Determination of Lignin in Solution

Another spectroscopic method for the determination of lignin in solution is that utilizing fluorescence spectrophotometry. This technique is reportedly capable of detecting lignin in concentrations two to three orders of magnitude 

Of the methods for determining lignin in solution based on a specific chemical reaction, that involving nitrosation, the so-called Pearl-Benson method, has found the widest application. In this procedure, reaction of the phenolic units in lignin with acidified sodium nitrite leads to the formation of a nitrosophenol which, upon addition of alkali, is tautomerized to an intensely colored quinone mono-oxime. The absorbance of the latter structure is measured at 430 nm and related to lignin concentration by calibration with a standard lignin. The procedure described below is essentially that developed by Barnes et al. (1963), who modified the original Pearl-Benson method (Pearl and Benson 1940) to improve its sensitivity. 

---

In organizing the subject matter for this chapter, it was found expedient to divide the presentation into two main subsections, one dealing with the analytical methods applicable to lignin when initially present as a component of a solid mixture and the other focusing on the determination of lignin in solution. 

The understanding of the macromolecular properties of lignins requires information on number- and weight-average molecular weights (Mn, Mw) and their distributions (MWD). These physico-chemical parameters are very useful in the study of the hydrodynamic behavior of macromolecules in solution, as well as of their conformation and size (1). They also help in the determination of some important structural properties such as functionality, average number of multifunctional monomer units per molecule (2, 3), branching coefficients and crosslink density (4,5). 

It is unclear how accurate this method is for the determination of lignin content in grasses, since a fraction of the lignin from grasses appears to be readily soluble in acidic solutions, such as used in this procedure. This would result in underestimation of the actual lignin content (Hatfield and Fukushima, 2005). 

A number of spectral methods for the determination of lignin consist essentially of totally dissolving the sample in a suitable solvent and measuring the UV-absorbance of the solution at a wavelength characteristic for lignin, usually 280 nm. Among the solvents used to dissolve the lignocellulosic material... 

Jurkiewicz S (1977) Problem of the effect of interfering factors in the quantitative determination of lignin compounds in aqueous solutions Przegl Papier 33(1) 30-35, Abstr Bull Inst Pap Chem 48 1245... 

Methods that attempt to directly measure the sulfonate content of lignin in pulps must overcome the challenge of distinguishing sulfonate from accompanying carboxylate groups. One such method (Sjostrom and Enstrom 1966, Sjostrom et al. 1965) is based on the concept that, under acidic conditions, benzidinium ions form salts only with the sulfonate groups. The pulp is saturated with an aqueous solution of benzidine hydrochloride and washed with water to remove excess reagent the benzidinium ions are eluted with hydrochloric acid and determined by UV spectrophotometry. 

Bogomolov BD, Pivovarova VA, Gorbunova OF (1986) Determination of lignin and ether-soluble substances in aqueous solutions by fluorimetry. Khim Drev (Riga) 2 54-57 Bolker HI, Somerville NG (1962) Ultraviolet spectroscopic studies of lignins in the solid state. 

Wet Chemistry and Chromatography. For the analysis of wood, the Klason lignin test, performed in concentrated sulfuric acid, is the accepted method for the determination of lignin content. We performed similar tests using also trifluoroacetic acid (TFA), the results of which are almost identical to those of the Klason tests. TFA has the advantage of allowing further analysis of the saccharides in the acid soluble fraction, as it can easily be evaporated from the solution. 

In solution, lignin is most conveniendy analyzed quaUtatively and quantitatively by uv spectroscopy. Typical absorptivity values, D, at 280 nm for milled wood (MW) lignins and other types of lignins are Hsted ia Table 4. These values are used for quantitative determination of the lignins ia suitable solvents. 

Certain derivatives of the lignin sulfonic acids can be determined directly in water. The nitroso derivatives, which are easily formed in solution, can be determined by differential pulse polarography [438]. Vanillin can be formed by alkaline hydrolysis [439] or alkaline nitrobenzene oxidation [440], extracted into an organic solvent and determined by gas chromatography. 

---