Polarography antioxidants

It is of interest to examine the development of the analytical toolbox for rubber deformulation over the last two decades and the role of emerging technologies (Table 2.9). Bayer technology (1981) for the qualitative and quantitative analysis of rubbers and elastomers consisted of a multitechnique approach comprising extraction (Soxhlet, DIN 53 553), wet chemistry (colour reactions, photometry), electrochemistry (polarography, conductometry), various forms of chromatography (PC, GC, off-line PyGC, TLC), spectroscopy (UV, IR, off-line PylR), and microscopy (OM, SEM, TEM, fluorescence) [10]. Reported applications concerned the identification of plasticisers, fatty acids, stabilisers, antioxidants, vulcanisation accelerators, free/total/bound sulfur, minerals and CB. Monsanto (1983) used direct-probe MS for in situ quantitative analysis of additives and rubber and made use of 31P NMR [69]. 

Applications As the basic process of electron transfer at an electrode is a fundamental electrochemical principle, polarography can widely be applied. Polarography can be used to determine electroreductible substances such as monomers, organic peroxides, accelerators and antioxidants in solvent extracts of polymers. Residual amounts of monomers remain in manufactured batches of (co)polymers. For food-packaging applications, it is necessary to ensure that the content of such monomers is below regulated level. Polarography has been used for a variety of monomers (styrene, a-methylstyrene, acrylic acid, acrylamide, acrylonitrile, methylmethacrylate) in... 

In short, polarography can be used for the analysis of C—N, C—O, N—O, O—O, S—S, and C—S groups and for the analysis of heterocyclic compounds. Also, many important biochemical species are electroactive, such as vitamin C (ascorbic acid), fumaric acid, vitamin B factors (riboflavin, thiamine, niacin), antioxidants such as tocopherols (vitamin E), A-nitrosamines, ketose sugars (fructose and sorbose), and the steroid aldosterone. 

In addition, at least two commercially available antioxidants have been shown by differential cathode-ray polarography to exhibit reduction waves Santonox R gives a poorly shaped wave at -0.6 V in an electrolyte consisting of ammonia and ammonium chloride in methanol-water [98] and 3,5-di-t-butyl-hydroxytoluene gives a wave at -0.65 V in aqueous sodium or lithium hydroxide [99]. In both cases, 40 ppm of analyte gave a current which was adequate for quantitative analysis. 

The procedure of Kuta and Quackenbush [121] can be modified for the determination of relatively simple organic peroxides such as benzoyl peroxide, para-tert-huty perbenzoate and lauroyl peroxide in PS, and would no doubt be suitable for the determination of other types of peroxides. In these procedures a suitable weight of polymer is dissolved in toluene and then an equal volume of 0.6 M lithium chloride in methanol is added. Precipitated polymer is removed by centrifuging and peroxides in the filtrate are determined by cathode-ray polarography. Polymerisation additives, styrene monomer or antioxidants in the polymer do not interfere in the polarographic procedure. A procedure for the determination of down to 20 ppm para-tert-hu.ty perbenzoate in PS is given next. 

Uhde and co-workers [34, 35] have studied the migration of 4,4 -thiobis-6-tert-butyl-w-cresol (Santonex R) from plastics utensils into sunflower seed oil. Sunflower seed oil that had been stored in vessels of polyethylene containing this antioxidant was diluted (3 5) with pentane and extracted with acetonitrile containing 5 % of water. The concentrated acetonitrile extract (or an ethanol solution of the residue on evaporation) was snbjected to thin-layer chromatography on Kieselgel with hexane-ethyl acetate (10 3) as solvent. To detect the antioxidant (down to 0.1 ppm) the plate was sprayed with 3,5-dichloro-p-benzoquinonechlorimine solution. To determine the antioxidant, the zone at Rf = 0.44 (located by means of iodine vapour) was removed and treated with fuming nitric acid sulfuric acid (1 1). The nitro-derivative of the antioxidant was determined in the product by polarography after the addition of urea and sodium acetate [35],... 

McBride and Evans [10] developed a rapid voltametric method for the estimation of antioxidants and tocopherols in oils and fats. The sample solutions were prepared by dissolving the oil or lard sample in an appropriate solvent, e.g., in most cases 0.12 M sulfuric acid in ethanol - benzene (2 1). The solutions were analysed with use of a linearly varying potential and a stationary, planar vitreous-carbon electrode, with a standard calomel electrode and a platinum-wire counter-electrode. Separate peaks were obtained for a-, y- and 8-tocopherol the peak for the P-isomer, was superimposed on that for the y-tocopherol. BHA (>10 ppm) can be determined in vegetable oil under the same conditions, provided that 8-tocopherol is absent. Kohler and co-workers [11] described a polarographic method for the determination of 4-4 thiobis(BHT) in food. The antioxidant was first nitrated, preferably with fuming nitric acid - concentrated sulfuric acid (1 1) at 20 °C for 1 hour. The polarography was carried out on the resulting solution after dilution and addition of urea and sodium acetate buffer solution. The Ey for the nitrated compound was -0.54 V versus the... 

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