mizes surfaces and thus the adsorption of ever-present moisture.
Table 11. Comparison of Water Found in Polymer Granules by DuPont Moisture Meter and Silylation Procedure Silylation, p p m
287. 267 1501; 1550, 1240 180. 127 2631; 1959 123 135, 103 236 210, 262
ACKNOWLEDGMENT
Moisture meter, ppm
The author thanks J. R. McDivitt for valuable advice and John Sutherland for checking out the method and running innumerable analyses.
27 7 1430 133 1793 109 21 2 26 5 360
LITERATURE CITED (1) John Mitchell, Jr., "Treatise on Analytical Chemistry," Part 11, Vol. 1. I. M. Koithoff, P. J. Elving, and E. 6.Sandell, Ed., lnterscience Publishers, New York-London. 1961. (2) F. A. Keidel, U.S. Patent 2 630 945 (1958). (3) F. A. Keidel. Anal. Chern., 31, 2043 (1959).
be possible to reduce this limit through the use of very dry solvents and silylating reagents which have not contacted traces of moisture during manufacture and packaging. This static method for the determination of water mini-
RECEIVEDfor review September 17, 1975. Accepted November 10,1975.
Determination of Acetates and Acetyl Groups by Digestion of Samples in Perchloric Acid Followed by Either Nuclear Magnetic Resonance Spectrometry or Distillation and Potentiometric Titration A. A. Schilt" and G.
D. Martinie
Department of Chemistry, Northern Illinois University, DeKalb, Ill. 60 1 15
Methods are described for the determination of acetates and acetyl compounds based upon digestion with perchloric acid followed either by quantitative NMR analysis or by distillation and titration of the liberated acetic acid. Interference by codistilled perchloric acid is avoided by either of two methods. One utilizes ion-exclusion chromatography to remove perchloric acid prior to titration of acetic acid. The other involves a potentiometric titration after addition of dioxane to enable precise differentiation between acetic and perchloric acid contents. Applied to pure compounds, the methods gave results accurate to within 1% in most instances. Results are reported together with a study of digestion products for various samples.
Acetyl and acetate determinations commonly involve alkaline or acid hydrolysis to yield acetic acid, distillation of the liberated acetic acid, and titration of the acid with standard base ( I ) . Reagents most frequently used for the hydrolysis step include p -toluenesulfonic acid, sulfuric acid, and alcoholic potassium hydroxide. Many problems arise in such determinations including slow hydrolysis, oxidation of C-methyl groups to acetic acid, collection of volatile acids other than acetic in the distillate, and difficult distillation and inefficient recovery of the acetic acid ( I , 2 ) . Some procedures designed to overcome the problem of incomplete separations involve vacuum or steam distillation, use of traps or bubblers, or use of ion-exchange or other chromatographic techniques. Instrumental techniques have also been applied to circumvent, in certain cases, the need for distillation or even the initial hydrolysis step ( I ) .
The work reported here concerns an investigation of the effectiveness of perchloric acid digestion in place of the usual hydrolysis step for the determination of acetyl and acetate compounds. This approach seemed promising in view of the report by Smith ( 3 )that acetic acid is totally resistive to hot, concentrated perchloric acid, while formic acid is readily and completely oxidized. Thus formyl and formate groups should not interfere in acetyl and acetate determinations. Also, unlike the Kuhn and Roth method ( 4 ) which depends on the use of an oxidative mixture of chromic acid and concentrated sulfuric acid, the proposed method should be free of risk of oxidation of acetic acid. Inglis reports that recoveries of acetic acid are slightly low when distilled from chromic acid ( I ) . Furthermore, since the oxidation properties of hot, concentrated perchloric acid are generally superior to chromic-sulfuric acid mixtures, it seemed promising to explore perchloric acid digestion as an alternative method for liberating acetic acid quantitatively from C-methyl and acetyl compounds, As a part of the present study, some alternatives and refinements in the distillation and titration steps were also explored.
EXPERIMENTAL Reagents. All chemicals, including samples subjected to analy-
sis, were reagent grade and used as received without further purification. Apparatus. A Varian A-60-A NMR spectrometer equipped with a variable temperature controller and drilled quartz sample tubes was used for recording NMR spectra. Potentiometric titrations were performed using a Corning Model 7 p H meter equipped with glass and saturated calomel electrodes. The same instrument and electrodes (inserted in a Teflon flow-through cell) were used to
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monitor effluents in ion-exclusion chromatography. The ion-exclusion column was 200 cm long, of 2.5-cm inner diameter, and contained Dowex 50 W-X8 (50-100 mesh) resin in the hydrogen form. Digestions and distillations were carried out behind a safety screen using an apparatus consisting of a 100-ml distillation flask, fitted with a Bethge distillation head (Item No. 370, G. Frederick Smith Chemical Co., Columbus, Ohio), and an 18-inch condenser. Heating was supplied by an electric heating mantle. Procedures. For possible hazards and safety precautions in using perchloric acid, see Results and Discussion section. Three different procedures were devised and evaluated for the determination of acetate and acetyl groups in organic compounds. Procedure A involves a digestion step followed by NMR analysis without an intermediate separation; Procedure B involves digestion, distillation, ion-exclusion chromatography separation of acetic from perchloric acid, and titration; and Procedure C involves digestion, distillation, and titration. Details of each follow. Procedure A . The sample for analysis was weighed into the distillation flask, the still was assembled, and 10 ml of distilled water and 30 ml of 72% perchloric acid were added. With the three-way stopcock of the distillation head set to retain distillate, heating was initiated and the acid was concentrated in the flask until the temperature reached 200 "C. At this time, the stopcock was opened slightly to begin returning the distillate slowly to the flask and heating was continued at reflux for 15 min. After cooling to room temperature, the contents of the flask were diluted to exactly 100 ml with distilled water, taking care to wash down the sides of the digestion flask and Bethge apparatus so as to include all of the solution. The integral of the NMR peak of the methyl protons of acetic acid ( & = 2.1 ppm) was recorded for the resulting solution a t a controlled temperature of 20 "C and compared to those obtained with standard solutions measured under identical conditions of R F power, gain, filter, and all other parameters. Concentration of acetic acid in the unknown was deduced from the calibration curve obtained from the knowns. Procedure R. A weighed sample was treated with 15 ml of distilled nater and 35 ml of 72% perchloric acid in the 100-ml distilling flask and digested for approximately 15 min, during which time the stopcock on the Bethge distillation head was closed to retain distillate. At the end of the digestion period, when the flask temperature reached 203 "C and approximately 20 ml of distillate had been collected, the distillate was transferred quantitatively, along with distilled water washings also, to the ion-exclusion chromatography column. The acetic acid content was eluted with distilled water at a flow rate of 1 ml/min, using both pH and NMR to monitor successive 10-ml portions of eluent. Those fractions containing acetic acid free of perchloric acid (tubes numbered 14 through 19) were combined and titrated to the phenolphthalein end point with standard sodium hydroxide. Procedure C. The sample was weighed into the distillation flask, the still was assembled, and 2 ml of distilled water and 10 ml of 72% perchloric acid were added. With the stopcock of the Bethge apparatus closed to retain distillate, the sample was digested for 15 min, allowed to cool, and 18 ml of 6 M sodium hydroxide was carefully added. Heating was resumed until approximately 20 ml of distillate in all had collected in the Bethge receiver. The distillate was removed and chilled in an ice bath. An additional 20-ml portion of distilled water was added to the still, and collection of distillate was resumed. As distillate arrived in the receiver, it was allowed to rinse the inner walls of the receiver and to flow into the flask containing the first 20 ml of distillate collected. After a total of 40 ml was collected, the distillate was diluted with an equal volume of dioxane and titrated potentiometrically with standard sodium hydroxide. From the difference between the first inflection point due to perchloric acid and the second due t o acetic acid, the amount of acetic acid was determined.
RESULTS AND DISCUSSION T h e i n e r t n e s s of acetic acid t o oxidation b y hot and conc e n t r a t e d perchloric acid w a s confirmed b y boiling a m i x t u r e of a k n o w n a m o u n t of acetic acid in 72% perchloric acid for 2 h r at t o t a l reflux in t h e B e t h g e a p p a r a t u s . Analysis revealed that 99.2% of the acetic a c i d remained. Acetic acid d i d n o t prove i n e r t however t o m i x t u r e s of nitric and perchloric acids. Digestion w i t h a 1:20 m i x t u r e of concentrated nitric a n d perchloric a c i d s resulted i n g r a d u a l oxidat i o n of acetic acid, with only 20% r e m a i n i n g a f t e r 8 h r of r e fluxing at t h e boiling point. V a n a d i u m ( V ) a n d c e r i u m ( I V ) , 448
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Table I. Determination of Acetate C o n t e n t of P-DGlucose Pentaacetate by T h r e e Different Procedures Procedure
Sample wt, g
A A
1.359 1.317 0.0702 0.0817 0.0788 0.0802
B B C C
_
Acetate Content, mg Calculated
10.2
10.0
10.0
0.0530 0.0619 0.0596 0.0607
_
_
~
Error, %
Found
10.3
-1.0 0.0
0.0526 0.0620 0.0603 0.0603
_
_
-0.8
4.2 +1.2 -0.7
_ ____ ~
Table 11. Determination of Acetyl G r o u p s a n d Acetate in Selected Pure C o m p o u n d s by Procedure C Millimoles CH,CO Compound
Weight, g
Theory
Found
Error, %
Acetic acid Acetylsalicylic acid Acetanilide Pentaerythritol tetraacetate Isoamyl acetate Triacetin Triacetin Butyl acetate Dehydroacetic acid Anthrahydroq u i n o n e diacetate
0.0525 0.3240 0.2316
0.875 1.798 1.713
0.87
-0.6
1.78 1.72
-1.0
0.2236 0.1839 0.1455 0.2728 0.2157 0.3757
2.939 1.413 2.000
-1.3 -0.9 +0.5
3.750 1.857 2.230
2.90 1.40 2.01 3.71 3.02
-1.4 +35.4
0.2741
1.680
1.71
+1.2
1.83
+0.4
-1.1
f r e q u e n t l y used as catalysts, p r o v e d to be w i t h o u t effect o n the stability of acetic acid t o w a r d s hot and c o n c e n t r a t e d perchloric acid, p e r m i t t i n g use of t h e s e oxidation catalysts if desired w i t h o u t loss of acetic acid. Various s t u d i e s revealed that acetic acid c a n be q u a n t i tatively recovered b y distillation, but o n l y if appreciable a m o u n t s of perchloric acid a r e also codistilled. T h i s need n o t be a serious limitation, because f u r t h e r s e p a r a t i o n or selective m e a s u r e m e n t of the acetic acid is possible. In f a c t , codistillation affords the a d v a n t a g e of improving speed and completeness of acetic acid recovery. T h r e e m e t h o d s of d e t e r m i n i n g acetic acid i n t h e presence of perchloric acid were investigated. T h e NMR metho d , based o n the i n t e g r a t e d a r e a of t h e signal d u e to acetyl p r o t o n s (6 = 2.1 p p m vs. TMS) gave a relative standard deviation of 1.43% for a 70% perchloric acid solution 0.850 M i n acetic acid and 4.2% for a similar solution 0.085 M i n acetic acid. T h i s m e t h o d is less sensitive and less precise t h a n t i t r a t i o n m e t h o d s . It requires careful a d j u s t m e n t s of d e t e c t o r p h a s e and d e t e c t o r zero. F u r t h e r details of q u a n t i t a t i v e analysis b y NMR a r e described by K a s l e r ( 5 ) .The a d v a n t a g e s of t h e m e t h o d a r e i t s simplicity and t h e f a c t that perchloric acid does n o t interfere, thus obviating need of a s e p a r a t i o n step. The o t h e r t w o m e t h o d s involved t i t r a t i o n of t h e acetic acid, o n e in t h e presence of perchloric acid, t h e o t h e r a f t e r s e p a r a t i o n b y ion-exclusion c h r o m a t o g r a p h y . Of t h e s e two, the f o r m e r is superior in s p e e d a n d precision. Based on five replicate d e t e r m i n a t i o n s , the d i r e c t titration method gave relative s t a n d a r d deviations of 0.52% for 0.858 M and 1.3% for 0.088 M acetic acid, both solutions also 1 M in perchloric acid. Results o b t a i n e d b y e a c h of t h r e e m e t h o d s , P r o c e d u r e s A, B, a n d C, in the d e t e r m i n a t i o n of a c e t a t e c o n t e n t of a p u r e s a m p l e of P-D-glucose p e n t a a c e t a t e are compiled in T a b l e I. P r o c e d u r e A r e q u i r e d larger s a m p l e sizes for o p t i m u m precision. Smaller s a m p l e s , safer to t r e a t w i t h perchloric acid, yielded satisfactory r e s u l t s b y e i t h e r P r o c e d u r e B or C. Of the t h r e e , P r o c e d u r e C is the m o s t generall y applicable and m o s t practical.
Table 111. Substances Remaining after Digestion of Samples Sample
Residual substances (mol/mol sample)
Kemarksa
A Acetic acid (0.23); Acetone (0.77) Acetylacetone A Butyric acid Acetic acid (0.04) A EDTAb None Acetic acid (0.01); Acetone (0.88);Ethanol (0.91) Ethyl acetoacetate A, B C 2-Heptanol Acetic acid (0.10) C Acetic acid (0.30);Unknown acetyl compound 2-Heptanone A Lactic acid Acetic acid (0.04) D Nitroethane Acetic acid (0.01) C Acetic acid (0.41);Unknown acetyl and methyl compound(s) Pinnacol 1,2-Propanediamine Propanediamine (0.66) 1,2-Propanediol Acetic acid (0.09) Sodium formate None Sodium propionate Propionic acid (1.00) a A : Charring occurred; B: Vigorous reaction accompanied by vapor loss; C : Tar formation; D : Explosive reaction. b Ethyl enedinitrilotetraacetic acid. Procedure C requires the addition of an equal volume of dioxane to the distillate prior to potentiometric titration with standard base. This greatly improves the sharpness of the first inflection point (corresponding to neutralization of perchloric acid), permitting much greater precision in determining the additional amount of base necessary to titrate the acetic acid content. Another innovation incorporated in Procedure C is the partial neutralization of the perchloric acid prior to the distillation step. This aids in minimizing the amount of perchloric acid codistilled without adverse effect on recovery of acetic acid. One may safely omit partial neutralization prior to distillation, hut a larger volume of standard base is then necessary in the titration of the perchloric acid. Procedure C gave the results listed in Table I1 when applied to a variety of pure compounds containing one or more acetyl groups. Dehydroacetic acid was the only one to give a disparate result, presumably because of partial conversion of its C-methyl group to acetic acid. Acetanilide, acetylsalicylic acid, and anthraquinone diacetate yielded volatile digestion products that solidified in the condenser and receiver during distillation. These proved to be nonacidic and did not interfere in the titration. Further tests on the solids indicated that they were chlorinated aromatic compounds. The product from acetanilide yielded a mass spectrum identical to that of a pure sample of tetrachloroquinone. Results of a study of possible interferences and troublesome compounds are summarized in Table 111. Relatively large samples (1-2 g) were treated with 35 ml of 50% perchloric acid and 100 mg of vanadium pentoxide, heated a t reflux (145 "C) for 30 min in the Bethge apparatus, and examined by NMR spectrometry to determine acetic acid and any other products. Although very effective for hydrolysis of N - and 0-acetyl groups, the perchloric acid digestion procedure proved inefficient for the conversion of C-acetyl and C-methyl groups to acetic acid. T o compensate for incompleteness one could apply empirical corrections in the same manner as is done in previously described methods ( I ) ; however, it would be preferable if conditions could be found that lead to quantitative oxidation of such groups to acetic acid. Unfortunately, when more concentrated perchloric acid was used, or when the acid was allowed to concentrate by distillation and the distillate return then re-
sumed, explosive reactions resulted with certain compounds (nitroethane, acetophenone, acetylsalicylic acid, and acetylacetone). Formic acid and formates can interfere in the determination of acetic acid if the digestion time is not sufficiently long. Total destruction of 0.5 g of formic acid in 1 2 ml of 50% perchloric acid required 25 min a t refluxing temperature. Oxidation is slow primarily because formic acid is largely vaporized so that only a small fraction is present in the digestion mixture a t any given time. It is imperative to stress here the possible hazards involved in digesting too large or unknown samples. For example, an explosion occurred when nitroethane was digested, causing the top of the still to be blown off. Samples should be tested using small amounts initially before subjecting them to analysis, and a safety screen should be used at all times. For details of safe handling of perchloric acid, the reader is referred to articles by Smith (3, 6) and by the Analytical Methods Committee of the Society for Analytical Chemistry ( 7 ) . The advantages afforded by perchloric acid, particularly in Procedure C, justify any extra precautions necessitated by its use. Because Procedure C is applicable to small samples, any explosion hazards can be minimized. Digestion time is shorter than the times involved in the various hydrolysis methods currently used. The high acidity, boiling point, and oxidative strength of perchloric acid facilitate release of acetic acid, and formates do not interfere. Distillation of acetic acid can be accomplished rapidly and completely, and codistilled perchloric acid does not interfere when the titration is performed potentiometrically.
LITERATURE CITED (1) A. S. lnglis in "Treatise on Analytical Chemistry", Part II, Vol. XIV, I. M. Kolthoff and P. J. Elving, Ed., Interscience, New York, N.Y. 1971. (2) J. P. Dixon, "Modern Methods of Organic Microanalysis". D. Van Nostrand, London, 1968. (3) G. F. Smith, Anal. Chin?. Acta, 8, 397 (1953). (4) R. Kuhn and H. Roth, Ber., 66, 1274 (1933). ( 5 ) F. Kasler, "Quantitative Analysis by NMR Spectroscopy", Academic Press, London, 1973. (6) G. F. Smith, Analyst (London),80, 16 (1955). (7) Analytical Methods Committee, Ana/yst (London), 84, 214 (1959).
RECEIVEDfor review August 4, 1975. Accepted October 23, 1975.
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