INDUSTRIAL AND ENGI-VEERING CHEXISTRY
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Vol. 19, No. 7
A Hydrolysis Number Determination for Wood Cellulose’ By L. F. Hawley and 1,. C. Fleck U S. FOREST PRODUCTS LABORATORY, MADISON, \\‘IS.
A +tewanalytical method f o r the further characterization of the Cross and Bevan cellulose isolated f r o m wood by chlorination. T h e method is shown to be valuable in analyzing partly decomposed wood awd particularly i.iz determining the composition of p u l p wood and of the corresponding pulp in such a w a y that the yields of p u l p can be calculated. H E Cross and Bevan chlorination method for determining cellulose, as modified by various investigators, has been widely used and has proved of great value in studying cellulosic materials. It has been unsatisfactory, however, in that the composition of the different aggregates isolated, and called “cellulose,” varied widely according t o the materials from which they were isolated. One method of rendering the Cross and Bevan determination more complete has been to make special tests of the cellulose aggregate after its isolation in order to obtain information concerning its composition. One such testnamely, the determination of alpha-, beta-, and gammacellulose-has been useful, especially the alpha-cellulose determination on nearly pure celluloses. These determinations are not so satisfactory, however, when made on Cross and Bevan cellulose isolated from wood, since so large a proportion of the crude cellulose from wood consists of betaand gamma-cellulose, neither of which really means much more than “not alpha-cellulose.’’ Moreover, the common assumption that alpha-cellulose from wood is necessarily the same as alpha-cellulose from cotton is incorrect. It is even doubtful whether these determinations characterize the cellulose from a purely chemical point of view or whether the variations may not be due to the physical size of the cellulose aggregate. Another method of characterizing Cross and Bevan cellulose has been the determination of the pentosans contained in it. Although the accuracy of the pentosan determination is questionable, it has given certain comparative results of much value. However, the pentosans are not the only variable constituent of Cross and Revan cellulose, and the assumption that pentosan-free cellulose (meaning total Cross and Bevan cellulose minus pentosans) is a pure, homogeneous unit is incorrect, and the conception of such an aggregate has not been of much value in the chemistry of wood. Again, it has been shown that mannose residues may constitute a considerable proportion of the Cross and Bevan celluloses isolated from softwoods. The mannose determination, however, is not a simple one. It has never been applied as a regular method for characterizing Cross and Bevan cellulose and would not afford a complete characterization in any event.
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Development of Method
An analytical method for any kind of cellulose aggregate from wood must be empirical because of the difficulty of removing all impurities from an insoluble, solid, colloidal substance of this type. Such a procedure, however, although not scientific, yields valuable information when it is properly standardized and intelligently interpreted. The Cross and Bevan method for determining cellulose is scientifically useful only to the extent that the analyst real1 Presented before the Cellulose Division at the 72nd Meeting of the American Chemical Society, Philadelphia, Pa., September 5 to 11, 1926.
izes the variant composition of the aggregate isolated. d skilful analyst with a well-standardized method can get yery satisfactory check results by this method, and the carbohydrates isolated certainly have some common properties which are sharply differentiated from those of the other carbohydrates in the wood. It has seemed, therefore, that advances in the analysis of the cellulosic constituents of wood could best be made by continuing the isolation of a crude cellulose aggregate by a well-standardized chlorination method and then making special examinations of the aggregate to determine variations in its composition. One of the characteristic differences between wood cellulose isolated by chlorination and pure cotton cellulose is the ease with which certain portions of the wood cellulose can be hydrolyzed. This difference has been recognized in connection with the hydrolysis of wood and the making of pulp and paper, but it appears never to have been suggested as the basis of an analytical method. Some such analytical method should be of great value in wood analysis, especially when applied to partly decomposed wood, such as decayed wood or the products of pulping or hydrolytic processes. For instance, in recent studies of the decomposition of wood during the pulping process, it has been found that the cellulose is slowly removed; but as there was no satisfactory method by which the changes in the composition of the cellulose could be traced, the question was left unanswered whether or not the material isolated by chlorination and called cellulose was of the same composition throughout the reaction. The cellulose is reduced in the same way during the decay of wood, but aside from pentosan determinations there has been no satisfactory method for determining what changes, if any, occur in the composition of the cellulose while part of it is being removed. A hydrolysis number determination for cellulose has been in use for some time, but since it was designed for determining the state of purity of very pure samples of cotton cellulose, it would not be suitable where a large proportion of the cellulose was readily hydrolyzable. It therefore seemed desirable to use more severe conditions of hydrolysis. Miller and Swanson2 carried out graded hydrolyses of wood by boiling samples for 6 hours with different concentrations of dilute hydrochloric acid. They found an apparently constant point in the hydrolysis where no more cellulose was hydrolyzed, even though the concentration of acid was increased from 0.75 per cent to 3 per cent. Hence a concentration of, say, 2 per cent acid would have made a satisfactory condition for an analytical method, except that it was desirable to decrease the time required for the hydrolysis by using a more concentrated acid. It was also believed that the degree of hydrolysis obtained, even by the most severe conditions used by Miller and Swanson, was still too mild for use in an analytical method of this kind. For instance, many hydrolytic attacks on wood which might be made in commercial processes or by natural forces would probably have 2
THISJOURNAL, 17, 843 (1925).
July, 1927
ISDC'STRIAL A-VD ESGIXEERIA'G CHEXISTRY
greater hydrolytic effect than the 3 per cent of hydrochloric acid, and any change caused by a stronger acid, higher temperature, or longer time could not be measured by an analytical method using the milder hydrolysis. It was hoped that a t -ome higher point a flat place in the curve of hydrolysiq would be found similar to that obtained by Miller and Swanson between 0.75 and 3 per cent hydrochloric acid. Figure 1 shows. however, that such a constant point was not obtained and that there is apparently no sharp dividing line between readily hydrolyzable and resistant cellulose. The middle line shows the amount of hydrolysis of a Cross and Bel-an cellulose from spruce wood when treated with different coiicentrations of wlfuric acid for 3 hours on a boiling-water bath. There is a fairly straight line relationship between acid concentration and amount of hydrolysis lvithin the limits of concentration used, but it should be noted that this line should be extended to zero, which mould require a very sudden break. The conditions used are, therefore, probably hydrolyzing all of the readily hydrolyzable material. The lower line shows the amount of hydrolysis of a Cross and Bevan cellulose which had been previously partly hydrolyzed. The two lines are practically parallel, showing that a mild preliminary hydrolysis has no effect on the final point reached by a more severe hydrolysis. This is further shown by the top line, which represents the residues of urihydrolyzed cellulose after both series of hydrolyses. Method
Since the method is to be empirical it does not make much difference what conditions for hydrolysis are chosen as standard so long as the hydrolysis is fairly severe and the residue is left in such condition that it can be readily washed, filtered, aiid weighed. Therefore, 15 per cent sulfuric acid was chosen as a suitable concentration. It was found also that the size of the particles of wood used, as well as the bleaching of the isolated cellulose. exerted conqiderable influence on the hydrolysis number determination. It is recommended, therefore, that unbleached Cross and Bevan cdlulose from 80- to 100-me~hsawdust be used.
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hydrolysis iiumber is 30.1 per cent, then the hydrolyzed portion of this cellulose is 30.1 times 60.5 = 18.2 per cent (still on the basis of the original wood), and the unhydrolyzed residue is 60.5 - 18.2 = 42.3 per cent on the same basis If the kind of pulp to he made from this wood contains 92.3 per cent Cross and Bevan cellulose, of which the hydrolysis number is 4.5 per cent, then the unhydrolyzed residue of the pulp is (100 - 4.5) 92.3 = 88.1 per cent. This uiihydrolyzed residue from the pulp corresponds exactly in amount and composition to the unhydrolyzed residue from the wood or, in other words, 42.3 per cent of the wood is the same as 88.1 per cent of the pulp and the pulp is therefore 42.3 = 48.0 per cent of the wood 88.1 The writers have not had the chance properly to test the accuracy of this method of determining pulp yields, as the hydrolysis number determination on the pulp wood has not been available. Table I gives some yields calculated on the basis of an assumed probable value of 43.75 per cent of cellulose resistant to hydrolysis in all the pulp-wood samples. Considering the possible variation in different lots of pulp chips, the calculated yields correspond very closely to the actual determined yields. Table I-Calculation of Pulpa Yields from Hydrolysis Number Determinations on Isolated Cellulose CALCD. CROSS AND
BEVAN CELLULOSE PULP SAMPLE
BISIS OF P U L P P e r cent 86 06
YIELD
HYDROLYSIS NUMBER
ASSUMING
STAB1.G CELLULOSE
CELLULOSE BASIS BASIS
OF P U L P
43.75 STABLE CELLULOSE I N WOOD
ACTU11, k-IllLD
P e r cent P e r cent P e r cent 3119-1 6.54 80 43 54.3 53.80 3131-1 5.83 90 88 48.14 48.50 96 50 7.00 51,Oli 50.10 85 67 3129-1 92 13 3127-1 6.37 88 95 49.18 47.40 95 01 6.11 88 2 1 49.60 4 8 .j0 93 93 3128-1 3120-1 7.24 50.33 47.90 93 66 86 89 a T h e preparation of these pulps has been described by Swanson a n d hlonsson, P a p e r T r a d e J . , 82, No. 4, 6 2 (1926).
Approximately 1 gram of the Cross and Bevan cellulose is heated with 100 cc. of 15 per cent sulfuric acid for 3 hours on a boiling-water bath. The residual unhydrolyzed cellulose is filtered on an alundum crucible (R A. 360), thoroughly washed with hot water, and then transferred to a tared alundum crucible (R A . 98), washed with hot water, alcohol, and ether, dried a t 105' C., and weighed The percentage loss in weight during the hydrolysis is the hydrolysis number
Application
Sherrard has already used this method with success in analyzing the residues of wood after repeated hydrolysij of the same sample. It has also been used in the analysis of decayed mood, giving valuable information which could not be readily obtained otherwise. These results will be published in the near future. Perhaps its most interesting and practical application is in connection with the analysis of pulp woods and wood pulps. The determination of cellulose by the Cross and Bevan method has long been criticized by the pulp chemist as giving no indication of the yield of pulp that is possible. During the delignification of the mood for the purpose of separating the fibers there is an accompanying undesirable hydrolysis of the cellulose, which reduces the yield of pulp considerably below the yield of Cross and Bevan cellulose. If, however, we know the yield and hydrolysis number of the Cross and Bevan cellulose from the original wood and from the pulp which is t o be obtained, we can calculate the probable yield of pulp from the wood. For instance, if a certain pulp wood contains 60.5 per cent Cross and Bevan cellulose whose
C O N C f N T R A T I O N OF A C I D
Figure 1
It has been very difficult, if not impossible, for a pulp mi11 to make accurate determinations of its yields of crude pulp during one day's operation, or that of one digester. It was only from monthly or less frequent inventories that any
IAVDUSTRIAL -4h'D ELVGISEERI NG C H E M I S T R Y
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yield figures could be obtained and those were usually on the unsatisfactory basis of cords of pulp wood. The manager must therefore wait until the inventory period to determine the effect of any voluntary or involuntary change in cooking conditions. By means of the determinations and calculations shown in Table I, it is now possible to determine pulp yields from the composition of the pulp and pulp wood with only the assumption of the mechanical losses. Such an analysis also shows the degree of hydrolysis to which the pulp has been subjected during cooking. The question might be asked whether the hydrolysis could not be performed on the pulp as well as on the cellulose isolated from the pulp. This can be done and the results used in the same sort of a calculation, provided that the lignin content of the pulp is also known. If, for instance, the loss of weight of a pulp on direct hydrolysis is 10 per cent, then the stable cellulose plus the lignin is 90 per cent, and if the lignin is 4 per cent, the stable cellulose is 86 per cent of the crude pulp. This figure for the stable cellulose can be used the same as before for comparison with the percentages of stable cellulose of the original wood to determine the yield of pulp.
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Table I1 shows calculations from the same series of pulps used in Table I, but with the loss in weight on hydrolysis determined on the pulp itself. This figure was then combined with the lignin figure to give the yield of pulp. The yields are seen to correspond very closely to those obtained from the hydrolysis number of the isolated cellulose. of Pulp Yields from Direct Hydrolysis of the Pulp (Figures in per cent) UNHY-
Table 11-Calculation
DROLYZED
PULP SAMPLE 3131-1 3129-1 3128.1 3120-1 3130-1
Loss
ON HYDROLYSIS
7.91 10.07 9.02 9.53 10.20
RESIDUE LIGNININCLUDING FREE LIGNIN LIGNIN RESIDUE 92.09 1.80 90.29 89.93 4.50 85.43 90.98 3.20 87.78 90.47 3.40 87.07 89.80 4.20 85.60
CALCD ACTUAL PULP PULP YIELD YIELD 48.23 48.50 51.21 51.06 49.84 48.50 50.24 47.90 51.10 50.20
Hydrolysis number determinations will be valuable in many other places. Pulps prepared by the alkaline and neutral processes, different species of wood, heartwood and sapwood, springwood and summerwood, other cellulosic materials than wood-in the analysis of all these the hydrolysis number determination should be included.
Detection of Lactic Acid in Presence of Other Organic Acids' By Frederick G. Germuth DEPARTMENT O F PUBLIC WORKS,BCREAU OF STANDARDS, BALTIMORE, LID.
HIS research was und e r t a k e n in the interests of the Revision Program of the United States Pharmacopeia, following the suggestion of the committee in charge of this work as to the desirability of further investigation of reliable cheniical methods for the detection of lactic acid, particularly in the presence of other acids of organic origin.
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Materials Used
A method has been described for the detection of lactic acid and lactates alone, or in the presence of other organic acids whose salts are largely employed for medicinal purposes. Lactic acid, in samples of 0.5 to 5 per cent concentration, in increments of 0.5 per cent, gives with 15 per cent solution of potassium thiocyanate (in the proportion of 0.5 cc. for each 1 per cent increase of lactic acid or lactate concentration) an orange or purplish coloration that is not discharged by saturated solution of mercuric chloride. Lactates treated in the same manner with the same proportions of reagents, and acidified with slight excess of concentrated hydrochloric acid, give with potassium thiocyanate solution an orange or purplish coloration that is not discharged by saturated solution of mercuric chloride.
The salts of organic acids employed in the experimental work and chosen as being those in whose presence lactic acid or lactates (see Table I) were most likely to occur were all of U. S. P. quality. S o attempt was made to purify these further as it is this grade of medicinals which the practitioner indicates when prescribing and also the grade employed by the manufacturing chemist in the production of various preparations. The potassium thiocyanate was of C. P. grade. It was recrystallized from slightly ammoniacal solution before use, then recrystallized twice again. The amount of iron in the original substance was thus reduced from 0.001 per cent to 0.0001 per cent. The colorimetric procedure was employed t o determine the amount of iron present. The other jmpurities were as follows: sulfuric anhydride, 0.001 per cent; ammonia, slight trace; chloride, 0.0001 per cent; and iron, 0.0001 per cent. The lactic acid was the C. P. product (sp. gr. 1.20, 68.5 1
Received February 28, 1927.
per cent) containing the following impurities : chloride, 0.001 per cent; sulfuric anhydride, 0.001 per cent. This xws carefully tested for iron content, and was found to contain about 0.0001 per cent of iron. Purification was accomplished by distillation a t greatly diminished pressure and subsequent redistillation. T h e p u r i f i e d product contained but a faint trace of iron, and traces of chloride and sulfuric anhydride. The calcium lactate was of U. S. P. quality and no attempt was made to eliminate impurities with the exception of the iron. This was accomplished by solution of the salt and the regular precipitation method. The iron was reduced to less than 0.0001 per cent, as determined by the potassium ferrocyanide method. Method
The procedure consisted in preparing 5 per cent solutions of each of the compounds under consideration, and subsequently treating each sample with chemical reagents of the concentration generally employed in qualitative analysis. The results are given in Table I. These reactions indicated the improbability of identifying lactic acid or a lactate in the presence of other organic acid anions. A series of experiments was made to prove this, however, using a mixture comprising all the salts used in the previous experiments. The results are shown in Table 11. I n the foregoing experiments those compounds were em-
