Determination of Oxirane Oxygen in Salts of Epoxy Acids and in the Presence of Amines A. J. DURBETAKI' Becco Chemical Division, Food Machinery and Chemical Corp., Buffalo 7, N. Y.
FOxirane oxygen is determined in salts of epoxy acids and in the presence of amines in epoxy compounds including resins by an argentimetric method utilizing hydrogen bromide in acetic acid and silver nitrate, or a completely nonaqueous titration with hydrogen bromide and perchloric acid in acetic acid. Visual end points are employed and the titrations proceed at the speed of aqueous acid base titrations. The accuracy of both methods is within 1 %.
S
of epoxy acids have shown promise as stabilizers for halogencontaining polymeric substances against heat and light (6). Amines represent an important class of curing agents for epoxy resins. The direct hydrohalogenation methods (1-3) generally used in the determination of oxirane oxygen in epoxy compounds are not applicable to salts of epoxy acids and in the presence of amines because of the corresponding reaction of the salt and amine with the hydrohalogen acid to form the metal halide, and amine hydrohalide, respectively. Although potentiometric differentiation using hydrogen chloride in acetic acid (3) is possible in some cases, this method is limited in scope and accuracy because of the leveling exerted by acetic acid (Figures 1 and 2). The indirect acidimetric methods such as pyridinium chloride-pyridine, hydrochloric acid-dioxane, etc. (7), are subject to interferences from readily hydrolyzable compounds, if present, and from the buffering effect on the end point, exerted by weak bases such as amines. To overcome the interferences encountered because of the readily hydrolyzable materials in the application of acidimetric procedures, Jungnickel and associates suggested the argentimetric determination of the consumption of the chloride ion ( 7 ) . Later Shechter, Wynstra, and Kurkjy used this principle in the determination of oxirane oxygen in the presence of amines (8). Their method involved the use of 0.4N hydrochloric acid in diALTS
Present address, Central Research Laboratories, Food Machinery and Chemical Corp., P.O.Box 8, Princeton, N. J. 2024
ANALYTICAL CHEMISTRY
methyl formamide, reaction a t 98" C. in a pressure bottle, followed by potentiometric titration with silver nitrate. Stenmark recently determined epoxides in the presence of amines by the use of 0.2N hydrochloric acid in dioxane, followed by a Volhard titration of the unreacted chloride (9). The two methods presented determine oxirane oxygen in salts of epoxy acids and in the presence of amines with the speed and accuracy of aqueous acidbase titrations. Both procedures are based on the fact that addition of the salt of the epoxy acid in acetic acid results in the regeneration of the epoxy acid and the formation of the metal or amine acetate. Titration of this solution with 0.1N hydrogen bromide in acetic acid gives the corresponding bromohydrin and the metal or amine hydrobromide. In the first procedure, the hydrogen bromide consumed by the metal or amine acetate is determined by titration with aqueous silver nitrate. In the second, the metal or amine acetate is regenerated instantly by addition of excess mercuric acetate and titration of the regenerated metal or amine acetate with 0.1N perchloric acid in glacial acetic acid. The mercuric bromide formed and excess mercuric acetate do not interfere with the titration. Visual indicators are employed for the detection of the end point. REAGENTS AND APPARATUS
Perchloric acid in acetic acid, 0.1N, anhydrous. Mercuric acetate solution in glacial acetic acid, 6%. Dissolve 6 grams of analytical reagent grade mercuric acetate in 100 ml. of warm glacial acetic acid and cool before using. Sodium acetate solution, 0.1N. Dissolve 53 grams of anhydrous analytical reagent grade sodium carbonate in a liter of glacial acetic acid. Eosin Y indicator. Dissolve 0.5 gram in 10 ml. of distilled water. Reservoir burets, Karl Fischer type (1). PROCEDURE A
Dissolve 0.3 to 0.5 gram of sample in 10 ml. of glacial acetic acid in a 150-ml. Erlenmeyer flask equipped with a rubber stopper which has a round opening to accommodate the buret tip. Add
slowly, while stirring with a magnetic stirrer, 25 ml. of 0.1N hydrogen bromide in acetic acid dispensed from a Karl Fischer-type buret (1). The weight of the sample should be chosen so that not more than 24 to 24.5 ml. of 0.1N hydrogen bromide in acetic acid is required for the complete reaction. Add 20 ml. of 0.1N sodium acetate, 6 drops of Eosin Y indicator, 0.1 gram of gelatin, followed by 25 to 30 ml. of water, and titrate to the orange-crimson end point with 0.1N silver nitrate. Run a blank titration by the addition of 25 ml. of 0.1N hydrogen bromide in acetic acid to 35 ml. of 0.1N sodium acetate, and proceed as above. PROCEDURE
B
Dissolve 0.3 to 0.6 gram of sample in 10 ml. of glacial acetic acid or chlorobenzene in a 150-ml. Erlenmeyer flask equipped with a rubber stopper which has a round opening to accommodate the buret tip. Use glacial acetic acid for metal salts of epoxy acids and chlorobenzene for amine salts of epoxy acids and epoxy resins in the presence of amines. When glacial acetic acid is used as the solvent, the sample should be titrated immediately because ring opening of the oxirane oxygen will occur upon prolonged standing. Add 5 drops of crystal violet indicator and titrate the sample with O.IN hydrogen bromide to the blue-green end point. Add 10 ml. of mercuric acetate solution, stir, and titrate to the blue-green end point with O.IN perchloric acid. RESULTS AND DISCUSSION
A variety of salts of epoxystearic acid were analyzed (Table I). The metal salts were prepared according to the procedure of Greenspan and Gall (6), slightly modified to obtain pure compounds. The amine salts of epoxystearic acid were prepared by mixing equivalent amounts of epoxystearic acid and the corresponding amine in benzene, followed by removal of thc solvent under vacuum. All compounds used in the preparation of the salts of epoxystearic acid were analytical reagent grade and, where necessary, were further purified by recrystallization or fractional distillation. To check the applicability of these procedures in the determination of epoxy resins in the presence of amines, knoTvn blends of Epon 828, as a model
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Figure 1. Potentiometric titration using 0.2N hydrogen chioride in acetic acid (3) A. 6. C.
ferf-Octylamine epoxystearate Butylamine epoxystearate Piperidine epoxystearate
Table I.
resin, with N,N-dimethylbenzylamine, tert-octylamine, and di-n-butylamine were made and analyzed immediately (Table 11). Epon 828 is a low molecular weight condensate of epichlorohydrin and a bisphenol, produced by Shell Chemical Corp. Procedure A utilizing acetic acidhydrobromic acid and silver nitrate is gmerally applicable to all salts of epoxy acids and in the determination of oxirane oxygen in the presence of amines in epoxy compounds including resins. Procedure B utilizing hydrogen bromide in acetic acid and perchloric acid in acetic acid is applicable in the determination of a-epoxy groups in the presence of amines, and in lithium, potassium, sodium, barium, and strontium salts of epoxy acids. It is not applicable to magnesium, cadmium, and other divalent or trivalent metal salts. The metal acetates of these compounds formed upon solution of the epoxy salts in acetic acid give very poor ciid points, because of the high degree of association of such acetates in glacial acetic acid (6). This procedure is usually preferable where the hydrolysis of the bromohydrin proceeds a t a rather fast rate in solutions containing water. Like all hydrohalogenation methods, these methods are not applicable to epoxides containing a tertiary carbon atom (2), or epoxides like styrene oxide n.hich isomerize rather readily in acid media. Water and alcohols a t concentrations greater than 1.5% interfere in Procedure B because of the production of indefinite end points and alcoholysis and hydrolysis of the epoxide as side reactions. Although no end point interference can be encountered in the argentimetric procedure from the presence of water or alcohols, these compounds should be kept to a minimum
Epoxystearate Sample
Analysis of Salts of Epoxystearic Acid
Procedure A
4.98 4.36 Barium 4.67 Strontium 4 . 5 3 f 0.01 (3) Cadmium 5.13 &00.02(3) Magnesium 3 . 6 6 f0 . 0 2 ( 2 ) N,N-Dimethylbenzylamine 3.73&0.01(2) tert-Octylamine a Figures in parenthesis represent.No. of detns.
Sodium
II. Determination of Epoxy Resin in Presence of Amines Epoxy Compound Composition of Recovered, Wt. 7 0 0 Mixture, Wt. % Amine Resid A B
Table
43.30 50. 8d 26.36
56.7 49.2 73.7
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56.3 49.0 73.4
56.4 49.0 73.2
Calcd. from oxirane oxygen content. on 828 used as model compound. c ~N-dimethylbenzylamine. d tert-Octylamine. 8 Di-n-butylamine. 0
*E
Founda Procedure B
Theory
4 . 9 7 & 0.01 (4) 4.37 f 0 . 0 2 (4) 4.65 0 . 0 2 (4j
... 3 . 6 8 & 0.01 (3)
.
3.69&0.00(3)
4.99 4.37 4.69 4.52 5.17 3.69 3.74
oxirane oxygen of epoxystearic acid, either a poorly defined break was observed due to the amine acetate, or no break was observed, as for instance, with the n-butylamine salt. ACKNOWLEDGMENT
The author is grateful to Becco Chemical Division, Food Machinery and Chemical Corp., for permission to publish this work, to Joseph Bomstein for his review and criticism, and to members of the Research Department who prepared the epoxystearic acid. LITERATURE CITED
because hydrolysis and alcoholysis of the epoxide occurs. The accuracy of both procedures is generally better then 1%. The potentiometric titration of tertoctylamine, butylamine, piperidine, and barium salts of epoxystearic acid with 0.2N hydrochloric acid in glacial acetic acid was carried on according to the procedure described in a previous publication (3). Figures 1 and 2 demonstrate the leveling effect of acetic acid (4). The first break obtained is that due to the acetate ion, while the second is due to the epoxy group. Despite the considerable dserence in relative base strengths of the amines, barium acetate, and the
(1) Durbetaki, A. J., ANAL. CHEM.28, 2000 (1956). (2) Ibid., 29, 1666 (1957). (3) Durbetaki, A. J., J . Am. Oil Chemists' Soc. 33,221 (1956). (4) Fritz, J. S.. ANAL. CHEM.25. 407 ( 1953) .' (5) Zbid., 26,1701 (1954). (6) Greens an, F. P., Gall, R. J. (to
.,
Becco Cfemical Division, FMC), U. S. Patent 2,684,353 (July 20, 1954). (7) Jungnickel, J. L., Peters, E. D., Polgar, A Weiss, F. T., "Organic Analy&," qol. I, pp. 127-54, Interscience, New York, 1953. (8) Shechter, L., Wynstra, J., Kurkjy, R. P., I n d . En . Chem. 48, 94 (1956). (9) Stenmark, A., ANAL. CHEM.29,
d
1367,1809 (1957).
RECEIVED for review November 20, 1957. Accepted August 4, 1958. VOL 30, NO. 12, DECEMBER 1958
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