the concentration of aluminum in the sample and the decrease in absorbance, which is related in terms of sodium hydroxide concentration. Effect of Sodium Gluconate Concentration. A study was made to determine the concentration of sodium gluconate needed for the complete release of hydroxyl ion from the complex. A concentration of 7'lz Z sodium gluconate gave optimum results. Effect of Sodium Carbonate. Since sodium aluminate samples contain various concentrations of sodium carbonate, a study was made to determine any effect due to carbonate. In the presence of carbonate, there is a slight increase in absorbance due to the hydrolysis of the carbonate which produces hydroxyl ion. This effect was eliminated in the
semi-automatic determination by adding an excess of sodium carbonate to the reference and sample systems, thereby negating any effect of hydrolysis from the sample. ACKNOWLEDGMENT The authors thank R. C. Obbink and E. P. Cox for helpful discussions. The experimental assistance of B. G . Kunkle is greatly appreciated. RECEIVED for review December 22, 1969. Accepted March 23, 1970. Presented at the Anachem Conference, Detroit, Mich., September 1969.
Quantitative Determination of Ethoxyl in 0-Ethylcellulose and in Ethyl-Hydroxyethyl Cellulose Harry Jacin' and John M. Slanski Research Division, American Machine & Foundry Company, The Morehead Patterson Center, Springdak, Conn.
THEOBJECTIVE of this work was to develop a suitable method of analysis for ethoxyl groups in cellulose derivatives. The analytical procedures presently used are modifications of the Zeisel procedure (1) or some combination of the Zeisel method and gas chromatography (2-4). Among the many deficiencies of these methods are their complexity, slowness, and lack of specificity for the group of interest. Lemieux and Purves (5) describe a method where the ethoxyl group in 0-ethocel is oxidized to acetic acid with chromium trioxide solution and the resulting acid is distilled over and titrated with a known sodium hydroxide solution. The oxidation step is specific for ethoxyl groups ; however, the quantitative determination of the resulting acetic acid is difficult to achieve because of the need for its removal from the reaction mixture by distillation followed by a titration step. In the new procedure, the oxidation step is retained; however, the acetic acid formed is determined gas chromatographically, using aliquots of the reaction mixture, thus avoiding the need for distillation and titration. Materials examined by the new procedure and by the classical Zeisel method gave similar values for ethoxyl groups. The new procedure, however, is much simpler to carry out; it is also rapid, accurate, specific for ethoxyl groups, and is superior to the Zeisel procedure in that it can be used on mixed cellulose derivatives (ethylhydroxyethyl cellulose). EXPERIMENTAL Apparatus. Perkin-Elmer 800 gas chromatography unit with a flame ionization detector was used. A 6-foot by 1
Present address, Beech-Nut, Inc., Port Chester, N. Y . 10573
(1) "Methods in Carbohydrate Chemistry," Vol. 111, R. L. Whistler, Ed., Academic Press, New York, N. Y . , 1963, pp 305-14. (2) T. Misui and Y . Kitamura, Microcliem. J., 7, 141 (1963). (3) K. F. Sproek and M. D. Danyi, ANAL.CHEM., 34, 1527 (1962). (4) D. L. Miller, E. P. Samsel, and J. G. Cobler, ibid., 33, 677
(1961). (5) R. U. Lemieux and C. B. Purves, Can. J. Res., 25B, 485 (1947).
Table I. Calibration Curve for Determination of Acetic Acid by Gas Chromatography Acetic acid Sample per milliliter, mg Peak area AC-1' 1.00 14 AC- 1 1.00 15 2.42 41 AC-2 AC-2 2.42 42 AC-3 3.32 62 AC-3 3.32 62 5.41 111 AC-4 5.41 109 AC-4 155 AC-5 7.36 AC-5 7.36 151
inch 0.d. stainless steel column packed with Poropak Q-S 60180 mesh (Analabs, Hamden, Conn.) was used. Oven temperature was 190 "C; injection port temperature, 230 "C; detector temperature, 135 "C; nitrogen flow was 17 ml/min. The stream was split 80/20, the 20 parts going to the detector. A Honeywell 1-mV single pen recorder was used. Chart speed was liZ inch per minute. The areas under the peaks were determined with a planimeter. Reagents. Chromium Trioxide (99-99.5%) was purchased from Fisher Scientific, Fair Lawn, N. J., and used as supplied. The glacial acetic acid used was analytical grade. Procedure. A sample of 20-80 mg of cellulose derivative was weighed accurately into a glass-stoppered flask. From a volumetric pipet or buret was added 10.0 ml of 30% (w/v) chromic acid solution in water. The mixture was swirled gently to wet all the material and left at room temperature for 3 to 5 hr-it was shaken intermittently. Completion of the reaction was indicated by total disappearance of solid particles and the formation of a uniform solution. An aliquot of this solution, usually 3 p1, was injected into the gas chromatograph and the area under the obtained peak was subsequently determined. The level of acetic acid in the reaction mixture was determined from a standard calibration curve. ANALYTICAL CHEMISTRY, VOL. 42, NO. 7, JUNE 1970
801
Table 11. Determination of Ethoxyl in Ethylcellulose K-5000by Gas Chromatography Percent ethoxyl Acetic acid from By GC Wt Ethocel, mg/ml Peak area By Zeisel calib. curve, mg 37 36 39 43 43 67 70 80 79
3.81 3.81 4.07 4.33 4.33 6.32 6.32 7.32 7.32 Mean = 43.7
43.4 42.4 42.6 43.4 43.4 43.9 45.4 44.4 44.0
2.20 2.15 2.30 2.50 2.50 3.70 3.83 4.35 4.30
Difference -2.5 -4.7 -4.3 -2.5 -2.5 -1.4 +2.0 -0.2 -1.1
44.5 44.5 44.5 44.5 44.5 44.5 44.5 44.5 44.5
Re1 std dev = 0.0199 Re1 std dev = 1.99 Table 111. Determination of Ethoxyl Groups in EHEC Samples by Gas Chromatography Percent ethoxyl Acetic acid from By GC Calcd Mg per ml Area under peak calib. curve, mg
Sample
4.74 4.74 3.61 3.61 7.41 6.78 6.78 6.61 7.33
EHEC-75
EHEC-55
23 23 16 15 37 24 24 27 31
1.65 1.65 1.30 1.25 2.65 1.70 1.70 1.75 1.90
RESULTS AND DISCUSSION A series of acetic acid solutions ranging in concentration from one to ten milligrams per milliliter were prepared. Three-microliter aliquots of these solutions were injected into the gas chromatograph and the areas under the obtained peaks determined. The results of a typical run are shown in Table I. A curve constructed from the values in Table I gives a straight line in the range 2.5 mg per ml of acetic acid and beyond. There is some deviation from linearity in the zero to 2.0-mg range. An extrapolation of the curve shows that it passes through the abscissa. This is probably due to some absorption of the acetic acid on the Poropak. An ethylcellulose (K-5000) obtained from Hercules Co., Inc., Wilmington, Del., was examined for percent ethoxyl groups by the Zeisel procedure and by the new chromatographic method. A comparison of the obtained results is shown in Table 11. In the new chromatographic method the percent ethoxyl is calculated as follows:
% Ethoxyl
=
mol wt Ethoxyl mol wt Acetic acid Wt Ethylcellulose (mg)
mg Acetate acid X
x
100
A comparison of the figures in Table I1 shows that the gas chromatographic procedure gives results similar to the Zeisel procedure. The precision of the gas chromatographic method is very good (relative standard deviation 1.99%). A series of experiments was run where known amounts of acetic acid were added to ethylcellulose chromic acid solutions and the percent recovery of acetic acid was determined. The recoveries ranged from 97 to 106 and averaged 102.2 for a ten-run series. In order to extend the usefulness of this method to other cellulose derivatives, an attempt was made to determine the 802
ANALYTICAL CHEMISTRY, VOL. 42,
NO. 7, JUNE 1970
26.1 26.1 27.0 26.0 26.8 18.9 18.9 19.9 19.5
Percent difference
25.4 25.4 25.4 25.4 25.4 19.1 19.1 19.1 19.1
+2.8 +2.8 $6.3 +2.4 +5.5 -1.0 - 1 .o +4.2 +2.1
level of ethoxyl groups in ethyl-hydroxyethylcellulose (EHEC). From the labeling code of these derivatives, it is possible to arrive at the degree of substitution (DS). Thus EHEC-55 means that the DS for ethoxyl is 1.O, while the DS for hydroxyethoxyl groups is also 1.0. In the case of EHEC-64, the ethoxyl groups DS is 1.2, while the hydroxyethoxyl DS is 0.8. Using the DS information, one calculates the percent ethoxyl in EHEC as follows: Ethoxyl
=
45 DS mol wt EHEC
x
100
The conversion of the acetic acid value to percent ethoxyl has been described previously. A series of determinations of ethoxyl groups in a number of EHEC samples was carried out. The experimentally found values were compared with the calculated ones. The results are shown in Table 111. The figures in Table I11 indicate that the method described is suitable for the determination of ethoxyl groups in EHEC derivatives. From the presented evidence, it would appear that the described method is specific for ethoxyl groups and can be used for their determination in ethylcellulose as well as in mixed cellulose derivatives such as EHEC. Methoxyl groups in methylcellulose could not be determined when chromium trioxide solution was used as the reagent. However, the presence of methylcellulose did not interfere with the determination of ethoxyl groups in 0-ethylcellulose. Acetyl groups in cellulose acetate can be determined in this manner but the ester hydrolysis is carried out with sodium hydroxide followed by acidification of the mixture and determination of the acetic acid by gas chromatography. RECEIVED for review January 14, 1970. Accepted March 13, 1970.
