Titrimetric determination of calcium carbonate equivalence of wood ash

Abstract

Wood ash is a residual material produced during the process of biomass burning for energy. Spreading this material on the land is used as a means of its disposal. Wood ash contains carbonates which react to raise soil pH, and its regulation is based on ash calcium carbonate equivalence (CCE). The procedure suggested by the Association of Official Analytical Chemists (AOAC) for the determination of the CCE of agricultural limestone is routinely used for the determination of the CCE of wood ash samples. This procedure involves heating a weighed amount of sample with HCI and then back-titration of the remaining acid with NaOH. Unlike agricultural limestones, which are largely carbonates, some wood ash samples contain significant amounts of contaminants such as alumino-silicate clays which may consume acidity when heated with HCl. Significant consumption of acidity by alumino-silicates during CCE analysis would cause the determination of artificially high CCE values, as these alumino-silicates would not be expected to neutralize acidity under field conditions. This study investigated whether the AOAC method of CCE determination resulted in reproducible, accurate CCE values when applied to wood ash samples with various amounts of alumino-silicate contaminants. The accuracy of the method was calculated by comparing the CCE values with those obtained from a soil incubation method. Time of heating had little effect on the CCE value obtained. The back-titration step was reproducible from operator-to-operator despite the formation of a white precipitate before the end-point. The CCE values obtained from the titrimetric method were similar to those obtained from a soil incubation method.