While I agree that Rodinal mixed does not last, in most parts of the world with running water, air is added to the water to make it flow through pipes more easily. The result is more oxygen and bubbles in water than is normal; if using tap water, I generally either boil it and let it cool (removes oxygen bubbles) or I let it sit for more than 24 hours to get the air out prior to mixing. The obvious alternative is using distilled but that's a different discussion. The result is a sightly longer lasting working mixture but only by a couple of hours. I do find less spots on my negatives and more control from agitation.
Boiling water and filter it for photographic use is a cheap method to get rid of some oxygen and carbonates in the tap water. So the chemicals will keep longer.
Even better is to use destilled or R.O. (Reverse Osmose) water.
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