Microcoulometric determination of total inorganic sulphur in water by a hydroiodic acid reduction method

A simple and reliable method reducing sulfate to sulfide for multiple sulfur isotope analysis

“…This method uses a reducing solution made of 100 mL 57% HI and 13 g NaH 2 PO 2 , and a very small amount (1 mL) of reducing solution was demonstrated to be able to completely reduce a soluble sulfate salt (0.5–2.5 μmol) to sulfide within 2 h, thus minimizing the use of relatively expensive HI. In practice, nothing prohibits the recycling of the used reducing solution by adding a few mg of NaH 2 PO 2 in order to reduce I 2 back to I − in a boiling flask if the used solution turns brown . In addition, the reduction train avoids the use of a distillation apparatus, and multiple reduction trains can be operated at a time, making it easier to process multiple samples simultaneously.…”

“…After the reducing solution had been purged for 20 min, the reduction train was assembled (Figure ) and the reaction tube was placed in the block heater and heated at 124°C. At lower temperatures the reduction speed will be slow, while if the temperature is too high, an excessive amount of phosphine (PH 3 ) will be produced from the decomposition of NaH 2 PO 2 . For the alternative setup, the drying agent was in‐line with the cryogenic system, and the latter was set at −200°C to trap the reaction products.…”

“…In the reducing solution of Thode et al, high concentrations of HI seem to be the most important component of the reducing agent for complete sulfate reduction, and the presence of H 3 PO 2 or NaH 2 PO 2 increases the reduction speed by maintaining a high hydroiodic acid to iodine ratio which is one of the factors favoring the reduction . HCl is only of secondary importance and its presence is suggested to increase the acidity and volume, and reduce the use of relatively expensive HI .…”

“…The isotopic analysis is conventionally performed by reducing sulfate (SO 4 2− ) to hydrogen sulfide (H 2 S), converting H 2 S into silver sulfide (Ag 2 S), and fluorinating Ag 2 S to sulfur hexafluoride (SF 6 ) for isotopic composition analysis by isotope ratio mass spectrometry (IRMS) . The reduction from SO 4 2− to H 2 S is mainly achieved by two different reducing agents: tin(II) (Sn 2+ ) solutions and hydroiodic acid (HI)/hypophosphorous acid (H 3 PO 2 ) mixtures . The Sn 2+ solution is mainly applied to solid samples (e.g., minerals) with an optimum reaction temperature between 280 and 300°C, and the HI reducing solution can be applied to aqueous samples at 100–125°C .…”

“…2,6,11,12 The reduction from SO 4 2− to H 2 S is mainly achieved by two different reducing agents: tin(II) (Sn 2+ ) solutions and hydroiodic acid (HI)/hypophosphorous acid (H 3 PO 2 ) mixtures. [13][14][15] The Sn 2+ solution is mainly applied to solid samples (e.g., minerals) with an optimum reaction temperature between 280 and 300°C, and the HI reducing solution can be applied to aqueous samples at 100-125°C. 14 Currently, the most widely used reducing method in sulfur isotope geochemistry follows the reducing agent recipe (500 mL concentrated HI, 816 mL concentrated HCl, and 245 mL 50% H 3 PO 2 ) of Thode et al, 16 and uses a distillation apparatus similar to that described in Forrest and Newman.…”