INDUSTRIAL
AND
ENGINEERING CHEMISTRY
A N A L Y T I C A L E D I T 1O N PUBLISHED
BY
THE
AMERICAN
CHEMICAL
SOCIETY
HARRISON
E.
HOWE,
EDITOR
Analvsis of Cellulose Derivatives J
Determination of Total Combined Acyl in Cellulose Organic Esters LEO B. GENUNG AND RUSSELL C. MALLATTI Eastman Kodak Company, Rochester, N. Y.
Most of the published methods for the determination of combined acyl in cellulose derivatives have been restricted to acetyl in cellulose acetate. The three best methods, Eberstadt, alcoholic alkali, and Ost acid distillation, have been further investigated as to their applicability for general acyl analysis. The effects of their more important variables have been measured, and the precision, accuracy, and range of applicability of each are reported. In order to avoid errors due to the heterogeneity of the reaction mixture and to excess alkalinity or acidity, temperature, and time, these variables must be controlled within definite limits. It was
found that saponifications by the Eberstadt method are best run a t an initial alkali concentration of 0.25 N for 48 hours a t not higher than 35" C. This is the most accurate method, but is applicable to cellulose acetate and only certain other esters. Saponification methods using alcoholic alkali are applicable to practically all cellulose esters, but are less accurate and reliable. The most satisfactory conditions are 0.25 IVinitial concentration for 24 hours a t not higher than 30a C. The acid distillation method of Ost is restricted in its application, b u t it has special uses due to shorter elapsed time required for an analysis and to the fact that only volatile acidity is measured.
used similarly by Green and Perkin ( 5 ) . These methods are capable of yielding ood results if conditions of time, temperature, and alkali strength are carefully chosen. Furthermore, these alkalies attack a much wider range of cellulose esters than aqueous alkali. Zemplen (16) saponified cellulose acetate with traces of sodium methylate in refluxing absolute methyl alcohol, but his method is not applicable to quantitative analysis because the amount of alkali consumed is not stoichiometric. Rapid methods involving solution of the sample in pyridine have been described by Battegay and Penche (2') and also by Murray, Staud, and Gray (11). Aqueous alkali is added to the solution and it is heated for a half hour at not more than about 55' C. Roeper (14) has tried various other solvents, including acetone, acetone-water, and acetone-alcohol mixtures with aqueous alkali a t various times and temperatures. Concordant and reasonable results were obtained with certain esters, but these methods are not generally applicable nor accurate. The acid hydrolysis method was proposed in 1906 by Ost ( 1 0 , who suggested the use of strong sulfuric acid followed by steamdistillation of the liberated acetic acid, and also by Green and Of the saponification methods using aqueous alkali (8), the one Perkin ( 5 ) , who decomposed the ester with sulfuric acid in the devised by Eberstadt (4) working with Knoevenagel (6, 7) and presence of absolute ethyl alcohol and then distiIIed off ethyl modified somewhat by Murray, Staud, and Gray (11) has proved acetate and saponified this ester in the distillate. Various modiuseful and reliable when applied to cellulose acetate and to cerfications of these methods have been reported (8), but nearly all tain similar esters of low molecular weight organic acids. It this work has been restricted to the determination of combined breaks down, however, when applied to esters of the type of celluacetic acid. Abribat ( I ) dissolved and degraded cellulose acetate with cold concentrated hydrochloric acid and then hydrolose stearate and even to certain cellulose butyrates. This method involves swelling the sample with warm aqueous alcohol, lyzed off the acetyl groups by diluting the acid. The acetic acid liberated was titrated potentiometrically in the presence of hydrofollowed by addition of aqueous alkali and a long saponification at room temperature. chloric acid. Pilgrim ( I S ) used warm strong hydrochloric acid The use of sodium ethylate was proposed by Cross and Bevan to degrade the cellulose, and hydrolyzed the acetyl groups by progressive dilution of the acid under conditions mild enough to (9), and was successfully ap lied with modifications by Woodbridge (16)and Mork ( I O ) . ðyl or ethyl alcoholic alkali was avoid charring. The reaction mixture was then diluted to a known volume and a suitable aliquot titrated. The difference in titer between the sample and a blank was taken as a measure of 1 Present address, Department of Chemiatry, University of Rochester, Rochester, N. Y. the acetic acid liberated. 369
T
HE increasing studies of the various esters of cellulose and the ever-widening field of application of these esters to practical uses have increased the need for tested and reliable methods for the analysis of these products. It is the purpose of this paper t o present the results of accumulated experience and of special studies made on the most satisfactory and generally applicable of the methods available. The methods for the determination of total combined acyl in cellulose organic esters fall into two general classes: saponification with alkaline reagents or decomposition and hydrolysis b y acids. The methods in the literature, applying principally to the determination of acetyl or combined acetic acid in cellulose acetate, have been well reviewed b y Krueger (8) and by Murray, Staud, and Gray (II), and recent additional references are given by Marsh and Wood (9).
370
INDUSTRIAL AND ENGINEERING CHEMISTRY
These methods have the follon-ing inherent difficulties or objections: 1. Most of the methods, and particularly the practical and widely applicable ones, involve heterogeneous conditions for the saponification or hydrolysis. The physical form of the solid ester is thus an important variable. I t is often necessary to powder or reprecipitate the sample before reproducible and reliable results can be obtained. Since insoluble regenerated cellulose is formed in all these methods except the strong acid procedure, the final titration must be slon- enough to allow for complete soaking out of the excess reagent. These factors may cause low result's due t o slow rate of penetration of the reagent and incomplete reaction, or high results due to s l o ~ soaking ~ out of the excess reagent. 2. When this first difficulty is met by dissolving the ester in a solvent, such as pyridine or acetone, the method is limited to esters soluble in such solvents and whose solutions will tolerate the addition of a comparatively large volume of aqueous or alcoholic alkali. Kot all the esters of cellulose will meet these requirements. The back-titration must still be made in the presence of regenerated cellulose. 3. Cellulose and its derivatives form acidic decomposition products when heated excessively with alkalies or acids in the presence of air. Strong alkalinities and elevated temperatures must be avoided, and the variables of time, temperature, and reagent concentration must be balanced carefully if accurate results are to be obtained. 4. The precision of these methods is, of course, easily measurable, but the accuracy is very difficult to establish. The heterogeneous conditions met in saponification and back-titration and the difficulty of complete distillation of acids after acid decomposition tend t o produce low results if the conditions are not vigorous enough, while too severe conditions of reagent concentrations and tem erature produce high results. Furthermore, it is extremely difficurt to prepare a cellulose ester of known acyl content to serve as a standard. Simpler esters, such as ethyl acetate or a glucose acetate, cannot be used for this purpose, since they do not duplicate the heterogeneous conditions met in the analysis of cellulose esters.
These difficulties can be met by using tested procedures within limits of reagent concentration, time, and temperature which have been shown to be satisfactory. By taking these precautions, high results can be avoided. Low results are usually caused by lumpy or hard sandy precipitates which resist penetration by the reagent, and can be eliminated by powdering t'he sample, or preferably by reprecipitating from suitable solvents to get a soft fluffy product. The accuracy of a method can be determined by comparing the best results from one method with those by other methods after the variables and limitations of each have been studied. Long experience with standard types of cellulose esters, particularly the acetates, enables one to establish fairly accurate acyl contents for each type, and these materials can then be used as reference samples, if not for primary st'andards. By combining this information, practical limits of accuracy can be set up. I n the following sections, three methods which have proved satisfactory are given in detail with results of studies of the effects of their most important variables. The precision and accuracy attainable and the limik of applicability are also given for each method.
Vol. 13, No. 6
cess of about 1 ml. of acid is added. The alkali is allowed to soak out from the regenerated cellulose for several hours and more acid is added if necessary. Finally alkali or acid is added to establish the exact neutral point, and the per cent acyl is calculated somewhat as follom: [ (ml. of acid for blank) - (ml. of acid for sample) ]
x
(acid normality) (equivalent wt.) sample wt. X 10
=
% acyl
Most of the results given in the tables which follow were calculated to acetyl or to "apparent acetyl" (equivalent weight 43) even though some samples were known to contain other acyl groups. When a large number of samples is to be analyzed by this method, time and labor can be saved by dispensing the alkali from a dispensing pipet which delivers approximately 40 ml. Almost all the acid can be added from a dispensing pipet which delivers approximately 20 ml., and the titration can be
TABLEI. EFPECTOF TIMEOF SWELLING AT 50" TO 60' C. Sample Celluloseacetate 1 Cellulose acetate propionate 1 Cellulose acetate 3
Apparent Acetyl for Various Times of Swelling None 15 min. 30 min. 1 hour 3 hours 5 hours n /c
%
%
w
40.5 40.6 40.5 40.5 43.7 43.2
40.6 40.6 40.6 40.6 43.6
40.7 40.7 40.5 40.5 43.7 43.5
40.7 40.7 40.7
..
7 40.9 40.9 40.9 40.9 43.6
.. .. ..
0
", a 40.5 40.5 40.6 40.6
..
..
..
TABLE 11. EFFECTO F ALKALICONCENTR.4TIOX Sample Cellulose acetate 1 (40.5% acetyl)
Apparent Acetyl a t Various Effective Alkali Concentrations 0.25.\r O.5N 1,V
Time Hours 2.5
4 6
16 24 48 72 Cellulose acetate 2 (40.5% acetyl)
2.5 4 6 16 24 48 72
Cellulose acetate propionate 1 (40.5% apparent acetyl)
2.5 4 6 16
Eberstadt Method
24
Accurately weighed 1-gram samples of the thorPROCEDURE. oughly dried ester are placed in 250-ml Erlenmeyer flasks, and 40 ml. of 75 per cent ethyl alcohol are added to each. Alcohol denatured by the 3-A formula may be diluted for this purpose. The flasks are then heated loosely stoppered for 0.5 hour at 50" to 60" C. (A double-walled bath containing refluxing methyl alcohol in the jacket provides the right amount of heat for this pur ose.) Then 40 ml. of 0.5 N sodium hydroxide solution are a d i d and the flasks are heated for 15 minutes at 50' t o C. Blanks containing alcohol and alkali are run with each set of samples or at least on new reagents. The flasks are s t o p pered tightly and allowed to stand at room temperature for 24, or preferably 48, hours with occasional swirling. At the end of this time the excess alkali is back-titrated with standard 0.5 N hydrochloric acid, using phenolphthalein indicator, and an ex-
48 72 Cellulose acetate butyrate 1 (35.577aapparent acetyl)
2.5 4 6 16 24 4s 72
%
%
28.4 33.7 40.0 40.2 38.9 40.7 40.5 40.5 40.5 40.7
40.3 39.1 40.7 40.4 39.9 40.1 40.8 41 .O 40.9 41.0 41.2 41.2
40.5 40.5 40.5 40.5 27.8 29.3 38.4 39.5 37.6 39.9 38.9 40.7 40.5 40.5 40.5 40.5 40.5 40.5 34.7 35.5 40.1 40.1 39.4 38.9 39.8 40.7 40.6 40.7 40.4 40.6 40.4 40.4 24.8 28.5 34.1 34.7 29.8 34.0 35.0 35.5 35.5 35.5 35.5 35.5 35.7 35.8
..
.. 40.5 40.5 40.6 38.8 40.0 40.1 40.8 41.5 40.9 4i:2 41.2
.. ..
39.4 40.2 40.5 40.5 39.9 40.1 39.9 40.5 40.8 40 9 41.2 41.2
..
32.3 34.5 35.3 35.4 32.8 35:s 35.4 35.4 35.4 35.5 35.5
..
..
70
..
.... 4i:o 40.9 42.3 42.6 43.4 43.5 44.9 45.0
.. .. .. .. ..
4i:o 41.5 41.9 43:4 43.0 44.8
.. .. ..
.... ..
40:s 4i:3
41.6 43.4 43.4 41.2 41.7
.. .. .. .. ..
3i:2 38.8 35.9 36.8 35.9. 39:6 37 8
.. ..
June 15, 1941
ANALYTICAL EDITION
finished using ordinary burets. Only the difference between the buret readings for the blank and the sample titration enters into the calculation. EFFECT OF TIMEOF SWELLIXG. The effect of the time of swelling at 50" to 60" C. is shown in Table I. The above procedure was followed and only the time of swelling was varied as indicated. These data show that the time of swelling is not important, and in many cases this presoak could be eliminated. It does not add much to the manipulation and does no harm, however, so i t is a precaution well worth taking and is to be recommended. EFFECT OF ALKALICOXCENTRATION. The same procedure was again followed except that the alkali normality and the time of standing were varied as indicated in Table 11. The solutions added mere actually 0.5, 1.0, and 2.0 N , but since they were diluted with equal volumes of 75 per cent alcohol, their effective normalities at the start of the saponification were half these values. The accepted values for the acetyl or apparent acetyl contents-i. e., all the acidity calculated to acetyl-of the various esters are given in the first column. Comparison of these accepted values with the other data leads to the following conclusions: At an effective normality of 0.25, the accepted acetyl value is reached in 16 to 24 hours' reaction time, and this value is not raised even after 72 hours. At an effective normality of 0.5, the accepted value is reached in about 16 hours, but after 24 hours the value observed rises slowly above this accepted figure. At an effective normality of 1, abnormally high values are obtained even at short reaction times, and the precision is poorer than at lower alkalinities. The most satisfactory conditions for use in this procedure are saponification a t an effective initial normality of 0.25 for not less than 24 hours and preferably for 48 hours. A set of 24 samples taken at random was analyzed using these conditions; a 24hour saponification was sufficient for 18 samples, but 48 hours was required to get acceptable results on the other six. Consequently it is safest to allow 48 hours for all samples.
EFFECT OF TEMPERATURE. T h e same samples were tested by this method at several different temperatures and with varying reaction times to measure the effect of temperature on the accuracy of the results. I n all cases 0.5 N alkali was added, making a resultant initial concentration of 0.25 N , and the 50" to 60" C. heat treatment was omitted in the reactions run a t 0" C. T h e data obtained are given in Table 111. The results of these experiments indicate the following conclusions : Saponifications at 0" C. are incomplete and the results erratic for reaction times of less than 24 hours. Acceptable results were obtained on three of these four samples using saponification times of 24, or preferably 48, hours. Reactions run smoothly at room temperature, and accurate values are obtained in from 24 to 72 hours. There is apparently no tendency toward high results when the specified conditions of temperature and alkalinity are met. When the saponifications are run at an elevated temperature such as 60' C. very erratic results are obtained in short reaction times, and very high results are obtained after about 10 hours. This result is the same as is caused by too high alkalinity. In the previous section it was shown that the most satisfactory conditions of alkalinity and time were 0.25 N and 24 to 48 hours. These experiments show that room temperature (25' to 30' C.) is satisfactory, but temperatures above about 35' C. are to be avoided.
FORMATION OF ACIDS OTHER THANACETIC ACID. T h e apparent acetyl values in Tables I1 and I11 show t h a t too high values are obtained when the alkalinity and temperature exceed the specified conditions. A special experiment was run t o prove that this extra acidity is due to acids other than acetic.
TABLE
371
111. EFFECT O F TEMPERATURE
Sample Cellulose acetate 1 (40.5% acetyl)
0' C .
