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I used an 5% acetic acid solution and eventually my solution became a dark blue. After I was done using it after a few days it turned green. Didn’t really affect the plating, but it was definitely interesting to see. Also, DO NOT USE MORE THAN 12 V DC or more than a few amps, or the solution will get very very warm and can pose a serious danger, let alone ruining your plating job.
For a volume knob I would suggest a rheostat wired between one of the connections as if you were wiring a switch. However, I would personally suggest using a speaker with an outlet because it usually amplifies signals that would be traditionally small, such as from an mp3 device or a phone. They usually have volume meters on them. Using a hand wired rheostat would probably give bad sound quality.