Effect of Pretreatments of Wood on the Lignin Determination Distribution of Methoxyls in Wood J
GEO. J. RITTER
AND
JAMES H. BARBOUR, Forest Products Laboratory, Madison, Wis.
V
'ARIOUS treatments have been proposed (2, 5, 9) for wood previous to the lignin determination by the 72 per cent sulfuric acid method. The purpose of such pretreatments has been to reduce the likelihood of contaminating the lignin residue with decomposed extraneous materials. Pretreatments both with benzene-alcohol solution (5) and with alcohol-benzene solution and hot water (9) have been Fedommended for North American woods. A pretreatment with 0.5 per cent sodium hydroxide has been recommended for Australian woods ( 2 ) . Although the three proposed pretreatments have merit for removing certain kinds of extraneous materials, they are inadequate for removing other interfering extraneous substances found in some woods. Other modifications (6, 7 , 10, 11, 12) of the sulfuric acid method and a discussion (4) of the effect of the alkaline pretreatment have been published. G. M. Kuettel of the Forest Products Laboratory has demonstrated that other troublesome extzaneous materialsnamely, catechol tannins-interfere with the lignin determination. The present paper proposes a modification of the sulfuric acid method, which insures the removal of catechol tannins by means of a preliminary treatment of the wood. It is recognized that the method with its proposed modification may still be inadequate for a quantitative lignin determination of some woods having unusually large percentages of still other extraneous materials. The proposed modification is based on the fact that catechol tannins, which are incompletely soluble in the benzenealcohol solution and hot-water pretreatments previously described for North American woods, are dissolved by pretreating the wood with 95 per cent alcohol. When analyzing wood containing no tannins the proposed pretreatment of the wood with alcohol is, of course, unnecessary; however, when analyzing woods in which the tannin content is unknown it is a safeguard. The method recommended contains three preliminary extractive treatments of the wood. The first consists of an extraction with alcohol to remove the catechol tannins; the second, an extraction with alcohol-benzene to remove resins, oils, fats, and waxes; and the third, an extraction with hot water to remove the remaining water-soluble materials. As far as could be observed microscopically, the solid extraneous materials in the cell cavities of the unextracted wood were absent after extraction except for an occasional speck, thus indicating the removal of the visible extractives. The wood residue remaining after the three extractions is next treated with 72 per cent sulfuric acid to dissolve the cellulosic materials, leaving the lignin as a solid residue. A detailed description of the method follows.
Procedure for Determination of Lignin in Wood Approximately 2 grams of air-dry wood in the form of sawdust (60 to 80, or 80 to 100 mesh) are weighed accurately in a tared alundum crucible. The material is then extracted for 4 hours in a Soxhlet apparatus with 95 per cent alcohol. The material is next extracted for 4 hours with alcohol-benzene solution (1 to 2 by volume). The solvent is removed by suction, the residue 238
washed with alcohol by suction to remove the benzene and then extracted for 3 hours with 400 cc. of hot water in a hot-water bath, filtered, washed with hot water, and finally dried. The dried residue is transferred to a large glass-stoppered weighing bottle and is well mixed with 25 cc. of 72 per cent sulfuric acid at 20" C. at which temperature it is maintained for 2 hours. The resulting mixture is transferred to an Erlenmeyer flask, diluted with water to a 3 per cent acid solution, and then boiled for 4 hours under a reflux condenser. The residue is filtered on a tared alundum crucible, washed free of acid by means of hot water, dried, and weighed. The lignin content is calculated on the basis of the oven-dry unextracted sample of wood. In case a correction for ash is desired, the lignin is transferred to a tared platinum dish and the ash content determined in the usual manner.
Test of the Method The data obtained by the method just described were compared with those obtained according to two of the other modifications of the method previously mentioned (5, 9) and also those obtained according to another modification in which alcohol is substituted for hot water for extracting the wood. Comparisons were made on the basis of (1) lignin content of the wood, (2) the methoxyl content of the isolated lignin, and (3) the percentage of the total methoxyl that is recovered in the isolated lignin. The test was made on two tannincontaining woods-redwood heartwood and white oak. In Table I the lignin contents as determined by means of the proposed method are listed in column 2 ; those obtained by two previously described methods (5, 9) are listed in columns 3 and 5 , respectively; and those obtained by substituting hot alcohol for hot water as a pretreatment of the wood are listed in column 4.
Discussion of Results APPARENT LIGXINYIELDS. There is a positive increase in the apparent lignin yields in going stepwise in Table I from column 2 to column 5 . In themselves they offer no explanation as to whether the differences in yields are due to an inclusion of increasing amounts of contaminants as the yields increase or to a removal of decreasing amounts of lignin as the yields increase. They are useful, however, when correlated with the data in Table 11,in showing that the changes in the apparent lignin content after different extractive treatments of the wood are due to an inclusion of varying amounts of contaminants. Before it can be demonstrated that larger percentages of foreign materials are included in the higher apparent lignin yields, it is necessary to compare some characteristic chemical property of the apparent lignin residues, such as the methoxyl content. METHOXYLCONTENTOF THE LIGNN RESIDUES. The methoxyl content of the lignin was employed for characterizing the lignin residues, which mere isolated by the four modifications of the method as indicated in the column headings of Table I. The results for comparison are recorded in Table 11. The methoxgl content determined by the Zeisel method (1) is highest in column 2 and successively decreases in col-
JULY 15, 1935
ANALYTICAL EDITION
239
TABLEI, EFFECTOF REMOVAL OF EXTRACTIVES ON LIGNIN YIELDSOBTAINEDFROM WOOD
TABLE111. METHOXYL CONTENT OF LIGNINISOLATED FROM WOODSUBJECTED TO DIFFERENTEXTRACTIVE TREATMENTS
[Percentages based on oven-dry (105' C.) weight of the unextracted wood.] Lignin in Wood Extracted with: Alcohol, alcohol- AlcoholAlcohol benzene benzene and Alcoholand hot( and hot alcoholbenzene Species water water benzene only 1 2 3 4 5 29.76 31.13 31.75 32.50 Red wood (heartwood) 30.84 31.20 31.85 32.60 30.54 32.28 30.90 31.83 30,77 32.06 30.96 31.56 30.36 31.00 32.16 31.71 30.47 31.00 32.28 ... 30.58 31.02 32,OS ... 30.32 31.02 ... 32.03 30.49 31.02 31.99 ,.. 30.42 ... ... 30.31 30.44 Av. 3i:03 3i:74 32:24 22.99 23.35 24.74 26.04 White oak 22.56 23.50 24.87 26.14 22.81 23.11 24.80 26.16 22.76 23.48 24.81 26.17 22.92 23.15 24.70 26.07 25.99 Av. 22:81 23:32 24:78 26.10
[Percentages based on oven-dry (105' C.) weight of the unextracted wood.] Methoxyl in Lignin IBola,ted from Wood Extracted with: Alcohol, alcoholAlcoholAlcohol, Alcoholbenaene, benzene, alcoholbenzene Species water water benzene only 1 2 3 4 5 Redwood (heartwood) 4.15 4 17 4.23 4 10 White oak 4.58 4.46 4.56 4.67
. t .
TABLE11. METHOXYL CONTENT OF LIGNINISOLATED FROM WOODSUBJECTEDTO DIFFERENTEXTRACTIVE TREATMENTS [Percentages based on the oven-dry (105' C.) weight of the isolated lignin.] Methoxyl in Lignin Isolated from Wood Extracted with: Alcohol, Alcoholalcoholbenzene Alcohol, Alcoholbenzene, and alcoholbenzene Species and water water benzene only 1 2 3 4 5 Redwood (heartwood) 13.61 13.44 13.43 12.68 13.75 13.43 13.34 12.79 13.57 13.52 13.29 12.75 Av. 13.64 13.46 13.35 12.74 White oak 20.10 19.14 18.39 17.89 20.10 19.13 18.47 17.88 17.89 A ~ . 20:io ii:i3 ii:43 17.89
umns 3, 4, and 5 . These results considered in conjunction with those in the corresponding columns of Table I show that the higher methoxyl contents of the lignin residues are associated wkh the lower apparent lignin yields. Such data suggest the presence of larger amounts of materials having a low methoxyl content as the percentages of the apparent lignin yields increase. That such is the condition can be shown by calculating the methoxyl content of the various apparent lignin residues on a common basis as is shown in Table 111. These calculated data, made on the basis of the unextracted wood, result from multiplying the methoxyl content of Table I1 by the content of its parent substance, the lignin, in Table I. Inasmuch as the data in Table 111 are a composite of the methoxyl and the lignin yields, they therefore include any errors that might have been made during the methoxyl and the lignin determinations. A slight error in either of the procedures would account for the lowest methoxyl value for redwood lignin in column 5 of Table 111. It would seem that this value should equal the others or even be slightly higher since, besides methoxyls in the lignin, it might include methoxyls associated with larger quantities of extraneous materials than are present in the other lignin residues. Allowing for slight errors in the determinations, the four values for the methoxyl content of the lignin residues from each wood are in close agreement. In other words, they are within experimental accuracy. It is therefore concluded from these results that the difference in lignin yields between columns 2, 3, 4, and 5 in Table I is due to the presence of extraneous materials having very little, if any, methoxyl. The presence of tannins in the alcohol extract of the wood was shown qualitatively by means of soluble ferric salts, indicating that a t
TABLEIV. DISTRIBUTION OF METHOXYL IN WOOD [Unless otherwise specified, percentages are based on the oven-dry (105O C.) weight of the unextracted wood.] Wood Fractions Redwood White Oak 1. Methoxyl in unextracted wood 5.71 6.38 2. Methoxyl in extracteda wood 4.70 5.91 3. Methoxyl in extraneous fnaterittls (1 - 2) 1.01 0.47 4. Methoxyl in lignin from extradted wood 4.17 4.55 5. Methoxyl in holocellulose from extracted wood 0.53 1.36 (2 - 4) 6. Per cent of total wood methoxyl in extraneous materials (3 t 1) 7. Per cent of total wood methoxyl in ligninb (4 c 1) 73.00 17.70 71.30 7.40 8. Per cent of total wood methoxyl in holocellulose 1.5 J. 1) 9.30 21.30 9. Per-cent'of extracted wood methoxyl in holocellulose (5 i2) 11.28C 23.020 10. Per cent of extracted wood methoxyl in lignin (4 t 2) 88.72C 76.98C a Wood was extracted consecutively with alcohol, alcohol-benzene solution, then washed with alcohol to remove benzene, and finally extracted with hot water. b Lignin was isolated from wood extracted accoi.ding to directions in No. 1. 6 Calculated on the basis of the extracted wood.
least some of the tannins are removed by the proposed preliminary alcoholic treatment of the wood. It is appropriate to mention here that in some woods methoxyl groups are associated with the extraneous materials (3, 13) and the holocellulose (8) as well as with the lignin. The distribution of the methoxyls in these three wood fractions of redwood and white oak is shown in Table IV. DISTRIBUTION OF METHOXYLS IN WOOD. Methoxyl determinations were made on the unextracted wood; the wood extracted with alcohol, alcohol-benzene solution, and hot water; and the isolated lignin. The methoxyl content of the extraneous materials and the holocellulose was calculated by difference. Data in lines 1, 2, and 4 of Table IV are the result of experimentation. Those in lines 3 and 5 were obtained by subtraction; those in lines 6 to 9, inclusive, by calculation. In redwood the methoxyl content of the extraneous materials is high compared with that of the corresponding result in white oak (line 3). A possibility of removing some hemicellulosic material from the wood during the pretreatment with hot water is recognized. However, until that possibility is proved an experimental fact, the data pertaining to the methoxyl content of the extraneous material and the holocellulose should remain as recorded in Table IV. Lignin and holocellulose constitute the extracted wood, as described in a previous paper (8). A part of the methoxyl in extracted wood is associated with the holocellulose and the remainder with the lignin (8). In eytracted wood the percentage of the methoxyl combined with the redwood holocellulose is low compared with that combined with the white oak holocellulose (line 9); in extracted wood the percentage of the methoxy1 recovered in the redwood lignin is high compared with that recovered in white oak lignin (line 10).
Summary and Conclusions The accuracy of the determination of lignin in wood by means of the 72 per cent sulfuric acid method is affected by the amount and kind of extraneous materials incompletely removed from the wood by means of preliminary treatments.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Successive extractions of wood with 95 per cent alcohol, alcohol-benzene solution, and hot water make the 72 per cent sulfuric acid method for the lignin determination more adaptable to North American woods. If the kind and amounts of extraneous materials present in a given species of wood are known, the preliminary treatments can be modified accordingly. For example, if catechol tannins are absent the extraction with 95 per cent alcohol may be eliminated. The methoxyl groups of redwood and white oak woods are distributed among the extraneous materials, the holocellulose, and the lignin.
Literature Cited (1) Bray, M. W., Paper Trade J.,87, 59-68 (1928). (2) Cohen, W. E., and Dadswell, H. E., “Chemistry of Wood,
Part 1,” Melbourne, Commonwealth of Australia, Council of Scientific and Industrial Research, 1931.
VOL. 7, NO. 4
(3) Friedrich and Salzberger, Monats., 53 and 54, 989 (1929). ENG.CHEM.,Anal. Ed., 5, 105 (1933). (4) Harris, E. E., IXD. (5) Mahood, S. E., and Cable, D. E., IND.ENQ.CHEM.,14, 933 (1922). (6) Norman, A. G., and Jenkins, S. H., Biochem. J.,28, 2147 (1934).
(7) Peterson, C. J., and Walde, A. W., IND. ENQ.CHEM., Anal. Ed., 4, 216 (1932). (8) Ritter, G. J., and Kurth, E. F., IND.ENQ.CHEM., 25, 1250 (1933). (9)
Ritter, G. J., Seborg, R. M., and Mitchell, R. L., Ihid.,
4, 202
(1932). (10) Ross, J. H., and Hill, A. C., Pulp & Paper Mag. Can., 27, 15, 541 (1929). (11) Schwalbe, H., Papier-fahr., 23, 174 (1925). (12) Sherrard, E. C., and Harris. E. E., IND.ENQ.CHEM..24, 103 (1932). (13) Sherrard, E. C., and Kurth, E. F., J. Am. Chem. Soc., 51, 3129 (1929).
RECEIVED March 23, 1935. Preaented before the Division of Cellulose Chemistry st the 89th Meeting of the American Chemical Society, New York, N. Y., April 22 to 26, 1935.
Sampling and Analysis of Entrained Matter in Gases FRANK B. VARGA
M
AND
ROGER 8. NEWTON, Yale University, New Haven, Conn.
ANY industrial gases
erential p r e c i p i t a t i o n of one A method for the sampling and analysis constituent. This proved to be are not “clean” a t all of entrained matter in coke-oven gases has the case a t the C o n n e c t i c u t stages of a process, but been devised and tested at various points Coke Company, where the percontain entrained m a t e r i a l s during the purification of the gas. The which may be objectionable in centage of the water removed method accounts for the following fractions was greater than the percentfurther steps in the process or age of the tar removed, indicatwhich may be valuable when of the entrained material: water, volatile ing t h a t r a i s i n g the gas temseparated from the gas. Nuorganic constituents (benzene and naphperature 60 that all water would merous methods for the removal thalene), benzene-soluble constituents, be in the form of vapor would of this material are used, such and constituents not soluble in benzene increase the efficiency of tar reas scrubbing and electrical pre(coke breeze). The method is simple and moval. cipitation, and in order to measure the efficiency of this rethe results are accurate and reproducible moval, it is essential to have an Ekperimental within about 10 per cent. The method accurate and reproducible means offers a means of testing and checking the The sampling of the gas was for determining the amount of operation of scrubbers, Cottrell precipia c c o m p l i s h e d by a filtration entrained matter in a gas. The method similar to that given tators, and other devices for removal of problem is further complicated by the R e s e a r c h Corporation by the complex nature of the entrained matter. in an unpublished report. The suspended materials in many apparatus is shown in Figure 1. gases. This investigation was undertaken to determine the nature and quantities of such The sample tube, A , was of glass 20 em. (8 inches) long and entrained matter in gases a t various points a t the plant of the 1.6 cm. (0.625 inch) in diameter. The end was bent to a right angle and tapered slightly. The edges were ground so as t o Connecticut Coke Company in New Haven, Conn. present a smooth edge to the gas stream. The filtering medium Various methods have been devised for the sampling and consisted of three or more plugs of glass wool, held apart by short analysis of entrained matter in gases, but for the most part pieces of glass tubing and packed in as tightly as ossible. In they have been applied to cases where the entrained matter use, the last plug was never discolored by tar, inzicating that all entrained tar was removed by this filtering device. Some was “dry dust,” such as is found in flue gases a t temperapreliminary experiments were made using an extraction thimble tures above the dew point. The method of Klempt (I), in addition t o the glass-wool filter. This roved to be unneceswhich is similar to the one used in this investigation, was used sary, since there was no discoloration of thimble due to tar. only for determination of the entrained tar and the amount Furthermore, the pressure drop, when taking a sample through the thimble was very great and made it very difficult to obtain of water was not measured. Kone of the previously described samples if the as main was at a pressure less than atmospheric. methods was applicable to a case of such complexity as that The omission ofthe thimble simplified the a paratus considerably, found at the Connecticut Coke Company plant. The method which was a great advantage when it h a 1 to be carried to the t o be described was devised to afford a means of testing the tops of high towers and along elevated ipe lines. In some cases a second tube containing glass-woof plugs was included Cottrell precipitators with respect to the removal of both in series with the sampler. When this tube was maintained at entrained water and entrained tar. It is well known that the temperature of the main, there was no gain in weight, indisuch a precipitator will remove a high percentage of the total cating that all entrained matter was removed in the first filter. entrained matter from a gas stream, but there may be a prefTo obtain a sample, A was inserted into gas main B through
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