Determination of Total and Primary Hydroxyl in Cellulose Esters by Ultraviolet Absorption Methods CARL J. MALM, LEO J. TANGHE, BARBARA C. LAIRD, and GLENN D. SMITH Cellulose Acetate Development Division, Eastman Kodak Co., Rochester, N. Y. Different amounts of primary and secondary hydroxyl groups are formed during the hydrolysis of cellulose acetate, depending on the exact conditions of hydrolysis. As the properties of the hydrolyzed cellulose ester depend on the relative amounts of the different hydroxyls, this information is desirable for characterizing the ester. Ultraviolet absorption measurements have been used to measure quantitatively the amounts of carbanilate and of trityl intro-
duced into hydrolyzed cellulose acetate by reaction with phenyl isocyanate and trityl chloride, respectively. Measurement of the carbanilate content allows calculation of the total hydroxyl, and measurement of the trityl content allows calculationof the primary hydroxyl in the original material. These methods have been applied to a wide variety of cellulose esters, and their precisions have been evaluated using a typical sample of cellulose acetate.
A
PRIOR method (6)for determining total hydroxyl in cellulose esters depends on acetylating the hydroxyl groups with acetic anhydride. The exceea reagent is hydrolyzed and titrated to indicate the hydroxyl content. Since a liberal excess of reagent must be taken, this method depends on small differences between the relatively large titration8 of blank and sample. Phenyl isocyanate has been shown to react rapidly and completely with the hydroxyl groups in hydrolysed cellulose acetate in pyridine solution a t 100’ C. to form a carbanilate (N-phenyl carbamate). The excess phenyl isocyanate may be removed by precipitating and washing the product in alcohol. The amount of carbanilate introduced can be quantitatively determined from the ultraviolet absorbance a t 280 mp. Primary hydroxyl in cellulose esters has been determined by. tosylation-iodination (3, 7 ) and by tritylation (8). The amount of trityl introduced has been determined ( 5 ) by dissolving the tritylated product in concentrated sulfuric acid and precipitating the triphenylcarbinol in water. In the present paper the amount of trityl is determined from the ultraviolet absorbance a t 259 mp.
(4)
Equation 4 is valid when acetyl is the only subst.ituent in the starting material, and provided the degree of polymerization is not excessively low.
Table I. Time Series for Carbanilation of Cellulose Acetate Sample 1
Reaction Time, Hours 0.5 1
Carbanilate,
%
OH, %
2
17 .. R R
16.40 17.33
2.81 3.00
3 4
17.43 17.70
3.02 3.07
R- . ni ..
0.5 22.12 1 22.43 2 22.13 3 22.45 4 22.16 5 Interlaboratory study (9). b Average values, this laboratory. 2
4.06 4.13 4.06 4.14 4.07
Acetyl, % From BY carbanilate saponification 40.8 40.535 40.55 40.42b 40.5 40.5 ... 40.45 ... . . I
39.0 38.9 39.0 38.9 39.0
39.050 38.97b
... ... ...
TOTAL HYDROXYL BY CARBANILATION
The course of the reaction between cellulose acetate and phenyl isocyanate in pyridine a t 100’ C. was followed by quantitative recovery of product (see experimental section). The data in Table I indicate complete reaction after 1 hour or less, confirming the results of Hearon et al. (4). This is the method referred to aa a new procedure in the interlaboratory study on the determination of acetyl ( 9 ) , and the samples referred to in Table I are the same as used in that study. The reaction of phenyl isocyanate with a partially hydrolysed cellulose acetate may be represented by Equation 1.
To calculate the per cent total hydroxyl from the per cent carbanilate in the derivative, the hydrolyzed cellulose ester may be considered as a simple alcohol: R 4 H
4-CoHsNCO
+
CF,HS~\”--COOR
(5)
The per cent total hydroxyl in the starting material is given by
yo total OH
=
~
YI7X
100
(6)
and the per cent carbanilate, c, in the derivative is given by c=-
R
+119136 x
(2)
% total OH = and the per cent carbanilate, c, in the derivative is given by 288
-
119n 42n
+ 119n x
100
(7)
Combining Equations 6 and 7, eliminating R, gives the per cent total hydroxyl in the original cellulose ester in terms of the per cent carbanilate in the derivative:
The per cent acetyl in the cellulose acetate is given by
c =
100
14.3~ cp --- ”
Equation 8 is valid regardless of the nature of the acyl groups and ~. the degree of polymerization. The carbanilates absorb strongly _ _ in the ultraviolet where cellulose acetate is almost transparent. The spectrophotometric curves of cellulose acetate (sample 1, Table I ) and the carbanilate and trityl ether derived from it are given in Figure 1. These curves were made on a Cary recording spectrophotometer, bal-
(3)
Combining Equations 2 and 3, eliminating n, gives the per cent acetyl of the original cellulose acetate in terms of the per cent carbanilate, c, in the derivative:
188
189
V O L U M E 2 6 , NO. 1, J A N U A R Y 1 9 5 4 anced out with air in both beams. The peak a t 280 mp was chosen for quantitative measurement. At this wave length, the absorbance of cell plus solvent was the same as when cellulose acetat,e was present. Quantitative measurements were made with a Beckman Model DU spectrophotometer balanced with cell and solvent. 2.c
In deriving an equation to calculate per cent primary hydroxyl from per cent trityl, the reaction may be considered simply: R’-OH
+ T G l = C6HaN * R‘-OTr + CsHsN.HCI
(9)
where Tr represents triphenylmethyl (molecular weight 243) and R’ represents all of the hydrolyzed cellulose acetate except the primary hydroxyl. The per cent primary hydroxyl in the starting material is given by
% primary OH
=7 l7 x 100
(10)
R $17
and the per cent trityl, t, in the derivative is given by
1.5
t=-
R’
243 x 100 259
+
2.0 -
C
-250
300
250
3bu
Figure 1. Spectrophotometric Curves of Cellulose Acetate and Its Carbanilate and Trityl Ether Derivatives A. B.
acetate earbanilate -- - - Cellulose Cellulose acetate Cellulose acetate trityl ether - - - - Cellulose acetate
.4 series of ten samples with acetyl contents varying from 32 to 44c7, was carbanilated. The products were recovered quantitatively as “skin” precipitates by pouring alcohol into the reaction mixtures. Additional carbanilates were prepared from these ten samples, obtaining the products in the conventional flake form by pouring the reaction mixtures into alcohol. The absorbances of both sets of products were in close agreement. Figure 2 shows the relationship between the per cent carbanilate and absorbance a t 280 mp of a O . l ~ ,solution of the cellulose acetate carbanilate in methylene chloride-methanol (90 to 10 by weight). The data are well represented by a straight line over this range. Thus the per cent carbanilate may be obtained by multiplying the absorbance by 17.1.
1.5-
~
0
c
0
gm
13
1.0-
0.5-
0 0
1
I
20
30
40
% Carbanilate or Trityl Figure 2. Per Cent Carbanilate or Trityl us. Absorbance 0.1 yo solution of cellulose acetate carbanilate or cellulose acetate trityl ether in methylene chloride methanol (90-10by weight)
Table 11. Trityl Content of Cellulose Acetate Trityl Ethers Trityl, %
PRIMARY HYDROXYL BY TRITATION
The method of determining trityl content of cellulose acetate trityl ethers by dissolving the sample in concentrated sulfuric acid and precipitating the triphenylcarbinol by addition of water ( 5 ) has given generally satisfactory results. However, when the amount of trityl is low, the precipitated triphenylcarbinol does not coagulate readily and either clogs or passes through the filter. The trityl group absorbs strongly in the ultraviolet. Figure 1,B, shows the spectrophotometric curve of the cellulose acetate trityl ether derived from sample 1, Table I. Gravimetric trityl determinations and ultraviolet absorbance readings a t 259 mp were made on a number of samples. The results are plotted in Figure 2. Cellulose acetate has an average absorbance of 0.015 under these conditions, and the absorbance of the trityl ether is corrected by this amount. The absorbance varies directly with the trityl content. The per cent trityl may be determined by multiplying by 25.3 the corrected absorbance of a 0.1 % solution.
I
IO
Flake Precipitate Sample 3 4 5
Recovery of triphenylcarbinol 12.2 23.3 26.1
Skin Precipitate Recovery of cellulose acetate Ultraviolet trityl ether absorbance 12.2 12.9 23.7 24.2 27.4 27.0
Ultraviolet absorbance 12.5 23.0 26.5
,
Combining Equations 10 and 11, eliminating E‘, gives the per cent primary hydroxy] in the original cellulose acetate in terms of the per cent trityl in the derivative:
% primary OH
=
~
7.02t 100.4 - t
The quantitative recovery of product has also been used as a method for trityl determinations. Products obtained in this way are referred to in Table T I as skin precipitates in contrast to
A N A L Y T I C A L CHEMISTRY
190 the conventional flake precipitates. The data in Table I1 show the agreement among the methods for determining trityl content.
allowing most of the alcohol to evaporate a t room temperature, the precipitate was dried overnight a t 100' C. and weighed. Carbanilation for Ultraviolet Absorbance. A sample of approximately 0.5 gram of cellulose acetate wm placed in a 15-ml. PRECISION AND RANGE OF THE METHODS screw-cap bottle and dried a t 100' C. for 2 hours. After dissolving the ester in 5 ml. of anhydrous pyridine, ap rosimately 0.5 ml. In a study of the precision of these methods, sample 1 of Table of phenyl isocyanate was added and thorougily mixed in u ith a I was carbanilated and tritylated 32 times. The reactions were small stirring rod. For samples whose anticipated acetyl content was below 39%, the amount of phenyl isocyanate was increased by carried out in groups of two on different days, but the absorbances 0.1 ml. for each 2% acetyl below that amount. After 2 hours of were measured a t the same time (Table 111). The mean total reaction a t 100' C., the solution was diluted with 2 to 10 ml. of and primary hydroxyl contents were 3.026 and 1.134%, respecpyridine, depending on the viscosity, and the product was pretively. The standard deviations for a single value were 0.020 and cipitated by ouring, with stirring, into 95% alcohol. The precipitate was Atered on a small Buchner funnel and transferred to 0.030%, respectively a 4-ounce screw-cap bottle. Two half-hour washes on the shaker were sufficient to remove all the excess reagent. After drying a t -~ 100" C., a sample of 0.1231 gram was dissolved in 100 ml. (123.1 grams) of methylene chloride-methanol (90 to 10 by weight). Table 111. Precision of Total and Primary Hydroxyl The absorbance was measured a t 280 mp on a Beckman Model Determinations DU spectrophotometer (Serial Number 61361). Carbanilation of Highly Esterified Cellulose Acetate. A reacTotal OH, Operator tion container was constructed from a pair of 24/40 standard% taper (borosilicate) ground-glass joints. A test tube, appro+ A 1.19 1.16 3.01 3.01 mately 20 X 150 mm., was fashioned from the outer part of the 3.01 3.11 1.16 1.08 1 . 1 7 1.17 3.03 3.03 joint by closing the open end in a blast lamp. The inner part was 1.09 1.11 3.00 3.00 drawn to a constriction just above the ground glass and a short 1.16 1 . 0 9 3.04 3.04 length of 6-mm. tubing was sealed on. The stirrer consisted of a 3.05 3.09 1.14 1.18 length of 4-mm. glass rod bent a t a right angle of the bottom in 1.11 1 . 1 5 3.05 3.03 the form of a semicircle, The stirring shaft had sufficient cooling 3.04 3.03 1.16 1.15 capacity to prevent loss of pyridine when heated in an oil bath a t 3 . 1 0 3.01 1 . 1 3 1 08 1.11 1.11 3.03 3.02 115' to 120' C. A reaction period of 30 minutes with occasional stirring was found to be sufficient. By shortening the drying 3.00 3.02 1 . 1 1 1.16 1.14 1.12 3 . 0 0 2.83'' period of the starting material to 30 minutes and of the carban1.14 1.12 2.98 3.00 ilated product to 1 hour a t 110" C., the elapsed time can be re1.12 1 . 1 3 3 . 0 0 3.01 duced-if necessary to about 6 hours. This method was applied 1.10 1.17 3 . 0 2 3.01 1.07 1 . 1 4 to a highly acetylated cellulose acetate of the type currently used 3.01 3.02 for motion picture film ( 2 ) . Four determinations in parallel on a 3.026 1.134 Mean typical sample gave hydroxyl contents corresponding to 43.54, Average range of 0,034 0.023 43.52, 43.55, and 43.547, acetyl, respectively. pairs ~
~~
Standard deviation of single value, based on average 0.020 0.030 range of pairs a Value omitted, ae duplicate solutions prepared frotn thifi sample all gave low and variable results.
Both methods have been applied to cellulose acetat.es containing 30 to 44.8% acetyl and to a wide variety of commercial cellulose acetate propionates and cellulose acetate butyrates. Repeated absorbance measurements a t different concentratione confirmed the earlier work of Ayres ( 1 ) that the useful range of absorbance of the Beckman Model DU spectrophotometer extends from about 0.2 to 1.6.
Tritylation of Cellulose Acetate. The determinations based on quantitative recovery of product were made iri a manner similar to the carbanilations. A sample of 0.5 gram w.ay dissolved in 2.5 ml. of pyridine in a 125-ml. flask and allowed to react Rith 0.5 gram of trityl chloride for 24 hours. +itthe end of the reaction the solution was diluted with a sufficient pyridine to give a good precipitate. In the preparation of trityl derivatives for ultraviolet absorbance, 1 gram of cellulose ester was dissolved in 5 ml. of pyridine in a 15-ml. bottle and allowed t o react with 1 gram of trityl chloride. For samples having more than 0.50 hydroxyl per glucose unit, the amount of trityl chloride was increased according to the formula previously given (8).
EXPERIMENTAL
ACKNOWLEDGMENT
Carbanilation with Quantitative Recovery of Product. A sample of approximately 0.5 gram of cellulose acetate was placed in a tared 125-ml. glass-stoppered Erlenmeyer flask and dried for 2 hours a t 100" C. Reweighing after cooling gave the weight of starting material. The cellulose ester was dissolved by standing overnight in 5 ml. of anhydrous yridine. Then 0.5 ml. of henyl isocyanate was added, and the &sk was placed in a 100' electric oven. After the contents of the flask had come to temperature, the phenyl isocyanate was mixed in by tilting the flask from side to side several times. After 2 hours' reaction time, the flask, with the stopper slightly ajar, was cooled intermittently with tap water. During the cooling period the solution was distributed over the interior by rotating and tilting the flask to a pouring position. Care was taken to prevent the solution from coming in contact with the ground-glam stopper. The thoroughly chilled solution was so viscous that it flowed very slowly to the bottom of the flask. The product was precipitated by quickly filling the flask with 957, ethyl alcohol. Ideal precipitates remained transparent for several minutes and gradually became white and opaque. After half an hour the precipitate could be stripped in practically one iece from the interior of the flask using a narrow spatula. T t e supernatant liquid was decanted through a tared 60-d.sintered-glass funnel of coarse porosity (Uorning No. 36060). The precipitate was washed with four changes of alcohol, each of a t least half-hour duration. On the last wash the precipitate was transferred quantitatively to the funnel. After
The authors wish to thank D. H. Anderson for preparing the ultraviolet spectrograms and for assistance in developing the ultraviolet absorption methods.
8.
LITERATURE CITED
(1) Ayres, G. H., ANAL.CHEM.,21, 652 (1949). (2) Fordyce, C. R., J . SOC.Motion Picture Engrs., 51, 331 (1948). (3) Gardner, T. S., and Purves, C. B.. J . Am. Chem. SOC.,64, 1539 (1942). (4) Hearon, W. hl., Hiatt, G. D . , and Fordyce, C. R., Ibid., 65, 829 (1 943). (5) Ibid., p. 2449. (6) Malm, C. J., and Genung, L. B.. and Williams, R. F . , Jr., ANAL. CHEM.,14, 935 (1942). (7) Malm, C. J., Tanghe, L. J., and Laird, €3. C , J . Bm. Cheni. SOC., 70, 2740 (1948). (8) Ibid., 72, 2674 (1950). (9) Subcommittee on Acyl Analysis, Division of Cellulose Chemistry Committee on Standards and Methods of Testing, ANAL. CHEM., 24, 400 (1952). RECEIVBD for review June 22, 1953. Accepted October 7, 1953. Presented before the Division of Cellulose Chemistry at the 124th Meeting of the AMERICAN CHEMICAL SOCIETY, Chi08g0, m.
