The rate of metal catalyzed oxidation of sulfur dioxide in collagen surrogates

Abstract

Oxidation of sulfur dioxide (SO 2) in gelatine gels has been used as a model for the process in leather and other collagen artifacts. Oxidation rate constants for 5–30% w/w gelatine gels at 25 °C had a mean first order rate constant of 3.25 × 10 –6 s –1 (i.e. t 1 2 = 60 h). The water content of the gelatine has no effect on the observed rate of SO 2 oxidation. Copper can act as a catalyst, but is often rendered ineffective because it is bound to the gel. Significant rate increases only emerge at low gel concentrations and high free copper concentrations. Three copper ions bind to a single gel unit and it is all bound when the protein material (% w/w)/free copper (mol kg –1) value exceeds 100,000. The availability of free copper to promote catalysis is dramatically reduced as the gelatine concentration approaches 30% w/w. Concentrations of likely catalysts together with total sulfur were determined in 11 samples of historical parchment and leather using inductively coupled plasma (ICP) emission analysis: iron 0.058–1.513; copper 0.002–0.065; manganese 0.002–0.091; sulfur 1.962–28.273 (g kg –1 dry leather). Although sulfuric acid (5% w/w) causes degradation of gelatine, the presence of metals did not alter the rate of this process. Gelatine appeared to be a useful good surrogate for leather in this study.