Reduction of Organic Sulfur in Soils with Tin and Phosphoric Acid
The chemical nature of organic S in soils is not well understood. In this study, a Sn-H3PO4 reagent was used for reduction of organic S in soils to H2S for subsequent determination as methylene blue. The reduction of soil organic S was compared with that of model S compounds, and with the fractions reduced with HI and with Raney Ni. Results showed that, in general, most of the reduction of organic S in 13 Iowa and 7 Chilean surface soils occurred after distillation for 1 to 3 h, but some soils continued to release H2S for up to 10 h of distillation with this reagent. Expressed as a percentage of total organic S in Iowa surface soils, the amounts of Sn-H3PO4-reducible S ranged from 15 to 64% (avg = 35%) and from 42 to 91% (avg = 60%) for 1 and 10 h of distillation, respectively. The corresponding percentages of the Chilean surface soils were from 8 to 73% (avg = 25%) and from 29 to 100% (avg = 55%). The Sn-H3PO4-reducible S values were significantly correlated with organic S [r = 0.71*** (P < 0.001) and 0.88*** for 1 and 10 h of distillation, respectively]. The amounts of organic S potentially reducible (Sr) with this reagent ranged from 35 to 86% (avg = 61%) of organic S in Iowa surface soils. The corresponding values for the Chilean soils ranged from 33 to 100% (avg = 66%). The times required to distill 50% of Sr ranged from 0.3 to 1.7 h (avg = 0.8 h) for the Iowa soils, and from 0.4 to 9.2 h (avg = 3 h) for the Chilean soils. Studies with five Iowa soil profiles showed that the percentages of organic S reduced with Sn and H3PO4 in 1 and 10 h of distillation varied with depth of sampling and that these values were similar, lower, or greater than those obtained with the HI-reducing mixture.
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