Comparison of sampling methods for the determination of sulfur species in volcanic gases

Abstract:

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.