Success!
Starting with the exciting part first:
Results
This is a plot of the pH of my electrolyte over time as I alternately apply a +1.5V potential and a -1.5V potential. As you can see, I am able to achieve a reversable pH swing of around 0.35.
In this experiment, I am manually reversing the leads that are hooked up to my DC bench power supply. Switches happen at 11:15, 11:45, 12:30, etc.
Discussion
Two things I want to pull out of this chart:
First, you can see that there’s a slow, irreversible baseline increase in pH. I’m not sure why this is, but I highly suspect it has to do with the spontaneous formation of bismuth oxide. I’m not going to worry too much about it, as future versions of the experiment will be run in a de-gassed electrolyte, which may solve the problem.
Second is the relatively “shallow” swing that I’m showing here. This is due to the very small surface area of my electrodes (1cm^2) relative to the large volume of electrolyte (125mL). I expect to be able to show a wider swing by adjusting the size of the electrodes.
Chloride Loading
I found it interesting how this reaction is highly limited by the amount of chloride that can form on the electrodes: There’s a clear peak when the silver is fully loaded up with chloride, and a clear floor when it’s all released.
You can see this effect quite clearly if you start with plain bismuth and silver electrodes. The silver electrode has to ‘charge’ with chloride (raising the pH) before it can be ‘discharged’ (lowering the pH). If you begin with a silver chloride electrode, you can drive the pH down below the starting point.
pH Probe Upgrade
The key to getting this to work was upgrading to a high quality industrial-style pH probe hooked up to an Arduino.
More Electrodes
I’m making a new set of electrodes for the next round of experiments. I came up with some better techniques using a mortar and pestle, a scraper blade, and 3d-printed ‘templates’. If anyone would like a set of electrodes, please let me know-- I would be happy to send you a set.
