Comparison of sampling methods for the determination of sulfur species in volcanic gases
Thursday, 18 December 2014
We aimed to evaluate three methods for the determination of sulfur species in volcanic gases using an artificial fumarole. Among three methods, both cadmium sulfide and silver sulfide precipitation methods utilized the NaOH solutions containing Cd2+ and Ag+, respectively, in pre-evacuated bottles. Sulfur gases (H2S + SO2) from the artificial fumarole were dissolved into the NaOH solutions, and then H2S reacted with either Cd2+ or Ag+ to precipitate as CdS and Ag2S. Subsequently, the alkaline solutions were filtered to separate the precipitates from the solution. While the filtrate was analyzed on an ion chromatograph (IC) for SO2 concentrations, the precipitate was oxidized by H2O2 solutions and analyzed on an IC for H2S concentrations. In the third method (the iodine method), KI-KIO3 solutions in falcon tubes were employed in addition to NaOH solutions in pre-evacuated bottles. Once sulfur gases were collected by the NaOH solution, they were also sampled into the KI-KIO3 solution. In the iodine solution, H2S was oxidized and precipitated as elemental sulfur, whereas SO2 was dissolved as sulfate. Once both sampling were done, the total sulfur in the NaOH solution was determined on an IC. The KI-KIO3 solution was centrifuged to separate elemental sulfur from the solution. Following the oxidation of elemental sulfur into sulfate, the resultant solution was analyzed on an IC for H2S concentrations. For all methods, the H2S concentrations determined from the fumarolic gases at low H2S flow rates were lower than expected. Notably, the H2S concentrations determined by silver sulfide precipitation method were lower than those by cadmium sulfide precipitation method. This is because Ag2S(s) is more soluble than CdS(s) under our experimental conditions. Also, the H2S determination by cadmium sulfide precipitation led to the similar results to the iodine method. However, the measured SO2 concentrations from the fumarolic gases were in good agreement with the expected ones.