Stability of the anion complexes [Fe(CN)6]3- and [Fe(CN)6]4- and any risk of HCN evolution?
In the first half of the 1990s I had a question out on some online chemistry communities where you could ask questions and get feedback from other research scientists. This was before the www internet and its user interface as we know it today, and although I received several responses, I could not conclude from the overall feedback. So maybe today, with the help of the ResearchGate community I may have a chance to receive answers I may conclude from? :-)
Maybe even some of you have direct laboratory experience with this topic?
The background is the development of electrochromic materials for utilization in electrochromic windows, where I electrochemically synthesized various combinations of the electrochromic materials polyaniline (PANI), prussian blue (PB) and tungsten oxide (WO3). In that connection PANI films which have been electrochemically deposited onto indium-tin-oxide (ITO) glass plates in aqueous solutions of aniline in sulphuric acid (e.g. 0.02 M aniline and 0.5 M H2SO4) were used as substrate electrodes for the following PB electrochemical deposition, in an electrolyte consisting of e.g. 0.5 M KHSO4, 0.001 M K3[Fe(CN)6] and 0.001 M Fe2(SO4)3. This combined PANI/PB electrode was later on assembled in a complete window device with WO3 as the counter electrode (also electrochemically synthesized).
For more information about these details see e.g. the following two articles (which can be requested through ResearchGate): (a) B. P. Jelle, G. Hagen and S. Nødland, ”Transmission Spectra of an Electrochromic Window consisting of Polyaniline, Prussian Blue and Tungsten Oxide”, Electrochimica Acta, 38, 1497-1500, 1993, and, (b, e.g. chapter 7.3) B. P. Jelle, ”Solar Radiation Glazing Factors for Window Panes, Glass Structures and Electrochromic Windows in Buildings - Measurement and Calculation”, Solar Energy Materials and Solar Cells, 116, 291-323, 2013.
The question is: How stable is the anion complexes [Fe(CN)6]3- and [Fe(CN)6]4- in an acidic environment and may there be any chance of hydrogen cyanide (HCN) evolution?
This especially for the synhesis of PB coatings and in principle also for the electrochromic switching of the complete windows (e.g. when by error applying an all too high voltage and maybe causing hydrogen evolution at one of the electrodes). Also bearing in mind that synthesis parameters (e.g. various chemicals, concentrations, voltage/current applications, etc.) may vary a lot. Is it possible to quantify any safe limits with respect to e.g. acidic concentrations and/or electrode voltage levels? With the poisonous HCN it is of course best to be on the safe side.
Note that for example, Wikipedia states that "Potassium ferricyanide has low toxicity, its main hazard being that it is a mild irritant to the eyes and skin. However, under very strongly acidic conditions, highly toxic hydrogen cyanide gas is evolved, according to the equation: 6 H+ + [Fe(CN)6]3- = 6 HCN + Fe3+", from the following weblink: https://en.wikipedia.org/wiki/Potassium_ferricyanide.
And see also:
https://en.wikipedia.org/wiki/Ferricyanide
https://en.wikipedia.org/wiki/Ferrocyanide
https://en.wikipedia.org/wiki/Potassium_ferricyanide
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