Unsuitability of Cr(II) reduction for the measurement of sulfides in oxic water samples.

After developing a highly sensitive method for detecting acid-volatile sulfides (AVS) in oxic freshwaters, we hoped to apply that method to measuring a different class of dissolved reduced sulfur compounds, chromium-labile sulfides (CLS). A popular method for measuring this pool of sulfides in sediments relies on reduction dissolution of metal sulfides by Cr(II) and has been employed by researchers for over 15 years. Here, we demonstrate that this method is inappropriate for measuring CLS in oxic freshwaters in which sulfate concentrations are large relative to the dissolved metal sulfides. We observe the reduction of sulfate by Cr(II), and this presents a significant interference.