INDUSTRIAL AND ENGINEERING CHEMISTRY
940
content (saponification value calculated as acetyl) and free hydroxyl, using Equations 10 and 13 to obtain values for N , and Na and then substituting these in Equations 11 and 12. The values given in Table VI were calculated in this way from the analytical data given and are compared with analyses by the partition method (IO). These data show that satisfactory agreement between the methods is attainable. CORRELATION WlTH PHYSICAL PROPERTIES.Free hydroxyl content of cellulose acetates, acetate propionates, and acetate butyrates in both units may be correlated with melting point, specific gravity, sorption of moisture, and solubilities in solvents or plasticizers by comparing Figures 1, 2, 3, and 4 with the triangular diagrams published by Malm, Fordyce, and Tanner (9).
Summary The free or unsubstituted hydroxyl content of cellulose derivatives can be measured by acetylation with acetic anhydride in pyridine under controlled conditions. The effects of time and temperature of reaction and of anhydride concentration and excess have been measured, and the procedure haa been set up to utilize the optimum conditions. The results obtained may be expressed as per cent hydroxyl, or if certain other analytical data are known the hydroxyl content may be expressed in terms of free hydroxyl groups per glucose unit of cellulose. Equations are given for making these calculations for cellulose acetates, acetate propionates, and acetate butyrates. The method has been found useful in checking the accuracy of other analytical methods, correlating the hydroxyl content with various physical properties of cellulose esters, and analyzing multicomponent derivatives, one component of which may be calculated by difference. The deviation from three hydroxyls per glucose unit, as found by analysis, may be used to calculate an average degree of polymerization, in the case of very low viscosity cellulose derivatives.
TABLE VI. MIXEDESTER COMPOSITION USINQFREEHYDROXYL DATA
Sample
X
V
Y
Observed Apparent Free acetyl” hydroxyl
Calculated Acetyl Butyryl
Observed by Partition Method Acetyl Butyryl
%
%
%
%
lo
%
35.0 32.6 32.5 31.2 31.2
0.14 2.51 2.57 3.96 3.99
1.3 2.4 2.3 3.8 3.8
55.6 49.8 49.8 45.2 45.2
0.8 2.6 2.5 3.9 4.0
55.6 49.5 49.9 45.1 45.0
z. Apparent acetyl is total saponification value calculated as acetyl, equivalent weight 43 (4).
Acknowledgment The authors gratefully acknowledge the assistance of Jamea Harper, who prepared the triangular charts which correlate hydroxyl content with composition.
Literature Cited (1) Bryant, W. M. D., Mitchell, J., Jr., and Smith, D. M., J. Am. C h m . SOC., 62, 1-3 (1940). (2) Christiansen, B. E . , Pennington, L., and Dimick, P. X.,IND. ENQ.CHEM.,ANAL.E D . , 13, 821-3 (1941). (3) Freed, M., and Wynne, A. M., Ibid., 8, 278-9 (1936). (4) Genung, L. B., and Mallatt, R. C., Ibid., 13, 369-74 (1941). (5) Gloor, W. E., IND.ENG.CHEM.,29, 691 (1937). (6) Hess, K . , and Ljubitsch, N., Ber., 61, 1460-3 (1928). (7) Kaufmann, H. P., and Funke, S., Ibid., 70, 2549-54 (1937). (8) Kraemer, E. O., and Lansing, W. D., J. Phys. Chem., 39, 15367 (1935).
(9) Malm, C. J., Fordyce, C. R., and Tanner, H. A., IND. ENQ. CHEM.,34, 430-5 (1942). (10) Malm, C. J., Nadeau, G. F., and Genung, L. B., IND. ENQ. CHEM.,ANAL.E D . , 14, 292-7 (1942).
Vol. 14, No. 12
Marks, S.,and Morrell, R. S.,,AnaZyst, 56, 428-9 (1931). Meyer, Hans, “Analyse und Konstitutionsermittlung organischer Verbindungen”, 5th ed., Berlin, Julius Springer, pp. 328-39, especially p. 336, 1931. Normann, W., and Schildknecht, E., Fettchem. Umschau, 40,
.
194-7 (1933). (14) Peterson, V. I,., and West, E. S., J. Bzol. Chem., 74, 379-83 11927). , (15) Reinhart, F. W., and Kline, G. M., IND.ENG.CHEM.,32, 18593 (1940). (16) Schaefer, W. E., IND. ENG. CHEM.,ANAL. ED., 9, 449-50 (1937). (17) Smith, D. M., and Bryant, W. M. D., J. Am. Chem. Soc., 57, 61-5 (1935). (18) Ibid., 57, 841-5 (1935). (19) Smith, D. M., Bryant, W. M. D., and Mitchell, J., Jr., Ibid., 61,2407-12 (1939). (20) Staudinger, H., “Die hochmolecularen organischen Verbindungen”, Berlin, Julius Springer, 1932. (21) Verley, A., and Bolsing, Fr., Ber., 34, 3354-8 (1901). (22) Wagner, R. H., Newsome, P. T., and Sheppard, S. E., Eastman \ - - -
Kodak Company, unpublished data. (23) West, E. S., Hoaglund, C. L., and Curtis, G. H., J. Biol. Chem., 104, 627-34 (1934). (24) Wilson. H. N., and Hughes, W. C., J. SOC.Chem. Id.,. 58.. 7 4 4 (1939). (25) Zerewitinoff, Th., Ber., 40, 2023 (1907); 2. anal. Chem., 68, 321-7 (1926). PRESENT~D before the Division of Cellulose Chemistry at the 104th Meeting of the A M ~ R I C A CHEMICAL N SOCIETY, Buffalo, N. Y.
Determination of Small Amounts of Combined Sulfur CARL J. MALM AND LEO J. TANGHE Eastman Kodak Company, Rochester, N. Y.
A method has been developed for the determination of very small amounts of sulfur in cellulose derivatives, based on oxidation with nitric acid and fusion with potassium nitrate. Samples of cellulose acetate have been prepared containing both large amounts of salt sulfates and combined sulfate and the application of dilute hydrochloric acid rinses to these esters showed that salt sulfate is readily washed out, while combined sulfate is retained. It is thus possible to differentiate between combined and noncombined sulfate, since two short rinses of the finely ground cellulose acetate with 0.1 per cent hydrochloric acid have been found sufficient to remove the latter. Hydrochloric acid rinses of a distilled water processed cellulose acetate reduced the sulfur content only when the sample was dried before acid rinsing, indicating that some of the combined sulfate was split off during the drying.
S
ULFURIC acid is widely used as a catalyst in the
commercial production of cellulose esters and small amounts of sulfur remain in such esters after precipitation, washing, and stabilization. The amount of sulfur in a cellulose ester is of considerable importance because it exerts an effect on the stability of the ester towards heat and on the physical properties after aging. Sulfur may be present in a cellulose ester as combined sulfate resulting from the reaction between sulfuric acid and the cellulose, and as noncombined sulfate such as sulfate salts from the wash water. Another source of sulfate in the latter
December 15, 1942
ANALYTICAL EDITION
form may be the cleavage of originally combined sulfate. The determination of the sulfur content of a cellulose ester naturally gives the total sulfur content from both these sources. However, the amount chemically combined is really the significant value, since small amounts of sulfate salts from the wash water are not harmful to the stability of the cellulose ester. A method by which the amount of chemically combined sulfate may be determined is outlined belon. To carry out this work a reliable method for the determination of small amounts of sulfur in cellulose esters was neqessary. The methods available from the literature (1, S) for the determination of sulfur were not satisfactory without modification because of the minute amounts involved. Upon increasing the size of sample, the main problem was the removal or destruction of the organic matter. Simple ashing resulted in considerable loss of sulfur. The following method is a modification of the procedure described by Zahnd and Clarke (6) and by Klingstedt (8). The oxidation of sulfur to sulfate and the destruction of the organic matter are accomplished by digestion with concentrated nitric aqid, followed by fusion of the residue with potassium nitrate. The nitrate is removed by evaporation with hydrochloric acid and the sulfur precipitated as barium sulfate in the usual way.
Determination of Sulfur A sample of 13.7 grams of cellulose ester is placed in a 500-ml. round-bottomed flask with a standard-taper ground-glass joint. After adding 0.5 gram of potassium nitrate, a small crystal of Carborundum to prevent bumping, and 60 ml. of concentrated nitric acid, the flask is heated under a reflux condenser on a hot plate in the hood. The ester dissolves readily in the hot nitric acid and a copious quantity of nitrogen dioxide is liberated during the first few minutes of heating. After refluxing overnight the spent nitric acid is distilled off over a Bunsen flame, care being taken to keep the flask constantly in motion and t o interrupt the distillation when 2 or 3 ml. of liquid remain, The oxidation is continued by further heating t o reflux on the hot plate for 4 hours after adding 5 grams of potassium nitrate and 10 ml. of concentrated nitric acid. The solution is then carefully evaporated to dryness over a low flame. After all the liquid has been boiled out, a hotter flame is applied to fuse the potassium nitrate. At this point the residue should be light colored or have only local dark areas; if the residue begins to blacken the oxidation is incomplete and the fusion should not be attempted. In such a case, an additional 10 ml. of concentrated nitric acid should be added and the sample boiled another hour. As the molten potassium nitrate cools, the flask is kept in motion to distribute the crystals. The major portion of the nitrate is displaced by evaporation to dryness with 20 ml. of concentrated hydrochloric acid. The residue of otassium chloride is heated strongly to drive out all the liquid gut not to the point of fusion. After ailowing to cool, the residue is dissolved in 100 ml. of distilled water and the solution filtered to remove any suspended matter. The volume of filtrate and washings should be about 200 to 250 ml. The solution is acidified with 0.5 ml. of concentrated hydrochloric acid and after heating on the steam bath, the sulfate is precipitated by addin 5 ml. of 5 per cent barium chloride solution. The sample is alfowed to digest on the steam bath overnight and the barium sulfate filtered off on a porous-bottomed crucible. The percentage of sulfur in the sample is numerically equal t o the weight of barium sulfate in grams, after correction for the blank.
Discussion of Method Certain details in the procedure have been incorporated to eliminate the danger of explosions during the final stages of the evaporations with nitric acid. There is danger of explosion if the first evaporation is carried completely to dryness, especially if the flask is not kept in motion. The interruption of this evaporation at a volume of 2 or 3 ml. overcomes any danger of explosion a t this point. During the fusion of the potassium nitrate at the end of second evaporation an explosion may result if too much or-
941
ganic matter remains. If the residue is black a t this stage further oxidation with nitric acid is necessary. A portion of the potassium nitrate is added at the start to prevent loss of sulfate due t o local overheating during the first evaporation. It is not all added a t the start, to avoid the danger of too much potassium nitrate and organic matter towards the end of the first evaporation. The length of time of the initial heating period may be shortened from overnight to 2 hours, but this leaves more organic matter and does not provide as great a margin of safety during the subsequent evaporations. The amount of barium sulfate obtained from samples of cellulose acetate is usually from 10 to 30 mg.; so special care must be taken that none adheres to the beaker during filtration and washing. On this account it is advisable to use beakers free from scratches for the precipitations. If the solution is stirred during the addition of the barium chloride, care should be taken not to scratch the glass with the stirring rod. Because of this effect and because the amount of barium sulfate is so small that no visible precipitation occurs for several minutes, the procedure was adopted of adding the barium chloride solution rapidly and without stirring. A blank must be run on the reagents used and values of 1 to 2 mg. of barium sulfate have been commonly obtained. T o promote uniformity of the blank i t has been found advantageous to prepare a composite of several bottles of nitric acid and also a stock solution of potassium nitrate in nitric acid (500 grams of potassium nitrate plus 1000 ml. of nitric acid). 'Approximately 12 ml. of this solution contain 5 grams of potassium nitrate and 10 ml. of nitric acid. Experiments were carried out to determine whether the complete oxidation by fusion with potassium nitrate was necessary. Samples were diluted with water after the &st evaporation to dryness and the sulfate was precipitated a t this point. Other samples were precipitated without displacing the nitrate by chloride. I n both instances slightly lower results were obtained. The precipitates filtered more slowly and were sometimes black on ignition. An attempt was made to reduce the elapsed time by precipitation in the presence of picric acid (4), but this method does not seem to be applicable where such small amounts of precipitate are involved. Filtration of the barium sulfate after boiling for 10 minutes after adding the barium chloride gave only 40 per cent of the amount of precipitate obtained after digestion overnight. Filtration after digestion for one hour on the steam bath gave 75 per cent of the total sulfate. The determination of sulfur has been applied t o sufficient variety of cellulose derivatives to permit some generalizations regarding its applicability. Samples of cellulose acetate propionate and cellulose acetate butyrate are more difficult to decompose than cellulose acetate, but the conditions outlined are sufficiently severe to oxidize these materials. The method is applicable to cellulose itself, but occasionally more drastic conditions must be imposed. If the initial evaporation cannot be carried to a volume of 2 to 3 ml. without the deposition of sludge, as much as possible of the nitric acid is boiled off and replaced by fresh nitric acid. The mixture is then boiled for an additional 2 hours and evaporated again. Cellulose ethers can be handled in the same manner as cellulose. Cellulose esters containing a large amount of higher acyl, such as stearyl, rasist complete oxidation by this method. Neither is the method applicable to aromatic sulfonic esters of cellulose.
Removal of Noncombined Sulfur During the course of this work it was found that treatment with simple reagents under very mild conditions reduced the
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942
Vol. 14, No. 12
alone caused a comparable reduction in sulfur content. TABLEI. SULFURCONTENT AFTER WASHING TREATMENTS Furthermore, repeated rinses with dilute hydrochloric acid Boiling without intermittent drying did not cause further reduction Distilled 0.1% in sulfur content. Time 0 . 1 % HCI 0.05% NaOH Water Detergent Hours
Determination of Combined Sulfur
Commercial Cellulose Acetate 0
0.5 1 2
;
0.0141 0.0075 0.0080 0.0082 0.0077 0,0079
0.0141 0.0065 0.0062 0.0062 0.0069 0.0072
0.0141 0.0089 0.0073 0.0075 0.0071
0.0141 0.0080 0.0062 0.0086 0.0077 0.0074
Cellulose Acetate High in Salt Sulfate
Cellulose Acetate High in Combined Sulfate 0 0.5
1 2 4
7
0.392 0.356 0.349 0.553 0.356 0.371
0.392 0,350 0,344 0,350 0.366 0.360
0.392 0.170 0.119 0.087 0.077 0.073
0.392 0.324 0.322 0.329 0.314 0.298
sulfur content and that repeated treatments with the same reagent in many cases soon yielded limiting values for the sulfur content. When these treatments were applied to esters high in salt sulfate and combined sulfate, respectively, it was found that the former is readily removed whereas the latter is retained. A sample of cellulose acetate high in salt sulfate was prepared by hydrolyzing a cellulose acetate esterification dope to acetone solubility. To each gallon of water used for precipitation and washing were added 30 ml. each of 15 per cent magnesium sulfate and calcium chloride solutions. The cellulose acetate high in combined sulfate was prepared by esterifying acetone-soluble cellulose acetate with a limited amount of chlorosulfonic acid in pyridine solution. The amount of sulfate introduced depends on the amount of chlorosulfonic acid used and on the amount of water in the system. In one experiment 200 grams of cellulose acetate were dissolved in 1500 ml. of pyridine and a solution of 10 ml. of chlorosulfonic acid in 50 ml. of dioxane was added slowly with stirring. After dilution with acetone the product was precipitated and washed in distilled water and contained 0.392 per cent of sulfur. The reagents selected for the washing treatments were 0.1 per cent hydrochloric acid, 0.05 per cent sodium hydroxide, boiling distilled water, and a 0.1 per cent detergent solution (diglycol oleate). The details of a typical washing series are as follows: A 250-gram portion of the cellulose ester was finely ground and stirred for half an hour at room temperature with 20 parts of 0.1 er cent hydrochloric acid. The ester was then pressed dry on a giichner funnel and one fifth of the wet cake was taken as the first sample, given two distilled water rinses, and dried. The remainder was given further rinses in 20 parts of 0.1 per cent hydrochloric acid. In this way Sam les were obtained after 0.5, 1, 2, 4,and 7 hours. Fresh acid was! solution was supplied after 0.5 and 1 hour and at hourly intervals thereafter.
Table I summarizes the results obtained in the removal of sulfur by these rinses from a commercial cellulose acetate, a cellulose acetate high in salt sulfate, and a cellulose acetate high in combined sulfate. All these treatments rapidly remove about half of the sulfur from the commercial cellulose acetate, but the dilute hydrochloric acid rinse most effectively reduces the sulfur from the cellulose acetate high in salt sulfate. Combined sulfate is not removed by the hydrochloric acid rinse. The cellulose acetate high in combined sulfate lost a small amount of sulfur during the first hydrochloric acid rinse, but this represents originally combined sulfate split off during the drying treatment, since washing the dried sample with distilled water
On the basis of these results, a recommended procedure for the removal of noncombined sulfate from cellulose acetate is to give the finely ground sample two half-hour rinses with 20 parts of 0.1 per cent hydrochloric acid, followed by distilled water rinses to neutrality. The sulfur remaining is combined and is determined as described above. However, in the case of more water-resistant cellulose esters such as cellulose acetate propionates and cellulose acetate butyrates which are almost fully esterified, a reprecipitation for dilute acetone solution into distilled water containing 0.1 per cent of hydrochloric acid was found to be a more efficient way of removing noncombined sulfate. The reprecipitation of samples of cellulose acetate from dilute acetone solutions into distilled water containing 0.1 per cent of hydrochloric acid was found to reduce the sulfur content to the same level as the series of hydrochloric acid rinses. For instance, the commercial sample of cellulose acetate used in Table I contained 0.0078 per cent of sulfur after reprecipitation in this manner. Rinses with dilute hydrochloric acid, applied to dried samples of cellulose acetate which had been precipitated and washed in distilled water, invariably caused reductions in the sulfur content. A commercial grade of acetone-soluble cellulose acetate was precipitated from the reaction mixture and washed to neutrality in distilled water. Half of the product, without drying, was fven repeated half-hour rinses in dilute hydrochloric acid. amples were withdrawn after each rinse, washed to neutrality, and dried. The remaining half was dried for 48 hours in a current of air at 70” C., then given a half-hour rinse in dilute hydrochloric acid, and washed to neutrality. This process was repeated several times to obtain a series of samples having had hydrochloric acid rinses with intermittent drying. The data in Table I1 show that the hydrochloric acid rinses after drying removed sulfur from distilled water-processed cellulose acetate. When the rinses were applied without drying, no such reduction of sulfur content was observed.
TABLE11. REDUCTIOK IN SULFURCONTENTOF CELLULOSE ACETATEBY HYDROCHLORIC ACID RINSESWITHOUT AND WITH INTERMITTENT DRYING Xo. of HC1 Rinses
0
1 2 3
Sulfur Content Without drying With drying % % 0.0170 0.0170 0.0108 0.0161 0.0083 0.0166 0.0063 0.0177
Literature Cited (1) Doree, C., “Methods of Cellulose Chemistry”, p. 277, New York, D. Van Nostrand Co., 1933. (2) Klingstedt, F. W., Z.a d . Chem., 112, 101 (1938). (3) Kruger, D., “Zelluloseazetate und die anderen organisohen Ester
der Zellulose”, p. 217, Dresden and Leipzig, Theodor Steinkopf, 1933. (4) Lindsly, H. C., IND.ENQ.C H ~ MANAL. ., ED.,8, 176 (1936). (5) Zahnd, H., and Clarke, H. T., J. Am. Chem. Soc., 52, 3275 (1930). PRESENTED before the Division of Cellulose Chemistry at the 104th MeetCHEMICAL SOCIETY, Bu5al0, N. Y. ing of the AMERICAN
