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The Bunsen reaction is a chemical reaction that describes water, sulfur dioxide, and iodine reacting to form sulfuric acid and hydrogen iodide:

2H2O + SO2 + I2 → H2SO4 + 2HI

This reaction is the first step in the sulfur-iodine cycle to produce hydrogen. The products separate into two aqueous layers, with the sulfuric acid floating on top, and a mixture of hydrogen iodide and unreacted iodine on the bottom.[1] While the two layers are generally considered immiscible, small amounts of sulfuric acid may still remain in the hydrogen iodide layer and vice versa. This can lead to unwanted side reactions, one of which precipitates out sulfur, a potential obstruction to the reaction vessel.[2] The reaction is named after Robert Bunsen, who discovered it in 1853.[3]

A similar reaction is the basis for Karl Fischer titration.

Note that at sufficiently high temperatures, concentrated H2SO4 may react with HI, giving I2, SO2 and H2O, which reverses the reaction.[2] Many chemical processes are reversible reactions, such as ammonia production from N2 and H2, and removing the desired product will shift equilibrium to the right of the equation favoring reaction products as per the Le Chatelier principle.


  1. ^ Yanwei Zhang; Pingan Peng; Zhi Ying; Qiaoqiao Zhu; Junhu Zhou; Zhihua Wang; Jianzhong Liu; Kefa Cen (3 February 2014). "Experimental Investigation on Multiphase Bunsen Reaction in the Thermochemical Sulfur–Iodine Cycle". Industrial & Chemistry Engineering Research. 53 (8): 3021–3028. doi:10.1021/ie4038856.
  2. ^ a b Guo, H.F.; P. Zhang; Y. Bai; L.J. Wang; S.Z. Chen; J.M. Xu (3 June 2009). "Continuous purification of H2SO4 and HI phases by packed column in IS process". International Journal of Hydrogen Energy. 35 (7): 2836–2839. doi:10.1016/j.ijhydene.2009.05.009.
  3. ^ Sella, Andrea (3 February 2012). "Karl Fischer's titrator". Chemistry World. Royal Society of Chemistry. Retrieved 2 January 2015.