tantalum content, aliquots containing 3 to 5 grams of the alloy were required for each gravimetric analysis, in which the tantalum was precipitated by acid hydrolysis, ignited to the oxide, and weighed. For the ternary alloys, which contained about 0.5% of tungsten, a separation was required in the gravimetric method. Analyses of filtrate solutions from the gravimetric precipitations showed that a small amount of tantalum remained in solution. The extraction-colorimetric procedure described was ideally suited for the low percentages of tantalum, and no interference was caused by the tungsten. Because the results for the gravimetric determination of tantalum in the binary alloys are slightly lower than the colorimetric results, although the two methods give comparable results with ternary alloys, it is possible that the tantalum is not completely precipitated,
as shown by the analysis of the filtrate solution, and that a small positive interference is caused by tungsten in the gravimetric method. I n any event, the difference between the two methods is small, considering the low concentrations of tantalum used. ACKNOWLEDGMENT
Gravimetric analyses were performed by Owen Kriege and Ross Gardner of this laboratory.
(5) Ikenberry, L., Martin, J. L., Boyer, W. J., ANAL. CHEW 25, 1340 (l9?3): (6) Leddicotte, G. W., Moore, F. L., J. Am. Chem. Soc. 74, 1618 (1952). ( 7 ) Milner, G. W., Barnett, G. A., Smales, A. A., Analyst, 80, 380 (1955). (8) Palillrt, F. C., Adler, N., Hiskey, C. F., ANAL. CHEY. 25, 926 (19.53). (9) Stevenson, P. C., Hicks, H. G . , Ibid., 25, 1517 (1953). (10) Thanheiser, G., Mitt.Kaiser Wilhelm Inst.. Eisenforsch. Dusseldorf 22. 258 '(1940).' (11) Vanossi, R., Annales QSSOC. quim. argentina 42, 59 (1954). (12) Waterbury, G. R., Bricker, C. E., ANAL.CHEM. 29, 129 (1956). (13) Wernig, J. R., Higbie, K. B., Grace, -
\ -
LITERATURE CITED
Brandt, ITT. IT., Purdue University, West Lafayette, Ind., private communication, July 22, 1955. Ellenburgh, J. Y., Leddicotte, G. W., Moore, F. L., ANAL. CHEY.26, 1045 (1954). Harvey, A. E., Jr., Manning, D. L., J . Am. Chern. SOC. 72, 4488 (1950). Hunt, E. C., Wells, R. A., Analyst 79, 345 (1954).
I
I
J. T., Speece, B. F., Gilbert, H. L., Ind. Eng. Chem. 46,644 (1954).
RECEIVED for review March 19, 1957. Accepted May 8, 1957. Work done under the auspices of the Atomic Energy Commission.
Determination of Phthalate Esters in Propellants HARRY STALCUP, FRANK McCOLLUM', and C. 1. WHITMAN U. S. Naval Powder Factory, Indian Head, Md. b A general method for determination of phthalate esters in experimental propellants involves hydrolysis of the phthalate by anhydrous ethanolic potassium hydroxide after reduction of nitrate, nitro, and nitramine compounds with powdered zinc and acetic acid. The phthalate i s recovered quantitatively as the dipotassium salt and may b e determined either by weighing the salt, or by titration with perchloric acid dissolved in glacial acetic. This method has been applied to the determination of dimethyl phthalate, diethyl phthalate, dibutyl phthalate, and dioctyl phthalate, in the presence of nitroglycerin, dinitrotoluene, 2-nitrodiphenylamine, diphenylamine, ethyl centralite (centralite-1), and triacetin. Phthalate recovery from synthetic propellant mixtures agrees within 1 .O% of the amount added, except for dimethyl phthalate where losses were 5 to 6%.
S
the introduction of the complicated Thames (14) lead phthalate gravimetric method and the Kavanagh (6) phthalic acid titration, several procedures have become availINCE
* Present address, Quartermaster Research and Development Command, Natick, Mass.
able for the determination of phthalate esters in plastic and propellant formulations. Some procedures have the disadvantage of being designed for specific or limited application. The iodometric method of Butts and associates (1) for diethyl phthalate is based on dichromate oxidation of the ethyl alcohol produced after hydrolysis of the ester with alkali. The chromatographic method developed a t Picatinny Arsenal (IO), in addition to being specific only for diethyl phthalate, has the usual disadvantages of chromatographic techniques in that the absorbent and developing solvents must meet rigid specifications for reproducible results. Several methods based on the use of instruments have been published. The Whitnack and Gantz (15) polarographic technique and the Pristera (9) infrared technique have been successfully applied to propellants. However, they require expensive equipment not readily available in many analytical laboratories. The ultraviolet spectrophotometric method of Shreve and Heether (1.9) and the Shaefer and Becker (12) gravimetric methods appear to give satisfactory results for alkyd resins and lacquers. One of the most difficult problems associated with the analysis of propellants has been the elimination of inter-
ferences due to nitrate esters and aromatic nitro compounds. The Lamond (7) procedure, which employs ammonium sulfide for the reduction of the nitrated compounds, is rather involved. The Mullaly (8) technique, based on the earlier work of Dickson and Easterbrook ( 2 ) involves the reduction of nitroglycerin and nitro compounds with methanolic ferrous chloride. Wight (16) found that nitrate esters could be effectively destroyed by reduction with zinc in the presence of sulfuric acid, after which the phthalate ester could be precipitated quantitatively as the alcoholate of dipotassium phthalate by the Goldberg (3) modification of the Kappelmeier (6) technique. These methods, however, are applicable to only a limited number of propellant formulations. They cannot be used for propellants requiring complicated preliminary separation techniques. More recently Grodzinski (4) has described a method based upon a titanous chloride reduction of nitrate esters and nitroaromatic compounds from which the phthalate is separated by means of a liquid-liquid extraction with petroleum ether. The phthalate is saponified and determined by titration. This method is obviously not applicable to those formulations containing other esters which may be etherextracted, saponified, and titrated. VOL. 29, NO. 10, OCTOBER 1957
1479
anhydride kept in a chilled, glassstoppered Erlenmeyer flask. The resulting solution is diluted with 720 d. of glacial acetic acid and allowed to stand 2 weeks until the reaction between water and acetic anhydride is complete.
The method described in this paper has been applied t o the determination of dimethyl phthalate, diethyl phthalate, dibutyl phthalate] and dioctyl phthalate in the presence of nitroglycerin, dinitrotoluene, 2-nitrodiphenylamine, diphenylamine, ethyl centralite, (centralite-l), and triacetin. It may be used for dimethyl phthalate, but recovery values are only 94 to 96%. Six analyses can be completed in an 8-hour day by the average laboratory technician using standard laboratory equipment and reagents. The ground propellant is extracted with methylene chloride and the nitrate esters and nitro compounds in the extract are reduced with zinc and acetic acid. The phthalate ester is unaffected. A second extraction with methylene chloride separates the phthalate ester from zinc and other insoluble products. The phthalate is saponified with alcoholic potassium hydroxide and precipitated by the Kappelmeier (5) technique. The phthalate content may be determined either by weighing the dipotassium salt or by titrating it in a nonaqueous solution of perchloric acid dissolved in glacial acetic acid. The reduced nitrate and nitro compounds do not interfere.
The flask is stoppered and swirled occasionally for 15 minutes. While the flask is being swirled, about 5 grams of zinc dust are added in small portions. The hot solution is filtered through a medium-porosity fritted-glass Biichner funnel into a 500-ml. separatory funnel that contains 250 ml. of water. The flask and filter disk are rinsed thoroughly with two 10-ml. portions of hot 65% acetic acid and about 75 ml. of methylene chloride from a wash bottle. The separatory funnel is shaken for about 1 minute, and the two-phase mixture is allowed to separate. The lower layer (methylene chloride containing the phthalate) is transferred to a 500-ml. Squibb-type separatory funnel, Five milliliters of additional methylene chloride are added to the first separatory funnel and, without shaking, the lower solvent layer is transferred to the second separatory funnel as before. The latter operation washes out droplets of methylene chloride solution that are rich in phthalate. The dilute acetic acid solution is extracted twice more in the same manner, using 25-ml. portions of methylene chloride each time. The acetic acid layer is discarded. Fifty milliliters of 10% potassium carbonate solution are added to the second separatory funnel containing the cornbined methylene chloride extracts. The
PROCEDURE
Gravimetric. Approximately 2 grams of powder are weighed accurately, ground in a Wiley mill to 20mesh particle size, and introduced into a small paper extraction thimble. A plug of glass wool is inserted into the thimble and the latter dropped into a short Soxhlet extraction tube having a siphoning chamber capacity of 40 to 50 ml. The sample is extracted with methylene chloride for 3 hours under a water-cooled condenser using a 250ml. refluxing flask as a receiver. A steam bath heat source is preferable for safety reasons. Sufficient heat should be provided so that the solvent vapors will condense above the sample in an almost continuous stream. After extraction] the methylene chloride solvent is evaporated over a steam bath with the aid of a gentle stream of dry air, and 25 ml. of 65y0 acetic acid are added. The solution is heated t o about 70" C., and a few drops of saturated copper sulfate solution are added.
APPARATUS
Glass extraction thimbles to fit Soxhlet extractors. Small Soxhlet unit (40- to 50-ml. capacity siphoning chamber) Water-cooled reflux condenser to fit Soxhlet unit. Glass wool. Refluxing flask, 250 ml. Steam bath. Separatory funnels, 500 and 250 ml. Iodine flask, 250 ml. Electric oven or electric furnace for regulating the temperature a t 210' C.
Table 1.
Diethyl phthalate Dibutyl phthalate Dioctyl phthalate
REAGENTS
Solution of approximately 0.5N anhydrous ethanolic potassium hydroxide (potassium hydroxide purified from ethyl alcohol), The ethyl alcohol should not contain more than 0.5% water. Anhydrous ether. Potassium carbonate solution, 10%. Wash solution consisting of a 1 to 1 mixture (by. volume) of anhydrous ether-ethyl alcohol. The mixture should not contain more than 0.25y0 water. Methylene chloride. Acetic acid solution, 65y0 (by volume). Zinc dust. Copper sulfate solution (saturated). Crystal violet indicator solution (O.lyo in glacial acetic acid). 0.1N perchloric acid in glacial acetic acid. The standard solution of 0.1N perchloric acid in glacial acetic acid is prepared by adding 9 ml. of 70% pefchloric acid dropwise to 50 ml. of acetic
a
ANALYTICAL CHEMISTRY
Phthalate Added, Gram
Phthalate Found, Gram
Recovery, yo
0.2322 0.1541 0.2722 0.2139 0.2501 0.2512 0.2516 0.2616 0.2641 0.2511 0.2712 0.2608 0.2522
0.2179 0.1450 0.2180 0.2008 0.2469 0.2487 0.2499 0.2611 0.2624 0.2496 0.2715 0.2598 0.2517
93.84 94.09 93.84 93.88 98.74 99.01 99.32 99.81 99.35 99.41 100.10 99.62 99.82
Av. Recovery, %
93.91 99.02 99.52 99.85
Composition of propellant extract: nitroglycerin, ethyl centralite, and dinitrotoluene.
Table II.
Pro-
: ; : :ENo.
1480
Recovery of Phthalate Added to a Known Propellant Formulation
Two Grams of Propellant Extract" Plus Dimethyl phthalate
1
2
3
Determination of Phthalates in Propellant Formulations by Gravimetric and Volumetric Methods
Phthalate by Diff., %
Run No.
3.23
3.30
1 2 3
Nitroglycerin Ethyl centralite Dibutyl Dhthalate " -
8.50
8.50
Nitroglycerin 2-Nitrodiphen ylamine Triacetin Dioctyl phthalate
3.25
3.27
Ingredients in Extractable5 Nitroglycerin Ethyl centralite Dibutyl phthalate
Formula Percentage of
Phthalate
Phthalate Found, % GraviVolumetric metric method method
3.26 3.20 3.27 Av. 3 . 2 4 1 8.47 2 8.49 3 8.53 Av. 8 . 5 0 1 2
3.32 3.28 Av. 3 . 3 0
3.22 3.27 3.18 3.22 8.50 8.55 8.40 8.48
funnel is shaken vigorously until the acetic acid dissolved in the methylene chloride is neutralized completely. The solution is tested with indicator paper or a drop of phenolphthalein indicator solution to make certain that an excess of potassium carbonate is present. The methylene chloride solution is transferred to a 250-ml. iodine flask. Five milliliters of methylene chloride are added to the separatory funnel containing the potassium carbonate solution and, without shaking the funnel, the lower phase is transferred to the flask, thus washing out droplets of methylene chloride solution a t the lower tip of the funnel. The contents of the separatory funnel are extracted with 10 ml. of methylene chloride, and again the lower layer is transferred to the flask, care being taken to avoid carrying any potassium carbonate solution through the stopcock with the methylene chloride. The solubility of phthalates in potassium carbonate solution is so low that 10 ml. is sufficient to ensure complete removal. The methylene chloride solution in the flask is evaporated on the steam bath by a current of dry air until the odor of methylene chloride can no longer be detected. The flask is removed immediately from the steam bath. Twenty-five milliliters of 0.5N potassium hydroxide in anhydrous alcohol are added to the extracted material, 1%-hichshould be free of water and methylene chloride. The flask is stoppered loosely and heated in a constanttemperature water bath maintained at 60" to 70" C. for 1 hour. The stopper is removed, the flask is cooled to room temperature, and 25 ml. of dry ether are added. The flask contents are filtered with suction on a medium-porosity fritted-glass crucible, which has been previously weighed inside a tared stoppered weighing bottle. The weighing bottle should be kept inside the balance case. The crucible is washed with absolute ether-ethyl alcohol (1 to 1 ratio). Except during the weighing operation, the precipitate should not be sucked completely dry, particularly on days of high humidity, because moisture in the air might dissolve some of the highly water-soluble salt. The complete removal of excess potassium hydroxide is proved by testing the filtrate from the final washing with phenolphthalein. The crucible is placed inside the oven and the precipitate of dipotassium phthalate alcoholate complex is heated for 1 hour a t 210" C., thus converting it to dipotassium phthalate. The crucible is cooled for a t least 1 hour in a desiccator. The crucible is transferred to the tared weighing bottle and weighed again. The difference in weight between the first and second crucible weighings is the weight of the dipotassium phthalate plus potassium carbonate impurity. The precipitate, which may contain a snall amount of potassium carbonate, is dissolved in warm neutral distilled water and titrated with 0.0500N hydrochloric acid to the phenolphthalein end point.
The number of milliliters of acid times the normality of the acid times 0.1382 equals the grams of potassium carbonate. The weight of potassium carbonate found by titration is subtracted from the weight of the precipitate on the crucible to obtain the corrected weight of dipotassium phthalate. Calculation: Per cent phthalate ester = P x F x 100 W where
F
factor for converting dipotassium phthalate to the phthalate ester P = corrected . . . weight of dipotassium phthalate W = weightof samde F for dioc$ phthalate = 1.610 F for dibutyl phthalate = 1.148 F for diethyl phthalate = 0.917 F for dimethyl phthalate = 0.801 -= =
0.94 0.85
Volumetric. The same procedure is used as for the gravimetric method, including the washing of the precipitate of potassium phthalate. Suction is applied and the ethyl alcoholether wet precipitate is washed into a 250-ml. Erlenmeyer flask with 30 ml. of glacial acetic acid. Five drops of
crystal violet indicator are added and the solution is titrated with standard perchloric acid in glacial acetic acid. The change in color from violet to blue is detected readily even when the solution is colored. Because the coefficient of cubical expansion of acetic acid is high, a correction of 0.0011 ml. per ml. per degree centigrade should b? made. The perchloric acid solution was standardized a t 29" C. and the buret reading corrected to this temperature. Less than 0.02 ml. of 0.1N perchloric acid is sufficient to bring about the change from violet to blue. Calculation: Per cent phthalate ester
m x 20
where
v =
volume of perchloric acid
N = normality of perchloric acid
M = molecular weight of phthalate ester
W = weight of sample DISCUSSION
The method described in this paper is based on precipitation of the phthalate as the dipotassium salt by the Kappelmeier (5) technique, according to the following reaction: / -COOH
Table 111. Relationship of Strength of Saponifying Reagent to Percentage Phthalate Recovery
Nor-
mality of
Sap!& fying Reagent 0.20 0.50
% Recovery of Phthalate Diethyl Dibutyl Dioctyl phthal- phthalphthalate ate ate 95.62 96.58 93.30 99.80 99.45 99.60
100.30 99.71 99.32 99.80 100.32 100.20
99.70 99.51 100.20 99.55 99.76 101.30
Table IV. Gravimetric Determination of Diethyl Phthalate in Propellant Formulation Using KOH Reagent"
Normality of KOH Saponifying Reagent 0.17
Av., %
10.24 10.19 9.96 9.99 10.21 10.86 10.68 10.81
10.78
Value obtained by diff.
= 11.1%.
0.22
a
Diethvl Phthalke Found, yo
10.12
=
V X N X M
(--coo,
KOH
/ -COOK
+C?"
This procedure was tested for accuracy and precision by analysis of the extractables in a standard propellant composition ground to 20-mesh particle size t o which known amounts of the phthalate ester had been added. Recovery values are shown in Table I. Table I1 indicates the applicability of the gravimetric and volumetric methods to various propellant formulations containing different phthalates. The importance of the strength of the saponifying reagent is indicated in Table 111. The ease of saponification of phthalate esters varies directly with the carbon content of the alcohol in the ester. The minimum concentration of the reagent for quantitative recovery of diethyl phthalate appears to be in the vicinity of 0.5N (Table IV). The precipitated dipotassium phthalate was determined volumetrically by titration with a standard solution of perchloric acid in glacial acetic acid by the Goldberg ( 3 ) method, as shown by the following reactions:
2CHaCOOK
+
VOL. 2 9 , NO. 10, OCTOBER 1957
1481
CHaCOOK
+ HClOi
+
KClOi
+ CHaCOOH
Previous difficulties in attempts to standardize the perchloric acid-glacial acetic acid solution were overcome with the substitution of Bureau of Standards acid potassium phthalate for sodium carbonate as the primary standard. The use of crystal violet in place of bromophenol blue and bromocresol purple as the indicator, as suggested by Seaman and Allen (11) eliminated difficulties due to faulty end points. The lower recovery values for dimethyl phthalate may be due to its greater solubility in the aqueous solutions used during the extraction and neutralization operations. Based on a 94% recovery of dimethyl phthalate by
the proposed method (Table I) an empirical factor may be calculated as follows:
0.801 = 0.85 0.94 LITERATURE CITED
(8) Mullaly, RI. A. C., Zbid., 80, 237 (1955). (9) Pristera, F., ANAL. CHEY.25, 844 (1953). 10) Ribaudo, C., Campisi, J., Picatinny Arsenal Project EPO No. EP-13, Rept. 4, PA Serial KO.1742, Jan. 19, 1951. 1 1 ) Seaman. W..Allen, E.. ANAL.CHEM. 23,592 (1951). Shaefer, W. E., Becker, W. W., Ibid., 25, 1226 (1953). Shreve, D. D., Heether, M. P., Ibid., 23, 441 (1951). Thames, F. C., IND.ENG.CHEM., ANAL. ED. 8, 418 (1936). I
Butts, P. G., Prine, G. B., Kouba, D. L., Becker, W. W., ANAL. CHEM.20, 1066 (1948). Dickson, W.,Easterbrook, W. C., Analyst 47; 112 (1922).
'
Goldberg, A . I., IND.ENG.CHEM., ANAL.ED.16,198 (1944). Grodzinski,J., ANAL.CHEM.27, 1765
Whitnack, G. C., Gantz, E. S. C., ANAL.CHEX.25, 553 (1953). Wight, T. W., Naval Ordnance Inspection Laboratory, Caerwent, N.O.I.L. Rept. CR2/48 (May
(1955).
Ka pelmeier, C. P. A., Paint, Oil, $hem. Rev. 99. KO. 12,, 34,, 36 (1937). (6) Kavanagh, F., IND.ENG. CHEM., ANAL.ED.8, 397 (1936). ( 7 ) Lamond, J., Analyst 73, 674 (1948).
,
1948).
RECEIVED for review October 30, 1956. Accepted May 3,1957.
Reduct io n of NitrogIy ce rin by Cata Iy tic Hy d roge no Iys is in Analysis of Propellants for DioctyI Phthalate HARRY STALCUP, MAE I. FAUTH, JAMES 0. WATTS', and RICHARD W. WILLIAMS* U. S. Naval Powder Factory, Indian Head, Md.
b Spectrophotometric, gravimetric, and polarographic procedures are described for the determination of dioctyl phthalate in propellants. The interference caused by nitroglycerin is removed by catalytic hydrogenolysis, using platinum oxide or palladium black. When 2-nitrodiphenylarnine is determined ethyl alcohol is the extraction solvent and the absorption at 425 mk in an unreduced aliquot of the extract from the propellant is measured. For the determination of dioctyl phthalate by a spectrophotometric procedure, the interference due to 2-nitrodiphenylamine is removed by catalytic reduction. The reduced sample is passed through Amberlite IR-120. The phthalate is measured If a determination at 276 mp. of 2-nitrodiphenylamine is not required, the propellant is extracted with a pentane-methylene chloride azeotrope, and the extract subjected to catalytic hydrogenolysis. In the spectrophotometric and the gravimetric methods the phthalate is precipitated as the dipotassium salt by alkali saponification. The polarographic determination of phthalate is made directly from the filtered extract after catalytic hydrogenolysis.
T
investigation was undertaken to improve the gravimetric method for the determination of phthalates in propellants (10). In the original method, the nitroglycerin in the acetic HIS
1482
ANALYTICAL CHEMISTRY
acid extract or in the dried methylene chloride extract, acidified with dilute acetic acid, was reduced with zinc dust. After extraction and filtration, the phthalate was saponified with alcoholic potassium hydroxide and precipitated by the Kappelmeier (1) technique as the dipotassium salt. Kuhn (2) found that nitrate esters could be effectively reduced by catalytic hydrogenolysis. This fact was utilized in the development of the present method. The determination of phthalate esters in propellant formulations by spectrophotometric measurements has been reported by several investigators ( 3 , l O ) . Schroeder and coivorkers (4-8) included 2-nitrodiphenylamine as well as some of the phthalate esters in their extensive studies of the absorption characteristic of propellant ingredients. Schemes based on preliminary chromatographic separations for the spectrophotometric analysis of phthalates in propellants are described in their papers. Additional work on such methods is indicated (11, 19). The use of spectrophotometric methods in the analysis of phthalates is described by Shreve and Heether (9). Investigators a t Picatinny Arsenal (3) determined phthalates spectrophotometrically in propellants by a procedure in which nitroglycerin and nitroaromatic compounds were reduced with titanous chloride. After a series of extraction operations before and after alkali saponification the phthalate was
finally determined spectrophotometrically. Previous work by Watts and Stalcup ( I d ) , based on the earlier work of Kuhn ( 2 ) with amyl nitrate, demonstrated that the interference of nitroglycerin in the determination of phthalate esters in the double-base propellants could be eliminated by catalytic hydrogenolysis of the ester. I n more recent investigations, it was found that the phthalate in propellants which contain 2-nitrodiphenylamine could be determined directly by spectrophotometric measurement, provided that the interference from the reduction products of 2-nitrodiphenylamine could be removed. This is accomplished by passing the alcohol extract containing the phthalate and the reduced products through the cation exchange resin Amberlite IR-120. 2 - Nitrodiphenylamine is determined spectrophotometrically on an unreduced aliquot of the alcohol extract. When the determination of 2-nitrodiphenylamine in the same sample is not desired, a smaller amount of sample may be used and after suitable extraction and hydrogenolysis procedures, the phthalate may be separated by precipitation as dipotassium phthalate. The phthalate content may then be determined by spectrophotometric, polarographic, or gravimetric methods. Present addres8, American Instrument
Go., Silver Spring, Md.
2 Present address, Bureau of Ordnance, Washington, D. C.
