Originally Posted by
sagacious
I wasn't sure when/if this would come up, but this is probably a good time. A citric solution used on iron should not be used for cleaning copper, or vice-versa. Keep citric-for-iron and citric-for-brass solutions separate.
If you want to try an experiment, use a citric solution on brass until it turns blue. Then dip a perfectly clean piece of carbon steel into the solution-- bam! it'll plate copper on there instantly. It will do this at room temp for carbon steel, and at higher temps for stainless. Can be useful if you want to copper-plate something, but can make a real mess if you don't.