Hi, all.
I'm looking to quantify the concentration of dimethyl sulfoxide in a particular solution. This method will potentially be performed routinely. I've investigated several methods, including HPLC. I intend to use HPLC, but would like to practice a second method as a confirmation and/or alternative procedure. I found a paper that describes DMSO quantification via reduction using trifluoroacetic anhydride and sodium iodide, followed by titration of the resulting molecular iodine with sodium thiosulfate (W. Cisielski, W. Jedrzejewski, Z.H. Kudzin, R. Skowronski, and J. Drabowicz. A new method for the iodometric determination of sulphoxides. Talanta 34: 969-72 (1988)). This method seems fairly quick and easy, but there's one potential issue: TFAA reacts violently with water, and the DMSO is in an aqueous solution.
I have two concerns: (1) that the reaction will be dangerous, and (2) that water content will hydrolyze TFAA, preventing the reduction of DMSO. An 8-fold excess of TFAA is used in the paper protocol, and it describes adding an aqueous solution to the reaction mixture after reduction, so I'm thinking that (1) isn't
that cocerning if small volumes are involved. For (2), the DMSO solution volume that's titrated would be no larger than 7 mL, which by my calculations contains ~0.40 mol water, while the TFAA amount is ~0.0004 mol. Will water content totally destroy the TFAA, or is there still hope for successful DMSO reduction?
The paper references another article where DMSO reduction is accomplished with iodide in hydrochloric acid. Unfortunately, the reference is obscure and in another language, but I tried working out the mechanism/reaction on my own and came to:
(CH
3)
2SO + 2HCl + 2NaI
(CH
3)
2S + H
2O + I
2 + 2NaCl
In this, my proposed mechanism is protonation on the oxygen of DMSO, followed by iodide addition to the sulfur atom. A second protonation of the oxygen atom creates a "sulfyloxonium" ion where water leaves (similar to acid-catalyzed elimination of alcohols). A second iodide ion attacks the iodine atom bonded to sulfur, the two depart as molecular iodine, and DMSO is reduced to dimethyl sulfide. If this is true, then this reaction seems like it would be more amicable to water. Would this be a better route to go for my particular application? Iodine could be titrated as described in the first protocol.
Thank you!