I also thought about just adding sodium ascorbate for comparison but didn't like the idea; ascorbic acid is a weak acid with Ka of 4.3E-7 and 5.6E-11. I don't know how this affect as a mild buffer, especially when the local oxidation products and sulfite reaction is involved. I don't know if this is significant or not but rather avoid the issue by comparing buffered D-76d against buffered D-76C. It's not perfect but nothing is perfect in photography and still this is fairer comparison, and it is easy to do anyway. Another motivation for me to do this is that with a buffered version, drug addicted HP-5+ doesn't take progressively longer processing time even when diluted. (With HP5+ I want to see smooth gradation and pleasing grain rather than increased accutance, which it inherently has plenty.)(posted 8323 days ago)If you have sodium ascorbate at hand but not the acid version, I suppose you could add that to D-76Hd for comparison. If this turns out to be so tiny an issue that doesn't matter, it's good but I don't know that at this moment.
This can also be a problem with carbonate version, as sodium ascorbate and disodium ascorbate would make even more stable buffers at high pH.