This question from Jack is a good one. I hadn’t really considered it before, and finding information on it online is either tricky or expensive. In short, I don’t know.
I have used electrolytes with a wide range of pH (acidity and alkalinity) but had not been looking for the differences. Some of my favorites are phosphoric acid (pH = 1.7), ammonium phosphate (4.2) , and tri-sodium phosphate (12). Quite different pH’s, but all slam that phosphate ion against the titanium anode and drop off an oxygen atom. Borates work fairly well, too. I’ve read that alkali sulfates can be used. But personal experience says, stay away from nitrates and chlorides.
According to some guidelines/requirements for anodizing titanium medical implants, a strong alkaline should be used. I suspect that this is to guarantee that nothing living can be in the solution.
According to one for-fee article from 1985 (Studies on anodizing of aluminium in alkaline electrolyte using alternating current) found “Electrolyte pH was found to affect the growth of anodic films considerably.” But I didn’t buy it to see how. This article is not quite to the point because it a) was about aluminum, b) they used alternating current, and c) it focused on only part of the pH spectrum.
If anyone has played with pH in anodizing titanium, please let me know if you notice any anodizing differences by pH.