Sulfide concentration

The free sulfide in sediment pore water comprises the forms H₂S and HS^-. Pore water (3-5 ml) is squeezed from sediment under N₂ pressure and directly transferred into 1 ml ZnCl₂ (2 %). This step is carried out as soon as possible to keep contact with oxygen at a minimum. Samples can be stored frozen.

Samples are diluted (usually 1:100) to reach an absorption value below 0.8. Samples with low sulfide concentration are diluted at least two-fold to avoid too high Zn2+ concentrations (otherwise samples will turn pink).

Diamine reagent: dissolve XX g N,N-dimethyl-p-phenylenediamine sulfate and YY g ferric chloride in 500 ml cool 50 % (v/v) reagent grade hydrochloric acid.

Diamine reagent is added to the sample in a vial according to the diluted volume of the sample:

The sample is immediately stoppered, shaken, and left in the dark for 30 min for the colour reaction to complete. The addition of diamine leads to formation of a blue thiazine-dye.

The absorption is determined spectrophotometrically (l= 670 nm) within the following few hours (after 24 hours the color is no longer stable). Dissolved sulfide is preserved as ZnS and determined by the methylene blue method of Cline (1969).



Reference:

Cline, J.D. (1969) Spectrophotometric determination of hydrogen sulfide in natural waters. Limnology and Oceanography 14, 454-458.



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