Simple and rapid determination of thiol- and organic disulfide-sulfur in foliage and humic soil horizons

A new method for rapid determination of thiol- (R-SH) and disulfide (R-S-S-R′) sulfur in soil and foliage samples is presented. Using a silver sulfide electrode, the thiol sulfur content of a sample is determined by potentiometric titration with AgNO₃. After reduction with a mixture of NaOH and ascorbic acid also its disulfide S content can be quantified subsequent to neutralization of the reductive solution with citric acid. The method was tested with eight organic standards, six humic soil samples, and three Norway spruce needle samples. Disulfide S from standards with abstrictable H-atoms in α- or β-position to the disulfide bond could - with one exception - be detected completely. For the aromatic disulfides which allow only direct nucleophilic attack, recovery was at 75%. For the soil samples, 32 to 60% of the carbon-bonded S consisted of disulfide S, for the spruce needle samples 26 to 33%. The method provides satisfactory results for most studied standards and is applicable to various natural substances after adequate sample preparation.