Comparison of peroxide-oxidisable sulfur and chromium- reducible sulfur methods for determination of reduced inorganic sulfur in soil

The rational management of acid sulfate soils requires analytical methods that provide reliable and accurate data on the content of reduced inorganic sulfur; it is this fraction that produces acid during oxidation. This study compared the utility of the chromium-reducible sulfur method for determining the reduced inorganic sulfur content of soil materials with methods based on oxidation using hydrogen peroxide. The results presented here indicate that methods involving oxidation by hydrogen peroxide for the determination of reduced inorganic sulfur are subject to significant interference by even minor amounts of sulfate minerals and organic matter, resulting in inaccurate determinations of reduced inorganic sulfur contents. In the presence of even minor amounts of gypsum, methods involving oxidation using hydrogen peroxide underestimated reduced inorganic sulfur contents by up to 0·167% sulfur, whereas in the presence of organic matter these methods overestimated reduced inorganic sulfur contents by up to 0·077% sulfur per cent organic carbon. The resulting errors in the determinations of reduced inorganic sulfur by hydrogen peroxide methods were often larger than the action criteria that are currently used to identify acid sulfate soils. Consequently, there is a risk of misidentification of acid sulfate soils (either false positive or false negative) for soils with low reduced inorganic sulfur contents when hydrogen peroxide methods are used. In contrast, the results from the chromium-reducible sulfur method do not appear to be affected by interferences from either gypsum or organic matter and this method appears to be more suitable for the determination of reduced inorganic sulfur in soils than methods based on oxidation using hydrogen peroxide.