Assessment of peroxide oxidation for acid sulfate soil analysis. 2. Acidity determination

Total sulfidic acidity (TSA) and total potential acidity (TPA) are derived from peroxide oxidation of acid sulfate soil materials (ASS), and are measures of the sulfidic acidity and the net acidity (net acidity = sulfidic acidity + actual acidity – acid neutralising capacity), respectively. The TSA and TPA of 4 ASS materials were determined using a variety of peroxide oxidation procedures and compared with the sulfidic acidity and net acidity derived from the use of an acid–base accounting model. TSA and TPA values both varied greatly with each peroxide oxidation method used, and both measures were found to substantially underestimate (i.e. by 23–85%) both sulfidic acidity (as determined from the chromium reducible sulfur content) and net acidity (as determined by acid–base accounting). A major cause of this underestimation of acidity was the retention of acidity through the precipitation of jarosite during peroxide oxidation. Substantial clay mineral dissolution appears to have occurred during peroxide oxidation of the ASS materials, as indicated by increased soluble aluminium. Such dissolution may contribute to the underestimation of both sulfidic and net acidity for the ASS materials using peroxide oxidation methods. The loss of acidity to the atmosphere was identified as a possible additional interference. This study shows the peroxide oxidation methods examined here are subject to substantial interferences, which caused large underestimations of acidity, and consequently, are unable to reliably provide accurate measurements of sulfidic and net acidity in ASS materials.