2157
Anal. Chem. 1984, 56,2157-2160 (13) Sabot, J. L.; Bauer, D. R o c . Int. SolventExtr. Conf. 1979, 27 (ISEC 77, CIM Special Volume), 509. (14) Nedjate, H.; Sabot, J. L.; Bauer, D. Hydrometallurgy 1978, 3, 283. (15) Mazitova, F. N.; Khairullin, V. K. Zh. Obshch. Khlm. 1980, 50, 1718. (16) Kosolapoff, G.M.; Maier, L. "Organic Phosphorus Compounds"; WlleyInterscience: New York, 1976, Vol. 7. (17) Mlotkowska, 8.; Zwierzak, A. J . frakt. Chem. 1978, 320, 777.
(18) Handley, T. H. Anal. Chem. 1963, 35, 991.
RECEIVED for review February 29, 1984. Accepted May 21, 1984. ~ ~ is made to the~ for ~ support of this research.
coFREMMI
Spectrophotometric and Titrimetric Determination of Carboxylic Acid Anhydrides Krishna K. Verma* and Pramila Tyagi Department of Chemistry, R.D. University of Jabalpur, Jabalpur 482 001, India
Two redox methods are descrlbed for the determlnation of carboxylic acid anhydrides involving reactlon with elther an excess of 4-amlnophenol or a measured but excessive amount of sulfanllamlde and photometrlc tltratlon of N-acyl4-amlnophenol with 2-iodylbenroate by measurlng the absorbance of orange-red product at 444 nm or the residual amount of sulfanilamide Is determlned by tltratlon with chloramine-T In the presence of acldtfled potasslum bromlde using methyl red as vlsual indlcator. Mlxtures of certaln anhydrides have also been analyzed by the 2-lodylbenzoate method. The methods are rapid, preclse, and accurate. No correlation Is needed If carboxylic and mlneral acids are also present. Carboxyllc acld chlorldes also react quantltatlvely.
Earlier methods for carboxylic acid anhydrides were based on acid-base titrations making necessary a correction in anhydride content due to acid present. Simultaneous measurement was made by titrating the originally present acid plus that produced on hydrolysis of anhydride and then determining the anhydride from difference to a second titration of the sample with either sodium methoxide or alkali hydroxide after pretreatment of the test sample with aniline (1). Reaction with morpholine (2-4) or aniline (5) followed by titration of residual amine assays the anhydride independent of carboxylic acid impurities. However, limitations are due to maleic and citraconic anhydrides, as their acids (and others are acidic to with ionization constant greater than 2 X visual indicator (2), and unsaturated anhydrides which undergo addition reaction with excess morpholine (4). The aniline method is free from such shortcomings but the reaction with succinic anhydride occurs under vigorous conditions so that the initially produced anilide and any free succinic acid present also combine with aniline (5). Mineral acids interfere in both of the amine methods. In the presence of large excesses of anhydrous acetic acid, the resultant amide tends t o buffer the end point in the determination of acetic anhydride (2). Formation of hydroxamic acid and color development with iron(II1) has been used for organic esters, acid halides, anhydrides, and lactones (6). The calibration curves are not always linear with anhydrides (1). Two methods are described in the present paper which make use of redox reactions and are free from the difficulties mentioned above. It is observed that 4-aminophenol undergoes acylation exclusively a t its amino group when an excess of it is made to react with carboxylic acid anhydrides.
The N-acyl-4-aminophenol produces almost instantaneously an orange-red color on oxidation with 2-iodylbenzoate that absorbs maximally a t 444 nm. The titration can be followed by measuring the color produced. The second method also involves acylation of an aromatic amino group but now of the sulfanilamide which after acylation fails to undergo its typical disubstitution reaction with bromine (7). The amount of sulfanilamide consumed in acylation, as found by titration with chloramine-T in the presence of acidified potassium bromide, is a measure of anhydride.
EXPERIMENTAL SECTION Reagents. 2-Iodylbenzoate,0.01 M solution, was prepared by dissolving 2.8 g of the free acid reagent (synthesized by the method of Banerjee et al. (8)) in a slight molar excess of potassium hydroxide (0.7 g in 50 mL of water) and diluting to 1 L with water. It was standardized iodometrically O ~ I C 6 H 4 C o+~41-
+ 5H+
+
212 + IC~H,COZH+ 2Hz0
Alternatively, a known amount of analytical reagent grade paracetamol (N-acetyl-4-aminophenol) is photometrically titrated with the reagent (as described below),the two substances reacting on a 1:l molar ratio. The strength of 24odylbenzoate determined by two methods agreed within 0.2%. Chloramine-T, 0.02 M solution, was made by dissolving 5.62 g of sodium N-chloro-4-toluenesulfonamidetrihydrate in 1 L of water and standardizing the solution iodometrically (9). Sulfanilamide, 0.01 M solution (about 0.43 g), was dissolved in 250 mL of ethanol. Prestandardization is not necessary as a reference titration on the same volume of this solution is also carried out along with the analysis. 4-Aminophenol was a 2% solution in ethanol. Methyl red was a 0.02% indicator solution in ethanol. Apparatus. A Pye-Unicam SP 8-100 spectrophotometer was used. Samples. All anhydride samples were purified by either distillation or recrystallization, and their purity was checked by established methods (Tables I and 11).
Procedures. Photometric Titration with 2-Iodylbenzoate. An accurately weighted amount of sample containing about 1mmol of anhydride is combined with 10 mL of 2% 4-aminophenol and warmed in a boiling water bath for a minute, except for phthalic and camphoric anhydrides where heating for 10 min under reflux is necessary. The contents are cooled to room temperature and diluted to volume in a 100-mL calibrated flask with ethanol or acetone. Two to 5 mL portions of this solution are mixed in a photometric cell with 20 mL of acetone and 5 mL of 5% sulfuric acid and diluted to 50 mL with water. The absorbance of solution is set to zero or minimum at 444 nm and it is titrated photometrically with 0.01 M 2-iodylbenzoate. The absorbance increases initially and then becomes constant after the complete reaction.
0003-2700/84/0356-2157$01.50/00 1984 American Chemical Society
~
2158
ANALYTICAL CHEMISTRY, VOL. 56,NO. 12,OCTOBER 1984
Table I. Determination of Anhydrides by the 2-Iodylbenzoate Method
t
7% puritf (% CV) reaction reaction 2-iodylbenzoate comparison anhydride time, min temp, OC method method acetic
propionic
1 1
butyric
1 1
20 20 50 50 70 20 20 70 20 50 20 50
1
20
5
10
20 50 50 20 20
1
50
10 5
70 20 50 70 70 20 50 70 70
1
5 1
10 10
succinic
1
5 20
maleic
1
20 cinnamic
phthalic
2
2
camphoric
10 20 5 2 10 20
97.9 (0.4) 99.6 (0.2) 99.7 (0.2) 99.8 (0.2) 99.7 (0.2) 98.0 (0.3) 99.2 (0.1) 99.3 (0.2) 98.0 (0.4) 100.1 (0.1) 97.2 (0.3) 98.6 (0.2) 98.4 (0.3) 99.7 (0.1) 99.6 (0.2) 100.8 (0.2) 90.2 (0.3) 96.5 (0.5) 98.1 (0.3) 98.3 (0.3) 28.6 (0.5) 45.2 (0.8) 100.1 (0.2) 100.3 (0.4) 22.6 (0.4) 38.5 (0.8) 99.6 (0.3) 99.8 (0.4)
99.8b
W
0
5m 0.6
a
0 m
v)
99.4*
* 0.4
99.8c 0.2
99.0d 99.8c 370
410
450
530
490
WAVELENGTH C h m 1
98.0'
99.9c
99.5c
Average of six determinations; CV, coefficient of variation. Morpholine method (2). Aniline method (5). Morpholine-carbon disulfide method ( 4 ) . e Tetrabutylammoniumhydroxide titration (IO).
Flgure 1. Absorption spectra of chromogen: acetic anhydride, 4.36 X M in water (A) and in 50% acetone (B); maleic anhydride, 7.96 X M in water (C) and in 50% acetone (D).
sulfanilamide,diluted to 20 mL with ethanol in a 250-mL conical flask, and warmed in a boiling water bath for a minute. The contents are cooled to room temperature and combined with 10 mL of 5% sulfuric acid, 0.5 g of potassium bromide, 20 mL of methanol, and 3 drops of methyl red indicator, and the residual sulfanilamide is titrated with 0.02 M chloramine-T until the red color of methyl red is sharply bleached. More methanol is added if the brominated sulfanilamide precipitates during titration. A reference titration on the same volume of sulfanilamide is concurrently carried out
@
reaction reaction chloramine-T comparison anhydride time, min temp, OC method method 2 1
succinic
10 5 1
10 maleic
5 1
10 butyric propionic phthalic
1 1 1 20
20 20 70 70 20 70 70 20 70 70 70 70 70 70
98.6 100.1 100.2 (0.2) 100.1 98.0 99.3 (0.1) 99.5 98.2 99.5 (0.2) 99.8 98.5 (0.2) 99.9 (0.2) 0.0
where AV is difference in volume (mL) of chloramine-T (molarity
RESULTS AND DISCUSSION
% purity" (% CV)
5
mol wt 2
-AVM
M) used for reference and experimental titrations.
Table 11. Determination of Anhydrides by the Chloramine-T Method
acetic
amt of carboxylic acid anhydride (mg) =
99.9b
99.4c
With an excess of 4-aminophenol, the acylation reaction occurs rapidly a t its amino group yielding N-acyl-4-aminophenol which undergoes oxidation with 2-iodylbenzoate in acid medium in a 1:l molar ratio producing orange-red color that has maximal absorption a t 444 nm. The observation that chromogen is decolorized by reduction with ascorbic acid and reoxidized by 2-iodylbenzoate to the colored product and that 4-aminophenol acylated a t amino and phenolic groups both by reaction with excess of anhydride does not yield any color with 2-iodylbenzoate led to the color reaction
99.6c C02H
98.8d 99.74 100.lC
0.5
@Averageof six determinations. Aniline method (5). Tetrabutylammonium hydroxide titration (IO). d Morpholine method (2)*
The end point is determined graphically by plotting absorbance (corrected for dilution) against the volume of reagent added amt of carboxylic acid anhydride (mg) = (mol wt)VM where V is volume (mL) of 2-iodylbenzoate of molarity M . Visual Titration with Chloramine-T. A known weight of anhydride (0.02-0.1 mmol) is mixed with 2 to 10 mL of 0.01 M
O G N ' X R
+
@-IO
t
H20
C02H
4-Aminophenol itself does not produce any color with 2iodylbenzoate. Every saturated aliphatic anhydride examined in this work produces an orange color while maleic, cinnamic, and phthalic anhydrides give red products. The color deepening is ostensibly due to extension of conjugation. The wavelength of maximum absorption in both cases is, however, the same, Le., 444 nm. Acetone when used as a cosolvent during the color development produces a pronounced hyperchromic shift without any concomitant change in A,, (Figure 1). This effect reaches its optimum value a t about 30% (v/v) of acetone concentration. Use of this observation
ANALYTICAL CHEMISTRY, VOL. 56, NO. 12, OCTOBER 1984
Table 111. Determination of Mixtures of Anhydrides by 2-Iodylbenzoate Method
amt found,b mg ( % CV) I1 I
amt added," mg I1 I acetic
maleic
1.15 2.49 4.34
5.30 4.12 1.13
succinic
maleic
1.65 3.09 5.21
4.86 3.15 1.84
acetic
phthalic
1.47 2.96 3.63 5.04
6.25 4.50 2.99 1.82
succinic
cinnamic
1.64 2.89 3.72 5.08
6.76 5.88 4.31 2.95
acetic
camphoric
1.58 2.90 3.68 4.93
6.82 5.71 3.97 2.63
1.19 (0.3) 2.56 4.15 (0.3)
5.48 (0.2) 4.00 1.06 (0.3)
1.72 3.00 (0.1) 5.31
4.93 3.08 (0.2) 1.90
1.38 (0.3) 2.99 3.58 (0.2) 5.15
6.32 (0.2) 4.45 3.04 (0.4) 1.76
1.68 (0.2) 2.78 3.76 (0.1) 5.19
6.85 (0.3) 5.79 4.30 (0.2) 2.89
1.63 2.99 (0.3) 3.57 4.86 (0.4)
6.94 5.80 (0.4) 4.09 2.60 (0.4)
"Amount determined on the basis of percent purity of samples (Table I). *Average of four determinations. has been made in the sensitive and accurate measurement of absorbance in photometric titrations. Sulfanilamide undergoes a substitution reaction a t two positions ortho to the amino group with bromine produced in situ by oxidation of acidified potassium bromide with chloramine-T.
-
+
MeC6H4S02NC1Na 2HBr MeC6H4SO2NH2 NaCl
H2N
+
b 0
SO,",
+ Br2
+ 2HBr
Br
Sulfanilamide undergoes quick acylation with aliphatic anhydrides exclusively a t its aromatic amino group which then fails to consume bromine in a substitution reaction as above because of deactivation and steric effects. Thus, the following relationship is maintained:
( R C 0 ) 2 0 = H2NC,H4S02NH2= 2MeC,H4S02NC1Na The bleaching of methyl red at the end point is faster than any secondary reactions. Therefore unsaturated acids, e.g., maleic and cinnamic acids, do not show any bromine addition to their double bond and, when present with corresponding anhydrides, do not cause interference. In contrast to aniline (5), the reaction of sulfanilamide with succinic anhydride is faster but that with phthalic anhydride is only negligible under the present experimental condition. Though it has application only to reactive anhydrides, the advantage of sulfanilamide (over aniline) is that there is no reaction with carboxylic acids. N-Acylated aromatic amines (e.g., acetanilide) do not show ortho bromination, but the para position is still open for substitution though with a poor velocity that does not permit a direct titration. Amines with ortho substituents acylated slower than their unsubstituted isomers. This has therefore exluded the possible use of amines that either have free para position or have ortho substituents (meta isomers do not much alter the nature of amine as this position is neither active for electrophilic bromination nor has a direct influence on rate of acylation). Sulfanilamide is the reagent of choice; secondly, it is obtained in a state of high purity and keeps well. 4-Aminophenol is more reactive reagent and requires less time and lower temperature for complete reaction than does aniline (5))and free acids do not interfere. The reagent should always be added in 50-100% excess to anhydride so that the acylation reaction occurs only at the amino group (which has preferential reactivity than the hydroxyl group). The color development is almost instantaneous, stable for long periods and unaffected by several fold excess amounts of 2-iodylbenzoate. Several other common oxidizing agents (9) were also tried in place of 2-iodylbenzoate but none yielded any color with N-acyl-4-aminophenol, perhaps because the oxidation is carried further over to N-acyl-1,4-benzoquinone imine. Other iodyl compounds, e.g., iodylbenzene, also produce color but these were not employed mainly because of their insolubility in common solvents. The 2-iodylbenzoate method has a wider range of application. Attempts were made to analyze mixtures of anhydrides by this method. The titration curves (Figure 2) have two breaks, the first corresponding to reaction with orange color producing anhydride (all saturated aliphatic) and the second to that which yields red color (maleic, cinnamic, and phthalic anhydrides). When two orange color producing anhydrides are titrated, e.g., acetic and camphoric anhydrides, it is the low molecular weight anhydride that is titrated first. Results for the determination of mixtures of anhydrides are given in Table 111. Esters do not interfere with either method; however, acid chlorides react quantitatively and can be determined by anhydride methods. Table IV includes results for the de'termination of two such compounds. Attempts were also made to determine anhydrides by a direct (nontitrimetric) spectrophotometric method based on the 2-iodylbenzoate reaction. However, the color absorbance
Table IV. Determination of Acid Chlorides
acid chloride
carboxylic acid ( % added)
benzoyl
% acid chloride" ( % CV)
2-iodylbenzoate method
chloramine-T method
99.1 99.2 94.2 94.5 78.9 79.1 acetyl 98.6 98.4 acetic (10) 88.7 86.2 acetic (20) 78.4 78.0 a Average of six determinations. Titration with tripropylamine and sodium hydroxide ( 1 ) . benzoic (5) benzoic (20)
2159
comparison method 99.4b 98.8*
2160
Anal. Chem. 1984, 56, 2160-2165
(Figure 1). The color development rate is slow when water ratio is 30% or below, and in neat organic solvents negligible color is produced.
Registry No. Acetic anhydride, 108-24-7;succinic anhydride, 108-30-5; propionic anhydride, 123-62-6; butyric anhydride, 106-31-0;maleic anhydride, 108-31-6;phthalic anhydride,85-44-9; benzoyl chloride, 98-88-4;acetyl chloride, 75-36-5;cinnamic anhydride, 538-56-7;camphoric anhydride, 76-32-4;4-aminophenol, 123-30-8;sulfanilamide,63-74-1; 2-iodylbenzoic acid, 64297-64-9. LITERATURE CITED
mL OF 2-IODOXYBENZOATE
Figure 2.
Photometric titration curves of mixtures of N-acyi-4aminophenois: (A) acetic (1.58 mg) and phthalic anhydride (1.48 mg); (B) succinic (3.52 mg) and maleic anhydride (1.86 mg); (C) propionic (3.25 mg) and maleic anhydride (3.43 mg); (D) acetic (2.70 mg) and camphoric anhydride (6.66 mg).
is found to be critically dependent on the proportion of ethanol, methanol, acetone, or anhydrous acetic acid used as solvent for the acylation reaction, there always being observed a hyperchromic effect with an optimum shift with acetone
(1) Siggia, S.; Hanna, J. G. "Quantitative Organic Analysis via Functional Groups", 4th ed.; Wlley-Interscience: New York, 1979; pp 175, 223-243. (2) Johnson, J. B.; Funk, 0. L. Anal. Chem. 1955, 27, 1464. (3) Ruch, J. E. Anal. Chem. 1975, 47,2057. (4) Critchfield, F. E.; Johnson, J. B. Anal. Chem. 1958, 28,430. (5) Siggia, S.;Hanna, J. G. Anal. Chem. 1951, 23, 1717. (6) Goddu, R. F.; LeBlanc, N. F.; Wright, C. M. Anal. Chem. 1955, 27, 1251. (7) Verma, K. K.; Gupta, A. K. Anal. Chem. 1982, 5 4 , 249. (8) Banerjee, A,; Banerjee, G. C.; Bhattacharya, S.: Banerjee, S.;Samadar, H. J . Indian Chem. SOC. 1981, 58,606. (9) Berka, A.; Vulterin, J.; Zyka, J. "Newer Redox Titrants"; Pergamon: Oxford, 1965; p 37. (10) Lucchesi, C. A,; Kao, L. W.; Young, G. A,; Chang, H. M. Anal. Chem. 1974, 4 6 , 1331.
RECEIVED for review February 8,1984. Accepted May 1,1984. Thanks are due to the Council of Scientific and Industrial Research, New Delhi, for a Junior Research Fellowship to P.T.
Comparison of Analytical Pyrolysis Techniques in the Characterization of Chitin Arie van der Kaaden* and Jaap J. Boon F.0.M.-Institute for Atomic and Molecular Physics, Kruislaan 407, 1098SJ Amsterdam, T h e Netherlands Jan W. de Leeuw, Frits de Lange, and P. J. Wijnand Schuyl Department of Chemistry and Chemical Engineering, Delft University of Technology, Delft, T h e Netherlands Hans-R. Schulten and Ute Bahr Fachhochschule Fresenius, Dambachtal20, 6200 Wiesbaden, Federal Republic of Germany The results of various analytical pyrolysls techniques wlth on-llne and off-llne detectlon have been examlned wlth respect to characteristlclty and contents of Information, using chltln [poly-( 1+4)+3-~-2-acet amldo-2-deoxyglucopyranose] as a model polymer. Curie-polnt pyrolysis-low-energy electron impact mass spectrometry (cPy-EIMS), using normal and preheated telescopic glass tube sample holders, was applied, as well as Curle-point pyrolysis-gas chromatography/mass spectrometry, with electron Impact lonlration and chemlcal Ionization (cPy-GC/EIMS, cPy-GC/CIMS). Mlcro-oven pyrolysis-high-resolution fleld lonlzatlon mass spectrometry (Py-FIMS) was used. I n addltlon, milligram-scale pyrolysls combined wlth off-llne gas chromatographlc and gas chromatographlc/mass spectrometric detectlon was carried out. Chemlcal characterlzatlon of the pyrolysls products Indicates various anhydro-2-aceiamldo-2-deoxyglucosesubstances as the most representative for the chltln structural constltuents. I n this respect Py-FIMS and cPy-GCMS yleld patterns whlch characterize best the chltln structure.
A number of analytical pyrolysis techniques are presently
available (1). Preference largely depends on practical considerations: availability of instrumentation, information contents, time of analysis, sample capacity, possibilities for automatic sample handling, and data processing. The techniques differ especially with respect to the pyrolysis conditions applied and the detection of the volatile pyrolyzate. The pyrolysis conditions vary from microgram to milligram sample quantities, vacuum to inert atmospheric reaction conditions, and heating rates varying from OC/ms to OC/s. On-line and off-line analysis of the pyrolyzate is possible. Mass spectrometry is the most commonly used detection system, but is performed in several modes, viz. electron impact ionization (EI), chemical ionization (CI), field ionization (FI),and field desorption (FD). Gas chromatographic separation of the pyrolyzate, previous to mass spectrometric analysis, is also often applied. The recent developments in pyrolysis-mass spectrometry have been reviewed (2). The application of Curie-point pyrolysis-gas chromatography/mass spectrometry has been described in a number of reports (3-5). The aim of analytical pyrolysis methods is to obtain fingerprints of the original material, which can be compared with patterns of reference substances or model compounds. The
0 1984 American Chemical Society 0003-2700/84/0356-2160$01.50/0
