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I've read that bromothymol blue can be mixed in with the electrolyte in the demonstration of electrolysis using a Hoffman apparatus. The indicator clearly shows the local differences in pH for the anode and cathode. And the blue color provides good visual contrast between the fluid and the gases that evolve. But I've found this indicator tends to break down quickly into a colorless solution while operating the apparatus, and after awhile you lose the indicating effect as well. Does anyone know of a better choice of indicator to use that will last longer?

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I've never used it for that particular experiment, but bromocresol purple has a similar transition range and a very obvious colour change. The trouble with using indicators here is that many are organic compounds and are subject to redox reactions near the potentials at which electrolysis of water is occurring, so they break down or polymerize if they reach the electrodes. Bromothymol blue is known to oxidize at below 1 V and deposit on the electrode. Bromocresol purple tends to polymerize, but it seems to be at a relatively high potential (>1.5 V), so it may fare better.

It may also be possible to limit the diffusion of the indicator to the electrode surfaces. Perhaps using an indicator trapped in polymer beads or a block of agarose gel.

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