Sulfonate group, lignin

Finally, the sulfonate content of lignin is deterrnined by two main methods one typified by conductometric titration in which sulfonate groups are measured direcdy, and the other which measures the sulfur content and assumes that all of the sulfur is present as sulfonate groups. The method of choice for determining the sulfonate content of lignin samples that contain inorganic or nonsulfonate sulfur, however, is conductometric titration (45). 

The titration method is based on the acidity of phenolic hydroxyl groups and, under proper conditions, can be used to determine this group together with carboxyl or sulfonate groups that may be present coincidentally in lignin. An improved nonaqueous titration method in DMF developed by Pobiner (1983),... 

In the absence of any strongly acidic (for example, sulfonic) groups in the lignin sample, as indicated by the lack of an inflection near +130 mV, the equation for calculating the carboxyl content is further contracted to... 

Several methods of estimating the sulfonate content of lignin in pulp simply measure total sulfur content and assume that all the sulfur is present in sulfonate groupings. 

Methods of determining the sulfonate content of lignin fall into two main classes those typified by conductometric titration, in which sulfonate content is measured directly and those, as represented by combustion/ion chromatography, which measure the sulfur content and assume that all the sulfur is present as sulfonate groups is approximate in many ultra-high-yield sulfite pulps (Katz et al. 1984) but may lead to large errors (Samuelson and Westlin 1947, 1948, Regestad and Samuelson 1958, Schoon 1961, Janson and Sjostrom 1966, Katz et al. 1984). 

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. 

Beatson RP, Heitner C, Rivest M, Atack D (1985) Sulphite treatment of aspen Factors affecting the formation of carboxylate and sulphonate groups Pap Puu 11 702-708 Brauns FE (1952) The sulfonation of lignin In Brauns FE (ed) The chemistry of lignin Academic Press, New York, 359-393... 

Brauns FE, Brauns DA (1960) The sulfonation of lignin In Brauns FE, Brauns DA (eds) The chemistry of lignin Supplement volume Academic Press, New York, 313-386 Canadian Pulp and Paper Association (1970) Standard Method G28 In Standard Testing Methods, Technical Section, Canadian Pulp and Paper Association, Montreal, Canada Canadian Pulp and Paper Association Useful Method G 7U In Standard Testing Methods, Technical Section, Canadian Pulp and Paper Association, Montreal, Canada Cappelen H, Schoon N-H (1966) Determination of sulfonic acid groups in unbleached sulfite pulps Sven Papperstidn 69 322-325... 

Why then are these lignin sulfonates not used as a partial replacement for phenol in phenol-formaldehyde-based wood adhesives The first reason is that the presence of the sulfonate groups confers a water sensitivity to the adhesive. This sensitivity is exacerbated by the presence of water-soluble carbohydrates. A second reason is the low reactivity of the lignin sulfonates with formaldehyde and the consequent low level of crosslink density achieved in the final adhesive. A third reason is the molecular size of some of the lignin sulfonates. Large molecular weight material cannot penetrate the cell walls of the wood to form an adhesive continuum between contiguous wood particles. 

A problem that still has to be solved is the deficiency of water resistance. The sulfonate groups are so polar that the polymerizate is still water-soluble. As was mentioned before, water-soluble fractions of organosolv lignins became insoluble in water after enzymic polymerization (11) Therefore, we hoped that an addition of phenol-rich organosolv lignin would improve the water resistance However, as can be seen in the last line of Table I, this is not the case. 

Knowing that sulfonation of aromatic compounds is a reversible process, can you think of a way to remove the sulfonate groups from Kraft lignin The product would be an analogue of organosolv lignin. 

Lignin sulfonates also find application. Here the lignin acts as the hydrophobic residue and the sulfonate group as hydrophilic surface active group. 

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