Kinetics and mechanism of fixation of Cu-Cr-As wood preservatives. Part 4: Conversion reactions during storage
Precipitates simulating those produced in wood by preservative fixation reactions were prepared by the reduction of Boliden K 33 and Celcure AP solutions with hydrogen peroxide and hydrazine. The pH changes on aging at 20 and 50°C were studied and related to the chemistry of fixation previously described. Hydrolysis of copper arsenates may render arsenic acid temporarily water soluble pending precipitation by trivalent chrome liberated by the slow hydrolysis and reduction by wood of chromic chromates. As the reduction of chrome is the primary driving force for the fixation of Cu-Cr-As preservatives, pH changes were observed in sawdust treated with dilute CrO3 solutions under different temperature cycles. The pH is essentially independent of temperature during the first three days when chromic chromates are being formed, but the subsequent pH is highly temperature-sensitive. Part of this effect is due to hydrolysis and reduction and part to generation of acidic reaction products in the wood at higher temperatures.