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Reaction of Chlorine Dioxide and a lignosulfonic Acid
WHEY
technical cellulose pulps are bleached, oxidation of residual lignin derivatives by the bleaching agent usually occurs, and in the sulfite pulping process, all residual lignin is probably in the form of lignosulfonic acids. T o study the chemistry of these oxidations, chlorine dioxide, a newer bleaching agent. \$as used with a lignosulfonic acid irolated from spruce wood. T o avoid complications caused by presence of carbohydrates and chemical condensation of lignin by the temperature and acidity used in the sulfite pulping process ( 3 ) ,a special method for isolating lignin from the wood was usedwood meal was alternately oxidized with aqueous sodium paraperiodate a t 20’ C. and p H 4, and the oxidized carbohydrates extiacted with 0.1-Yaqueous sodium hydroxide (5). Four or five such treatments left the lignin as a friable. brown powder free of carbohydrates but someivhat oxidized. T h e most convenient method found for sulfonating this extracted lignin was stirring for 20 hours ~ i t ha n excess of 167, aqueous sodium bisulfite kept near 100’ C on a steam bath. A reflux condenser instead of a n autoclave was used. but nevertheless. the mixture retained its original pH of 5.2. T h e resulting insoluble sodium lignosulfonate, after extraction with cold, dilute hydrochloric acid, yielded a n insoluble lignosulfonic acid (7, 4 ) called stage I acid. Using the same equipment, a 2% suspension of this stage I acid in water was then converted to the soluble stage I1 acid by heating a t about 100”C. This mixture remained a t pH 2.5, and autohydrolysis for 15 hours was sufficient. T h e progress of both sulfonation and autohydrolysis was followedweight of undissolved portions a t any time was determined and also ultraviolet
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absorption of the soluble portion a t 280 mp. These two determinations checked. T h e resulting stage 11 lignosulfonic acid, isolated in 70 to 80Yc yield and stable only in aqueous solution, contained 2.3 to 2,7% of sulfur and was free of inorganic salts. Only 20% passed through a cellophane membrane when a solution was dialyzed for 24 hours against running tap water. T h e solution of lignosulfonic acid was kept nearly saturated with gaseous chlorine dioxide a t room temperature for 15 days, but even after this time, the dioxide was still being slowly reduced. A small amount (1%) of the product precipitated during the oxidation as a white solid containing 0.7y0or less of sulfur. After filtration, any by-product chloric acid in the liquors was reduced to hydrochloric acid by adding hydriodic acid a t room temperature; the liberated iodine was extracted with ether, and the hydrohalic acids Lvere removed as silver salts. Silver was precipitated from the filtrate bv adding the exact amount of hydrochloric acid, and the final liquor contained only sulfuric and oxylignosulfonic acids. About 90Yc of the original sulfonic acid groups were recovered in this way. T h e use of ion exchange resins in rhe separation was avoided. When aqueous barium hydroxide was added to the final liquor until pH 4 was attained, the sulfuric acid, quantitatively and selectively precipitated as barium sulfate, accounted for about half of the original sulfonic acid groups. T h e oxylignosulfonic acids remaining in the mother liquors were fractionally precipitated from aqueous ethyl alcohol or aqueous acetone as barium, calcium, and zinc salts, but no individual substance could be isolated.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Although the fractions varied in sulfur content from 0.5 to 4%, their ultraviolet absorption spectra at 280 mp uniformly failed to display the maximum characteristic of unoxidized lignins. All the fractions contained 5 to 7y, of chlorine, all but 0.5 EO 1.5% of which was slowly removed by cold dilute alkali. This treatment did not appreciably affect the sulfur content; therefore, chlorine dioxide had removed all the more labile sulfonic acid groups from the lignosulfonic acid, and had introduced chlorine, most of which had the same lability toward alkali as a n aliphatic halide. T h e high metallic content of the neutral oxylignos ulfonates shot+ed that they contained carboxylic as well as sulfonic acid groups. Thus, the sulfonic acid groups in the lignosulfonic acid appeared to be of t h o types. About half were removed as sulfuric acid, and not by oxidation as the expected, low molecular weight aliphatic sulfonic acids. T h e other half waq retained in a complex mixture of oxylignosulfonic acids, about 2070 of which could not be dialyzed through cellophane and which therefore retained a conriderable molecular weight. This observation showed that oxidation with chlorine dioxide failed to degrade all the lignosulfonic acid to substances that diffused readily. Therefore, diffusion phenomena must play a notable part i n technical bleaching with chlorine dioxide. T h e absence of a n ultraviolet absorption maximum a t 280 mp showed that determinations of residual lignosulfonic acids based on observation of this maximum ( Z ) , would be in error if the pulps had been bleached with chlorine dioxide by D. M. Smith. Acknowledgment
T h e authors wish to thank the Xational Research Council of Canada for the studentship and fellowship held during this research by D. M. Smith. literature Cited (1) Adler. E., Lindgren. B. 0 , Suensi, Pupperstzdn. 55,563 (1952). ( 2 ) Bethge, P. O., Gran, C., Ohlsson, K. E., Zbzd., 55, 44 (1952). ( 3 ) Cabott. I. M.. Purves. C. B.. Pzdb & Pupei M a g . Can. 57, No. 4, 151 (March 1956). Hagglund, E., “Chemistry of Wood,” Academic Preps, pp. 215 et seq. S e w York, 1951. Sacks, W., Ph.D. thesis, McGill University. Montreal, Can. (October 1954). Division of Cellulose Chemistry, Symposium on Lignin. 130th Meeting, ACS, Atlantic City. N. J., September 1956. 2
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D. M. SMITH1 and C. B. PURVES MsGill University and Pulp and Paper Research Institute of Canada, Montreal, Canada Present address, Stanford Research Institute, Menlo Park, Calif.
