrow region a t the stoichiometric end of the spectrum. When the fuels were burned using both primary and secondary air, combustion was classed as relatively complete, which merely implies combustion occurred under conditions approaching stoichiometric. When primary air was eliminated, the process was classed as relatively incomplete, which imp!ies self-sustaining combustion far removed from stoichiometric conditions. Bleed valves on the suction side of the exhaust pump were used to maintain atmospheric pressure in the combustion chamber during all stages of combustion. Burning was stopped when it was no longer possible to maintain atmospheric pressure because of the back pressure created by the collection in the particulate trap. The average length of run was approximately 15 minutes. Sample Compositing. The particulate material which collected in the burner enclosure was washed from the walls with water and transferred to the particulate collection thimble by suction filtration. The total particulate material thus contained in the collection thimble was suctionwashed with a small portion of diethyl ether which cleared the glass membrane of water and allowed the collection thimble to be used directly in a Soxhlet
