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At pH 8-12 and 25°C, 10-20% of lignin undergoes cleavage producing soluble species. In suspensions of lignin it is possible to follow the concentration changes of hydroxybenzaldehydes and α,β-unsaturated carbonyl compounds. These species can be determined by the recording of polarographic current-voltage curves directly in the suspensions, where the presence of suspended and colloidal particles does not interfere with the function of the DME. The base-catalyzed cleavage follows first-order kinetics. The pH dependence indicates that the rate determining step is not a nucleophilic attack by OH -, rather the rate determining step is preceded by a rapidly established acid-base equilibrium. The cleavage of natural lignins obtained enzymatically follows a pattern similar to that of rot wood lignins and surprisingly also to that of kraft lignins. The patterns of cleavages are different from those of lignosulfonates, Klason, periodate and expansion lignin. The three dimensional (tertiary) structure of lignin seems to play a role in its reactivity.During the last two decades, the number of applications of polarography in practical analysis decreased considerably. Nevertheless, there are some areas where polarography still fulfils the conditions to be the best method for the solution of a given analytical problem. One of such areas is the analysis of heterogeneous systems. This is facilitated by the low sensitivity of limiting currents obtained by DC or normal pulse polarography to the presence of colloidal or dispersed, slightly soluble particles. It is thus possible to carry out, without separation other than that of the largest particles, direct analyses in real time of electroactive species in the presence of colloids and solid particles. An example of such applications is the determination of electroactive species in suspensions of lignin.Lignin promises to be one of the materials of the future. The chemical industry was first based on coal and now relies on oil as the single most important raw material. Eventual exhaustion of oil and later of coal will have

Differential pulse polarography (DPP) is an excellent analytical method, but cannot be recommended for applications in the initial stages of mechanistic studies. Its application is also questionable in the analysis of samples containing varying concentrations of an organic cosolvent or of surface active compounds. The effect on limiting currents obtained by DC polarography and on peak currents obtained by DPP was demonstrated for reductions of various nitrobenzene derivatives. The similarity of changes in current for various nitrobenzenes enables the exclusion of the formation of adducts 01 micelles as the predominant effect. The decrease of the limiting currents in all cases is much smaller than the decrease of DPP peak currents, due to strong effects of the surfactants on the kinetics of electroreduction. For the analysis of natural products or of solutions contaiining varying amounts of organic cosolvent, the measurement of the DC polarographic limiting current is preferred. An addition of 30%# ethanol minimizes the effects of the varying concentration of surfactant on both DC polarographic limiting and DPP peak currents.

The polarographic method proved suitable for investigation of adsorption of acifluorfen (I) (5-[2-chloro-4-(trifluoromethyl)phenoxy]-2'-nitrobenzoic acid) on lignin. Polarographic reduction of acifluorfen in aqueous buffers (pH < 7) is complicated by adsorption of the reduction product. In acidic media the reduction of the protonated hydroxylamino group occurs at such positive potentials that only a single six-electron wave is observed. In buffers containing 33% (w/w) ethanol the effects of the adsorption are eliminated and the reduction of the hydroxylamino group occurs at more negative potentials, so that one four-electron and one two-electron wave are observed. Splitting of the four-electron wave at pH > 7 is attributed to a decrease in protonation rate of the reduction intermediate. KEY WORDS Pesticides, pokdrography, ligninIn forestry, the development of seedlings in nurseries is protected by application of various herbicides and pesticides. Such applications are regulated and, in principle, a minimal amount of pesticide should be applied. To estimate the minimal amount necessary for a successful application understanding the fate of the chemical in the environment is essential. The lifetime of a pesticide can be affected by various factors, such as its solubility in aqueous systems and its sensitivity to pH and illumination, as well as its reactivity with compounds frequently encountered in the environment. One factor that has not been considered in the past is the binding of pesticides by and their interactions with lignin, which is naturally present in abundance in forest litter and forest soil. We have recefitly pointed out [ 1-61 that lignin adsorbs both inorganic and organic materials.In this study we investigated the potential of polarography as an analytical method in a study of adsorption of the group of pesticides used in nurseries that contain a nitro group on an aromatic ring in a diphenyl ether. As the behavior of the studied nitro compounds was not deIn memory of Professor Gaston Pdtriarche. 'To whom correspondence should be addressed.scribed in the available literature, it was necessary first to obtain basic information about the nature of the limiting currents of these compounds, second to find optimum conditions for polarographic analysis, and third to study the adcorption. EXPEh!lMl3TM InstrumentationThe dc current-voltage curves were recorded using a Sargent Polarograph Mark XVL using a dropping electrode with natural drop-time and a Sargent-Welch Polarograph Mark 4001 with a controlled drop-time of 1 s. The dropping electrode had m = 1.87 mg/s and t, = 3.85 s at h = 100 cm in 0.1 M KC1 at 0.0 V (SCE).Preliminary studies were carried out in a Kalousekcell using a liquid junction separated saturated calomel electrode as a reference. Adsorption studies were performed in an H-type cell with a thermosrating jacket and a bottom stopcock for replacing the studied solution. pH measurements were carried out with a PHM 84 Research pH meter (Radiometer) with a 202B glass electrode. Chemicals and So...

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