(13) General Electric ~CO., X-ray Dept., Milwaukee, Wis., “X-ray Wavelengths for Spectrometer,” cat. no. A4961-DA (1959). (14) Glocker, R., Frohnmayer, W., Ann. Physik 76,369-95 (1925). (15) Hakkila, R. A., Waterbury, G. R., Develov. Avvl. Svtctru. 2. 297-307 (1963j. ‘ (16) Hughes, H. K., Hochsgesang, F. P., ANAL.CHEM.22,1243-58 (1950). (17) Klug, H. P., Alexander, L. E.. “X-ray’ Diffraction Procedures for Polycrystalline and Amorphous Materials,” pp. 281-90, Wiley, New York, 1954. (18) Knapp, K. T., Lindahl, R. H., .
V
I
Mabis, A. J., Advan. X-ray Anal. 7 , in press (1964). (19) Leroux, J., Ibid., 5, 153-60 (1962). (20).Liebhafsky, H. A,, Pfeiffer, H. G., Winslow, E. H., Zemany, P. D., “X-ray Absorption and Emission in Analytical Chemistry,” pp. 313-17, Wiley, New York. 1960. (21) Liebhafsky, H. A., Winslow, E. H., Pfeiffer, H. G., ANAL.CHEM.34, 282R294R (1962). (22) N. V. PhiliDs GloeilamDenfabrieken. ‘ S‘ci. Equipment Dept., Application Lab.; Eindhoven, Netherlands, “ 9 b l e of Mass Absorption Coefficients, Noreko Reptr. 9, No. 3 (1962). (23) Peed, W. F., Dunn, H. W., U. S.
Atomic Energy Comm. Rept. ORNG 1265 (1952). (24) Raag, V., Bertin, E. P., Longobucco, R. J., Advan. Electron Tube Techniques 2,249-59 (1963). (25) Stainer, H. M.,U. S. Bur. Mines Circ. 8166 (1962). 126) Victoreen. J. A.. .J. A.o.ol. Phus. 20. 1141-7 (1949). ’ (27) Wright, W. B., Jr., Barringer, R. E., U. S. Atomic Energy Comm. Rept. Y-1095 (1955). (28) Zingaro, P. W., Norelco Reptr. 2, 92-5 (1956). ‘ 1
\ - - I
RECEIVED for review September 16, 1963. Accepted November 12, 1963.
Nonaquelous Titration of Acidic Groups of Lignins. Titration in Pyridine Using Potassium Methoxide as Titrant STANLEY 0.THOMF‘SON and GORDON CHESTERS Department o f Soils, University o f Wisconsin, Madison, Wis.
b The acidic properties of dioxane and alkali lignins were investigated b y potentiometric titraticn, using a system with pyridine as solvent, potassium methoxide as titrant, and a platinized platinum-calomel electrode combination. Reproducibile results were obtained for all compounds used in evaluating the system, and for all lignin samples. Differentiating titration was possible witii some polybasic compounds. Sharp potential breaks were obtained for lignin samples. Acidic properties varied with lignin source and method of isolation. Some results indicate that the acidic hydrogen content of lignins might be higher than generally reported.
T
H E IMPORTANCE (OF ACIDIC GROUPS
relative to an understanding of the structure of lignin is well recognized, and several methods fcr their qualitative and quantitative decermination have been described (2, 3 ) . Methods that have received greatest emphasis in recent literature are those employing ultraviolet absorption, and conductometric and potentiometric titration. Goldschmid (9) developed a method for determining phenolic hydroxyl groups in lignin based on the difference in ultraviolet absorption of samples dissolved in neutral and ,alkaline solutions. Conductometric titration was employed by Sarkanen and Schuerch ( l a ) for determination of equivalent weights of lignin preparations dissolved in a n acetone-ethanol-water mixture. Conductometric titration was also employed by Gaslini and Nahum (8) who investi-
gated the acidic properties of alkali lignins in an aqueous lithium metaborate solution. A conductometric method using ammonia as solvent was successfully employed in the titration of thiolignins (7). Butler and Czepiel ( 4 ) determined the phenolic and enolic hydroxyl groups in lignins by potentiometric titration in a nonaqueous system with dimethyl formamide as solvent and potassium methoxide as titrant. Results obtained by the various methods for similar lignin samples might differ considerably. Consequently, the acidic properties of isolated lignins merit further research either through improvements of existing
i
SUCCINIC
‘7
EXPERIMENTAL
: 5z
I
methods or the development of new ones. This paper presents the results of a n investigation of acidic properties of dioxane and alkali lignins in a nonaqueous system that combines features of the method of Butler and Czepiel (4and , that developed by Cundiff and Markunas (5) for the nonaqueous determination of acids. The theory and practice of titration in nonaqueous systems has been reviewed extensively (1, 6, 11, IS). I n the present system, the samples were dissolved in pyridine and titrated with potassium methoxide, using a platinized platinum indicatingmodified methanol calomel reference electrode combination. Use of the method of Cundiff and Markunas as described (6) gave no distinct potential breaks in the titration of lignin preparations. The system of Butler and Czepiel ( 4 ) gave unsteady potentials.
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0 I m.
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YILLILQUIVALENTS O f TITRANT
Figure 1. Titration of benzoic and some polybasic acids
Apparatus. Beckman Zeromatic p H meter, Model 96; platinized platinum indicating electrode; sleevetype calomel reference electrode in which the saturated aqueous potassium chloride solution was replaced with a saturated methanol solution of potassium chloride ( 5 ) ; 10- or 25-ml. buret; tall-form weighing bottle or any suitable substitute fitted with three-hole rubber stopper; and magnetic stirrer (grounded) and stirring bar. Reagents. Pyridine, reagent grade; potassium methoxide in benzenemethanol mixture, prepared according to Fritz (6) and standardized against benzoic acid; and azo-violet indicator: a saturated solution of resorcinol in dry redistilled benzene. VOL. 36,
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of azo-violet indicator were added. The acidic impurities in the solvent were titrated with potassium methoxide to a blue end point. The sample, about 0.5 meq. of test compound or 50 mg. of lignin, was transferred to the sample holder and the rubber stopper with the electrodes and buret was positioned. After complete dissolution of the sample, the titrant, approximately O.lN, was added in increments of 0.1 or 0.2 ml., and the titration followed potentiometrically.
r MALEIC
RESULTS AND DISCUSSION
Test Compounds. Titration curves of benzoic, citric, oxalic, and succinic acid (Figure 1) were typical of those obtained with known compounds. The potential spans were usually 800 0 I m. YILLIEOUIVALENTS OF TITRANT
Figure 2. Differentiating titrations of dibasic acids
The system was evaluated by titration of several known acidic compounds of different pK values. Some of these compounds were structurally related to lignin on the basis of their aromatic nucleus and type of substitution. All test compounds with the exception of ethylvanillin and ferulic acid were reagent grade. Ethylvanillin was of unspecified purity; ferulic acid was of practical grade. Both compounds were obtained from the Matheson, Coleman and Bell Division. Lignin samples with the exception of Indulin AT were prepared as described elsewhere (14). Indulin was obtained from the West Virginia Pulp and Paper Co. Procedure. Forty milliliters of pyridine was pipetted into a clean dry weighing bottle, and six drops
Table 1.
Acid Content of Dioxane Lignins
Lignin source Scots pine Rye straw Bromegrass Oak leaves
Meq. H per gram
OH
5.80 7.00 6.70
9.9 11.9 11.4 12.0
7.05
%
Table II. Acid Contents of Alkali Lignins
Extraction tempera- Meq. H ture per gram
Lignin source
Oat straw Rye straw Scots pine
25' C. 90: C.
Boiling 25' C. 90' C. 25" C. 90'C.
Boiling
656
e
4.00
4.80 4.70 5.10 4.30 5.00 4.75 4.60
ANALYTICAL CHEMISTRY
OH
%
6.8 8.2 8.0 8.7 7.3 8.5 8.1 7.8
Titration curves of maleic and salicylic acid are shown in Figure 2. The two inflections indicate that differentiating titration by this system is possible with some compounds. On the basis of the dimociation constants of the known acidic compounds in water, it appears that differentiating titration of two acidic groups by the system can be successful only when the ratio of K1 to K2 is of the order of 10,000 or greater. This is evident from a consideration of the dissociation constants of oxalic and maleic acids which have K1 to K2 ratios of the order of 1000 and 10,000, respectively. The two acidic hydrogens of maleic acid were differentiated, while a single inflection was obtained with oxalic acid. Successful titration of the second acidic hydrogen of salicylic acid also shows that some very weak acidic groups may be determined by this method. Lignin Samples. Representative titration curves of lignin preparations with single inflections are shown in Figure 3. Although a gelatinous precipitate was produced during the titration of each sample, excellent curves with a potential span of approximately 600 mv. were obtained. I n most cases the potential increased by about 200 mv. in the early stages of the titrations but the potential breaks of 400 to 500 mv. were sufficiently sharp to permit ready determinations of the end points. Maximum value of the relative standard deviation of the titrations was *0.98%',. The curves shown in Figure 3 were those of lignin samples obtained from the respective plant material by a single (one step) 1,Cdioxane extraction. The data in Table I show that with the
MILLIEQUIVALENTS/QRAM
Figure 3.
Titration of dioxane lignins
to 900 mv., and the initial flatness of the curves resulted in very sharp inflections. Each of the polybasic acids gave a single inflection, indicating the limitation of the system in its use for differentiating titration. However, the inflection point of the curve of each compound was equivalent to the total content of acidic hydrogen. Ferulic and nicotinic acid, and vanillin and ethylvanillin, compounds structurally related to lignin, gave curves that were similar to those shown in Figure 1. Titration of syringic acid produced a single inflection that was equivalent to the first acidic hydrogen. KO definite end point was obtained with phenol.
UlLLlEPYIVA.LEU1SIGR.Y
Figure 4. Titration curves of lignins showing two potential breaks
exception of the samde isolated from Scots pine the cor tents of acidic hydrogen were similar for the different isolates. Data on the acidic properties of alkali lignins extracted a t different temperatures are presented in Table 11. I n the case of the rye straw and the Scots pine lignins where direct comparisons can be made, the acidic hydrogen contents of the alkali lignins (Table 11) were less than those of the corresponding dioxane lignins (Table I). I n general the temperahre of extraction had no appreciable effect on the amount of replaceable hydrogen in the alkali lignins. Small increases were obtained a t the higher temperE,tures for the oat straw samples but this trend was reversed for samples from rye straw and Scots pine. If 840 is assumed to be the best estimate for the molecular weight of the lignin building unit ( 2 , S), a titration of approximately 1.2 meq. of acidic hydrogen per gram of lignin would correspond to a ligr in building unit (molecule) containing one hydroxyl group. The data in Table 11 suggest that with the possible exception of oat straw lignin extracted a t 25’ C. and rye straw lignin extracted a t 90” C. the alkali lignins contained 4 hydroxyl groups per molecule-Le., approximately 4.8 meq. of acidic hydrogen per gram. The data in Table I suggest 5 hydroxyl groups per molecule of dioxane Scots pine lignin and 6 groups each for dioxane rye straw, bromegrass, and oak lignins. Differences n the number of hydroxyl groups as noted imply differences in structure of the lignin preparations concerned. Table I11 shows data obtained from the titration of dioxane oat straw and dioxane Scots pine lignins fractionally extracted by repeated dissolution from the two sources. Most fractions isolated from oat straw showed similar acidic properties, but 1 here were marked differences between the early and late fractions isolated frorr Scots pine. Examples of titraticn curves with two inflections are presented in Figure 4. Curves of the type shown were obtained with several lignin preparations. Acidic hydrogen value of 4.00 meq. per gram (Table IV) for the firrt inflection of the indulin curve is in clo3e agreement with the total values repor;ed by Butler and Czepiel (I),and by Gwlini and Nahum
Table 111.
Lignin source Oat straw
Acid Contents of Fractionally-Extracted Dioxane Lignins
Extraction time, hr.
Scots pine
Cumulative extraction time, hr.
Meq. H per gram
OH, %
3 7 15 31 63 127 255 1 3 7 15 31 63 127
6.69 5.77 5.71 6.05 6.08 6.45 6.31 4.95 4.04 4.39 4.39 5.46 7.43 7.69
11.4 9.8 9.7 10.3 10.3 11.0 10.7 8.4 6.9 7.5 7.5 9:3 12.6 13.1
2 4 8 16 32 64 128 1 2 4 8
16 32 64
(8) who, respectively, found 4.65 and 4.79 meq. per gram of indulin. Because
the total values of acidic hydrogen (Table IV) determined from these curves (Figure 4) were higher than any found for similar lignins in the literature, and curves of the type shown were not obtained for all samples investigated nor consistently for the same sample, there is some question as to whether or not the second potential break in each curve represents a true inflection-ie., one due to acidic hydrogen in the sample. If these breaks constitute true inflections, the curves indicate that lignins contain weakly acidic groups that were never detected by the use of prevously reported methods of determination. I n the case of the present method, the absence of consistency in the occurrence of the second inflection might be due to the methanol in the titrant (IO)or to the gelatinous precipitate produced during the titrations. The uncertainties concerning the second potential break are under investigation. LITERATURE CITED
(l).Beckett, A. H., Tinley, E. H., “Titration in Son-Aqueous Solvents,” 2nd ed., British Drug Houses, Poole, Dorset, 1957. (2) Brauns, F. E., “Chemistry of Lignin,” pp. 246-54, Academic Press, New York, 1952. (3) Brauns, F. E., Brauns, Dorothy A., “The Chemistry of Lignin,,’ Supplement Vol., pp. 244-66, Academic Press, New York, 1960. (4) Butler, J. P., Czepiel, T. P., ANAL. CHEM.28,1468 (1956). (5) Cundiff, R. H., Markunas, P. C., Ibid., p. 792.
~~
Table IV. Acid Content of Lignin Samples Showing Two Inflection Points
Meq. H Lignin samples per gram Indulin, AT 1st inflection 4.90 2nd inflection 5.10 Total 10.00 Dioxane oat straw 1st inflection 6.30 2nd inflection 5.55 Total 11.85 NaOH rye straw 1st inflection 3.35 2nd inflection 6.05 Total 9.40
OH
%
8.3 8.7 17.0 10.7 9.4 20.1 5.7 10.3 16.0
(6) Fritz, J. S., “Acid-Base Titration in
Nonaqueous Solvents,” G. Frederick Smith Chemical Co., Columbus, Ohio,
1952. (7) Gaslini, F., Nahum, L. Z., ANAL. C H E M . 31,989 (1959). (8) Gaslini, F., Nahum, L. Z., Svensk Papperstid. 62, 520 (1959). (9) G_oldschmid, O., Ibid., 26, 1421 (1904). (10) Harlow, G. A., Wyld, G. E. A., Ibid., 30,73 (1958). (11) Riddick, J. A,, Ibid., 28, 679 (1956). (12) Sarkanen, K., Schuerch, C., Ibid., 27, 1245 (1955). (13) Streuli, C. A., Ibid., 34,302R (1962). (14) Thompson, S. O., Chesters, G., Engelbert, L. E., Soil Sei. SOC.Am. Proc. 28, in press (1964).
RECEIVED for review September 23, 1963. Accepted November 20, 1963. This investigation was sup orted in part by a Federal Regional &ant under Project NC-55 for which the authors express their gratitude.
VOL. 36,
NO. 3,
MARCH 1964
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