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I'm not a chemist and I'm looking for a simple bistable and reversible electrochemical reaction that changes color, specifically:

  1. A liquid in a glass with two electrodes that when a voltage is applied, the liquid changes color.
  2. When the power is removed, the new color remains (at least for a few minutes/hours).
  3. When an opposite voltage is applied, the liquid changes to the original color and stays like that when power is removed.
  4. It's possible to repeat this process many times without having to replace the electrodes.

It would be ideal if the chemical components were easy to get and non-toxic. The colors don't matter at all (transparent to red, blue to green, orange to white, anything really).

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    $\begingroup$ You open to liquid crystals and LCDs? $\endgroup$
    – Todd Minehardt
    Commented Feb 18, 2022 at 15:15
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    $\begingroup$ Hi @ToddMinehardt! Correct me if I'm wrong, but with liquid crystals and LCDs you need continuous power to keep them in a specific state, unless some kind of bistable liquid crystal exists? $\endgroup$
    – Marcos
    Commented Feb 18, 2022 at 17:23

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You can carry out an electrolysis of a neutral solution containing sodium sulfate $\ce{Na2SO4}$ plus some ammonium acetate $\ce{CH3COONH4}$ as buffer, and some drops of bromthymol blue. The solution is initially green. A buffer is needed, as the "neutral" $\ce{Na2SO4}$ solution is always a bit acidic, probably due to the atmospheric $\ce{CO2}$. After some time, hydrogen and oxygen are produced on the electrodes. But the cathodic zone (around the minus pole) turns basic and the solution gets blue, because of the appearance of $\ce{OH-}$ ions in solution : $$\ce{2 H2O + 2 e- -> 2 OH- + H2}$$ Simultaneously the anodic region (around the positive pole) turns acidic as $\ce{H+}$ ions are produced and the solution turns yellow, according to the equation $$\ce{2H2O -> 4 H+ + O2 + 4 e-}$$ You may separate the two regions for example with a filter paper, allowing the ions to cross it. After some time, you obtain two separated solutions, one blue, and one yellow. If you remove the filter paper, the solution turns green again by stirring. You can inverse the polarity if you do want it, and start again.

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  • $\begingroup$ Hi @Maurice, this is the kind of reaction that I'm looking for, thanks for that! The only thing I can think of is that H2 and O2 are being produced from the solution, so eventually the liquid would evaporate completely. Covering the glass would make the pressure to rise a lot inside, unless the H2 and O2 could be combined to form water again? But I don't think this is a spontaneous reaction. $\endgroup$
    – Marcos
    Commented Feb 18, 2022 at 20:05
  • $\begingroup$ @Marcos. The amount of water to be destroyed would take rather rather long. Look. The electrolysis of $18$ g water produces $24$ liters gaseous hydrogen and $12$ liters gaseous oxygen, at usual temperature and pressure. You will have to wait many days before being able to see the level of your solution decrease. $\endgroup$
    – Maurice
    Commented Feb 18, 2022 at 22:22

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