OK, I am not a chemist but I will say this to even out the informal output.
A few years ago, in a deal, I got some VERY, VERY old (I would say about 40 years old) Rodinal that had turned as black as black coffee. There was considerable sediment at the bottom of the bottle. I saw this as a stellar opportunity to see just how 'good' old Rodinal was. First, I carefully filtered out all the sediment and was left with black liquid. Carefully, I tested its potency with a bit of exposed Pan F + and, simultaneously, compared another bit of similarly exposed Pan F + with 'new' Rodinal.
Here is what I found: with multiple attempts to get the old to perform as well as the new, I found that precisely one third more of the old was needed to achieve the same contrast as the new. That is not bad at all for something that looks defunct.
So, that means that if I want to make 240mL of a working solution developer I would dilute the new Rodinal at 80X (1 + 79). This equates to diluting the old Rodinal only 60x (1 + 59) to achieve the same result. (This is the same as saying 1.33mL of the old Rodinal is needed for each 80mL of working solution. (i.e., 1.33 + 78.67.)
Thus, I would need 3mL of new Rodinal to make a viable working solution of 240mL (3 X 80mL). I would need 4mL of old Rodinal to make an equally viable working solution of 240mL of the same potency and performance as the new.
David Lyga is very frugal and does not wish to waste developer when doing his 'clip tests'. His remedy to measure the tiny quantities needed for tiny working solution quantities for development (as little as 0.1mL Rodinal concentrate could, theoretically, be needed!) force him to ponder, seriously, just how bad would it be to store mildly diluted Rodinal (despite the severe warnings not to do that!!!). Not wanting to avoid yet another 'experiment' in order to educate myself, I did just that and now REGULARLY store this mildly diluted Rodinal (filled up in PET plastic or glass bottles: 1 + 9). I have found that this 'minor' dilution has no deleterious effect with ongoing performance; it is still strong enough to withstand oxidation, but, as a precaution, again, is stored air-tight. No, I take no chances and store this diluted Rodinal in airtight containers with glass marbles taking up the slack. Now I can to all my clip tests without having to mix larger, more serious quantities of the stuff. (Of course, the dilutions for working solution now need ten times more "Rodinal" than before.) - David Lyga