Response to Rodinal with Ascorbate - what am I doing wrong???

If you don't want to minimize the steps and get the good result, I recommend to buy sodium ascorbate. If you don't buy it, use hydroxide to neutralize. Or even simpler, switch to Ilfosol-S. If you are looking for a match for TMX, also try Microphen 1+1 or 1+3.

I do not understand why bicarbonate is recommended to neutralize ascorbic acid. Unless you want to make a very low pH developer this just makes things difficult. Here's a little review from freshman chemistry. Ascorbic acid's pKa is about 6, and that of bicarbonate is about 7.6. Remember if pH is near pKa, that compound is only about 50% dissociated. If you mix similar molar quantity of these, they'll result in somewhere near neutral pH region, meaning that some of each are dissociated, some of each are not dissociated. It doesn't quite look like a textbook buffer, but acts as a buffer. There is even more thing involved in this complicated buffer that affects its pH. The gas you saw is carbon dioxide from bicarbonate. Solubility of CO2 in water (to form carbonic acid) decreases as pH goes down, so when carbonic acid sees ascorbic acid, it gives up the gas. This removal of carbonic acid moves around the bicarbonate <--> carbonic acid equilibrium. This decreases titratable base, as well as the proton provided from the ascorbic acid. Yet there are some undissociated acid and base. What it means is that this ascorbic acid and bicarbonate mixture will pull the pH down from Rodinal's normal operating pH. Adding more of this mixture would pull the pH even farther, and Rodinal would cease to function at such a low pH.

Ascorbic acid can be used as a part of the buffering system if the developer is appropriately formulated. I generally prefer this approach for low pH developers. However, Rodinal is poorly buffered high pH formula that is rather intolerant of acidic disturbance. To Rodinal, even bicarbonate looks like an acid.

(ascorbic acid is diprotic but the second dissociation is not of interest unless you are looking at very high pH. even so, the theory is the same)

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