The determination of lignin in the analysis of woods

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Abstract
Current methods for determining lignin in wood by means of 72% H2SO4 are briefly reviewed and the present lack of uniformity in these methods is stressed. Preliminary hydrolysis with dilute H2SO4 has been shown to be ineffective as a general means of preventing the formation of carbohydrate condensation products during the isolation of lignin, unless precautions are taken to remove the products of hydrolysis soon after they are formed. On the basis of a study of the conditions governing the digestion of the wood with 72% H2SO4 the following procedure is suggested for determining the amount of lignin. After a preliminary extraction with alcohol-benzene (1:2), a 2 gm. sample of air-dry wood of approx. 10% moisture content is digested with 25 ml. of 72% H2SO4 at 10[degree] [plus or minus] 0.5[degree] for 5 hrs. (hardwoods) or 6 hrs. (softwoods). The acid is diluted with water to a conc. of 3% and the mixture boiled under reflux for 2 hrs. The lignin residue is then collected in an alundum crucible of porosity RA 360, washed until free from acid and dried at 105[degree]. When xylose, fructose and sucrose are treated with H2SO4 under the above conditions only negligible amts. of insoluble residues are obtained.

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