New Method for Determining Phthalate Esters in Propellants JOSEPH GRODZlNSKl Central Laboratory o f lsraeli M i l i t a r y Industries, Ministry o f Defense, lsrael
This worli was undertaken in order to develop a convenient and reliable method for the determination of phthalate esters in propellants. The method described is based upon a titanous chloride reduction of nitroesters and nitroaromatic bodies, which allows a sepaiation of phthalate esters, by a petroleum ether extraction, and their volumetric determination. This method has been applied to the determination of dimethyl phthalate, diethyl phthalate, dibutyl phthalate, diamyl phthalate, and diphenyl phthalate, in the presence of nitroglycerin, dinitrotoluene, diphenylamine, ethyl centralite, and ethyl acetate. The method allows one to determine the percentage of phthalate esters in smolieless powders with a precision within 0.02 to 0.03q0. I t is accurate and simple and requires a short operating time, and is therefore well suited for routine control work in industrial laboratories. A very convenient procedure for identifying the type of phthalate esters has also been developed.
D
saponification of the p o \ ~ t i c ~ i , .Then the free phthalic acsid is separated and titrated with :ilkxli according to the Iiavanaugh ( 4 ) proredure. The e s w t n r w of this met,hod is Ion., esperially it applied to powders containing nitroaromatic bodies 01' :i high percentage of nitroglycerin. This work was undert,aken in an effort to obtain a quicmk, rcliable, and generally applicalile method of determining phthalirt,esin propellantx, well suited for routine coutrol analysip. This method has been based upon H titanous chloride reduction of nitroesters and nitroaromatic. I d i e s which allows a separation of phthalates by a petroleum ethcr ext'raction. The phth:rl;ites neuare afterward saponified in an alcohol solution, previousl~~ tralized, and t,he alkali is retitwtcd.
t
LRING the past 10 years several papers have been pub-
lished dealing with the determination of phthalates. Their authors suggested polarographic ( 1 4 ) ) spectrometric ( 9 ) , volumetric (Z), or gravimetric methods (8, 1 1 ) . However, there has been up to the present considerahle need for a conr.enient and reliable method for determining phthalates in propellants xvhich requires a specific procedure, because of the interference of other ingredients, such as stabilizers, nitroesters, :tiid nitroaromatic bodies. All the previously published methods for the determination of phthalates in propellant,^ possess serious disadvanhges. The earliest, the lead phthalate gravimetric method, proposed hy Thames (18),is complicated and has been proved to be t,Iieoretically incorrect (10). The method of Lamond ( 5 ) applies an ammonium sulfide reduction procedure in order to separate the phthalates from the nitroesters and nitroaromatic bodies. It. is, however, lengthy and time consuming because of the numerous operations involved in the separation of phthalates from the nitroesters and nitroaromatic bodies, as well as in the preparation of the ammonium sulfide reagent, according to the special requirements of the method. Moreover, it,s disadvantages are not outweighed by its accuracy. The iodometric method of ethyl phthalate determination, developed by Butts, Prine, Kouba, :tnd Becker (1) is based on t,he oxidation of ethyl alcohol, which has been obtained from the phthalate, after its saponification. However, t,he traces of the residual solvents, if present i n the powder, will interfere, causing too high results. A partly indirect method has been adopted by an American .4rmy specification (6) for diethyl phthalate determination. According to this met'hod the phthalate together with the nitroglycerin is chromatographicallj-separated from the ethyl centralite aud the former is calculated from the difference after the titanometrir determination of nitroglycerin. This procedure has been suited to only one definite type of powder and possesses the disadvantages of an indirect determination. The recently published infrared spectrophotometric method of Prist,era ( 7 ) seems to outline a fast procedure. However, i t requires expensive equipment which is not usually available for routine n-ork in industrial laboratories. The latest acidimetric method of Tranchant (IS) suggests the liberat,ion of phthalic acid from potassium phthalate by the action of phosphoric acid on the latter, which has previously been formed as a reeult of the ether-extract,
y Figure 1. -
Linuid-liauid extraction apparatus
The method proposed in this paper has been applied to the determination of dimethyl phthalate, diethyl phthalate, dibutyl phthalate, diamyl phthalate, and diphenyl phthalate. Both the artificially prepared mixtures of phthalates with nitroglpcerin, dinitrotoluene, diphenylamine, ethyl centralite, and ethyl acetate as well as powders of known composition have been tested. The method proved to he uncomplicated and exact. APPARATUS 4ND REAGENTS
Liquid-liquid extraction apparatus (Fisher Scientific! Co., Catalog No. 9-573 with slightly modified dimensions). Thc izpparatus consists of an outer glass jacket with a bent side t,ube which is connected to an Erlenmeyer flask of 200-ml. capizcit,j-:
1765
1766
ANALYTICAL CHEMISTRY
The jacket encloses a narrow inner tube with a funnel top and an enlarged bulb-shaped lower end with small openings. Petroleum ether. Collect the fraction of boiling point 60' to 70" C. from the purified petroleum, boiling point, 60' to 90' C. Titanium chloride solution, 20%. Acetic acid, 84%. Standard hydrochloric acid, 0.1S. Alcoholic potassium hydroxide solution, approximately 0.212'. This should contain about 10% of water, which will prevent precipitation of the potassium phthalate formed in the reaction. RECOMMENDED PROCEDURE
Keigh 2 grams of previously ground propellant into a 250-ml. flask. Add 20 ml. of 84y0 acetic acid, fit the condenser into the neck of the flask, place it on a water bath, and heat for 20 minutes a t 100' C. Decant the liquid and filter into another 250-ml. flask. Repeat the extraction of the propellant with a second 20-ml. portion of 84% acetic acid. Filter the acetic acid solution and rinse the residue with several portions of acetic acid, using a total of 30 ml.
deflegmator and condenser, and evaporate the petroleum ether on a water bath. Rinse the deflegmator with 10 ml. of ethyl alcohol and transfer the alcohol into the receiving bottle. .4dd 4 to 5 drops of phenolphthalein, cool the alcoholic solution to between 0 and 5" C., add the potassium hydroxide solution dropwise, until a slight pink coloration appears. Xeutralize the excess of alkali with 1 to 2 drops of hydrochloric acid solution. Add exactly 20 ml. of the potassium hydroxide solution, fit a reflux condenser, and heat 45 minutes on a water bath at 100' C., then rinse the condenser with 50 ml. of distilled water free from carbon dioxide. Titrate the excess of alkali with hydrochloric acid solution. Carry out a blank titration.
(B
loo = Grams of sample X P
% phthalate ester
where
B = ml. of HC1 required for blank determination S = ml. of HCl reauired for back titration A- = normality of H c ~ molecular weight of phthalate ester K = 2 - K , innn ---P = purity of estimated phthalate expressed as a decimal. This is calculated from the saponification number; if the latter is not known. P is assumed to be 1.000.
.
Table I.
Hydrolysis of Phthalates during Reduction Procedure C.
Time of Heat,ing, Minutes, a t 2 at 100
Reduced nitroesters, % Hydrolyzed dimethyl phthalate, % Hydrolyzed dibutyl phthalate, 70
Table 11.
93.3 0.04 0.02
3 at 100 98.5
0.06
0.03
5 a t 60,
5 at 100
5 a t 100
99.9 0.11 0.05
100.0 0.12 0.05
Summary of Results Obtained on Synthetic Samp1es Phthalate Added, Gram
Phthalate Found, Gram
Recovery,
0.1348 0.1375 0.1845
0.1347 0.1375 0.1841
39.9 100.0 99.8
0.1280 0.1562
0.1283 0.1568
100.2 100.4
0.1220 0.1917 0.1766
0.1221 0.1923 0.1769
100.1 100.3 100.2
Dibutyl phthalate 1 gram of mixture E
0.1624 0.1625 0.1773
0.1624 0.1626 0.1780
100.0 100.1 100.4
Diphenyl phthalate i 1 gram of mixture B
0.1735 0.1561 0.1842 0,1673
0.1736 0.1555 0.1836 0.1670
100.1 99.6 99.7 99.8
Sample Dimethyl phthalate 1 gram of mixture A
+
+
Diethyl phthalate 1 gram of mixture A Dibutyl phthalate f 1 gram of mixture A
+
DISCUSSION AND RESULTS
then
%
Composition of mixture A. 90% nitroglycerin, 10% ethyl centralite. Composition of mixture B. 60% nitroglycerin, 35% dinitrotoluene, 3% ethyl acetate, and 2% diphenylamine.
Owing to a very rapid extraction procedure [adapted from Hirschhorn (S)],and to the comparatively small number of operations involved, the time of determining phthalates in propellants is greatly shortened, as compared with most other available methods. The extraction is directly followed by a titanous chloride reduction. The reduction of nitroesters and nitroaromatic bodies extracted from the porn-der is quantitative under prescribed conditions. During the reduction procedure the hydrolysis of phthalates is negligible (Table I). A summary of results obtained by analyzing the artificially prepared mixtures of phthalates with the usual smokeless powder ingredients is presented in Table 11. For all the analyzed samples the difference between the given amount of phthalates and that found by the analysis does not exceed zt0.40/0.
Table 111.
Sample
No. 1
After all of the phthalate solution in acetic acid is collected in the second flask, pass in carbon dioxide to expel the air. rldd 10 ml. of titanium trichloride solution for each 100 to 125 mg. of nitroglycerin or 75 to 100 mg. of dinitrotoluene present in the solution (the solution must remain intensively violet after the reduction is completed as well as in the course of the extraction). Place the bottle on a water bath and heat the solution successively 5 minutes a t 60" C. and 5 minutes a t 100' C.; cool the flask, and add 30 ml. of petroleum ether and 20 to 50 ml. of water. Insert a rubber stopper and shake the contents of the flask vigorously for 5 minutes. Transfer the contents of the flask into the outer jacket of the liquid-liquid extraction apparatus and add 30 to 50 ml. of water. Rinse the flask first with 3 to 4 ml. of acetic acid and then with 50 ml. of petroleum ether and add these solutions to the contents of the jacket. P u t the inner tube into the jacket. Take care that no acetic acid solution passes into the receiving flask. Connect the jacket with the condenser, and place the receiving flask on the heater. Isolate the upper part of the jacket by means of a thick glass-n-ool layer to prevent the condensation of the petroleum ether vapors on the walls of the outer jacket. Start the heating and adjust the boiling rate of the petroleum ether in the Erlenmeyer flask so that 4 to 5 drops of petroleum ether pass per second through the aqueous solution (for 8 hours). The 8 hours of continuous extraction under the stated conditions are sufficient for complete extraction of quantities of phthalate up to 200 mg. Then remove the receiving bottle, connect n-ith a
2
3
4
5
Summary of Results for Some Typical Smokeless Powders
Principal Ingredients Diethyl phthalate E t h y l centralite Nitroglycerin Kitrocellulose Dibutyl phthalate Diphenylamine Kitroglycerin Nitrocellulose Dibutyl phthalate Diphenylamine Dinitrotoluene Kitrocellulose Diamyl phthalate Ethyl centralite Dinitrotoluene Nitrocellulose Diphenyl phthalate Diphenylamine Dinitrotoluene Kitroglycerin Nitrocellulose
Phthalate Calculated from Difference,
Phthalate Found,
%
%
Average
2.6
2.61 2.63 2.63
2.62
G,2
6.19 6.15 6.14
6.16
5.0
5.00 5.01 5.03
5.01
3.3
4.02 4.05
4.03
5.2
5.03 5.01 4.99
5.01
Some of the results obtained for the analyzed propellants are contained in Table 111. For all the analyzed samples the results are reproducible within zt0.03% of the phthalate contents in the powder, and are also in accordance with those calculated from the difference between the total rreight of the sample and that of all the other ingredients.
V O L U M E 2 7 , N O . 11, N O V E M B E R 1 9 5 5
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The method permits the determination of the percentage of phthalates in practically all smokeless powders, now in use, with a precision within 0.02 to 0.03, h p a simple procedure, which requires only a short operating time. I D E N T I F I C 4 T I O S O F T Y P E OF PHTHALATE E S T E R
The following two methods dealing with the identification of the type of phthalate ester in propellants may be found in the literature on the subject: the infrared spectrophotometric method of Pristera (Y)and the method of Tranchant ( I S ) , which is based on a rather lengthy procedure consisting of the separation of free alcohols from the phthalate esters and identification of the former by their p-nitrohenzoic acid esters. By the present method it is possible to separate the phthalate ester qualitatively from the other propellant ingredients by a simple and convenient procedure. The separated phthalate esters may be conveniently identified by their refractive indices, by their saponification number, or by one of the other usually applied methods. The follon-ing procedure for the separation of phthalate esters from the other propellant ingredients is recommended. Reagents. Hydrorhloric acid-acetic acid solution. Mix 500 ml. of concentrated hydrochloric acid with 500 ml. of glacial acetic acid. Potassium dichromate solution. Dissolve 5 grams of U. S. Pharmacopoeia potassium dichromate in 370 ml. of distilled water and pour this solution s l o w l ~into 350 ml. of sulfuric acid (specific gravity, 1.84; a.C.S.). Sodium bicarbonate solution, 0.5%. Carbon tetrachloride, U.S.P.
Table 11..
1
Mixt.
2
Mxt.
3
RIist.
4
Prop.
5
Miut.
6
Rlixt.
7
Prop.
8
Prop.
9
Rlixt.
Refractive Indices of Phthalate Esters.
Pht, S G , C
Dimethyl phthalate P h t , D X T . DF.4 Dimethyl phthalate Plit. 1 - G C , Diethyl D S T . DF.i phthalate P h t . XG, C Diethyl phthalate P h t , h-G, DFA, Dibutyl C, DST pht lialate P h t , S G , ?JJ., Dibutyl p ht lialat e Plit YG, 3I.J., Dibutyl phthalate c' Pljt, D X T , D F I Dibutyl phthalate Plit, S G , DXT, Diisortms-l DF.4 c phtlialate
c
1.5138
1.5135
,, ,
1.5138
1.5135
, , ,
1.5019
1 5020
1.5010
1.5010
1.4900
1.4900
1.4900
1.4885
RIixt.
CONCLUSIONS
The isolated phthalate ester may non- be identified by its refractive index. Refractive indices of phthalate esters, isolated by the described procedure from artificially prepared mixtures and from propellants of known composition, have been determined in a Spencer Abbe type refractometer. Results are presented in Table IV. As is evident from Table IV, the pure phthalate esters have been separated from all the synthetic mixtures and propellants examined, except from those containing mineral jelly (for those the refractive indices are slightly diminished because of the traces of mineral jelly contained in them). Considering the differences in the refractive indices of the phthalate esters, normally used in the production of propellants, the purity of all phthalate esters separated is sufficient to identify them by their refractive indices.
..,
1.4925 1.4905 1.4925
1.4920
.,
1.4860
1.48130
,
,
,,
Diphenyl n p = 1.572 1.571 phthalate iibbreviation~. 3Iiut., synthetir mixture. Prop., propellant. P h t , pht,halate ester. S O ,nitroglycerin. C, ethyl centralite. DFA, diphenylamine. DNT, dinitrotoluene. lI..J., mineral jelly. 10
to wash out completely the remaining traces of diphenylamine (a strip of filter paper moistened with 1 to 2 drops of the carbon tetrachloride solution and of the potassium dichromate solution should give no traces of blue coloration). Then successively rinse thecarbon tetrachloride solution with 10ml. of sodium bicarbonate solution and with two 10-ml. portions of distilled water, and transfer to a 100-ml. distilling flask. Concentrate the solution to 5 to 6 ml. If it is colored, add a pinch of activated carbon and filtei into a small beaker. Evaporate the carbon tetrachloride completely. The phthalate ester remains on the bottom of the beaker. This procedure must be modified for a propellant containing ethyl centralite. I n this case the rinsing with hydrochloric acidacetic acid fnixture may be omitted as inefficient, the rest of the procedure remaining unchanged. Then in order to eliminate the ethyl centralite, the following additional operations are required. Add 25 ml. of the potassium dichromate solution, cooled previously to between 5' and 10" C., to the phthalate ester mixed with ethyl centralite, which remained in the beaker. Transfer the solution to a 150-ml. separatory funnel. Rinse the beaker with several portions of petroleum ether to a total of 50 ml. and add the petroleum ether solution to the separatory funnel. Shake it and separate the petroleum ether laaer. Rinse with two to three 5-ml. portions of potassium dichromate solution, cooled to between 5' and 10' C., so as to wash out the ethyl centralite completely. (The last portion of the potassium dichromat? solution should not give any change in its original coloration after having been vigorously shaken with the petroleum ether solution.) Then rinse and evaporate the petroleum ether solution a5 previously described (for the rinsing and evaporation of the carbon tetrachloride solution). The phthalate ester remains on the bottom of the beaker.
ACKNOWLEDGMENT
The author wishes to acknowledge the assistance of Lena Ahiasaf-Herzog, who carried out much of the experimental work. This paper is published with the kind permission of the General Director of the Israeli Military Industries.
Pht, S G . DXT, DFA, C
Procedure. Extract the propellant with acetic acid and reduce the extracted nitroglycerin or dinitrotoluene, as previously described, but without the precautions observed in the quantitative determination. After the reduction has been completed, transfer the solution to a 250-ml. separatory funnel, and add 30 ml. of concentrated hydrochloric acid and 40 ml. of carbon tetrachloride. Shake vigorously, allow the two layers to separate, and transfer the carbon tetrachloride layer to a 150-ml. separatory funnel. The procedure then depends on the type of stabilizer present in the propellant. If the propellant has been stabilized only with diphenylamine proceed as follow-s: Rinse the carbon tetrachloride solution with two to three portions of hydrochloric acid-acetic acid mixture, 10 ml. each, so as
LITER4TURE CITED
Butts, P. G., Prine, G. B., Kouba, D. L , and Becker, W. IT7., ANAL.CHEM.,20, 106G7 (1948). Goldberg, A. I., Ibid., 16, 198-200 (1944). Hirschhorn, I. S., ANAL.CHEM..19, 880-2 (1947). Kavanaugh, F., IND.ENG. CHEM., ANIL. ED., 8, 397-8 (1936). Lamond, J., AnaZyst, 73, 674-7 (1948). Office of Chief of Ordnance, Washington 25, D. C., hlilitary SDecification. MIL-P-10557 IOrd \ . Pristera, F., ANAL.CHEM.,25, 844-55 (1953) Shaefer, W. E., and Becker, W. W., Ibid., 2 5 , 1226-31 (1953). , Shreve, 0. D., and Heether, M. R., Ibid., 23, 441-5 (1951). (10) Smith, S. B., and Strempfer, J. F., Ibid.. 16, 416 (1944). (11) Swann, M. H., Zbid., 21, 1448-53 (1949). (12) Thames, F. C., Ibid., 8, 418-9 (1936). (13) Tranchant, J., Mhm. poudres, 35, 303 (1953). (14) Whitnack, G. C., and Gants, E. St. C., ANAL.CHEM.,25, 553 (1953). ~
RWEITTDfor review .January 25, 1955. .4ccepted August 8, 1955.
