Just was poking around the web, curious that the pKa for THH did not seem to be stated on this site.

Found these two sources that indicate a pKa of 9.67:

Metabolomics Innovation Centre
Chembase.cn

It is very close to harmaline (pKa 9.8 )

I can't vouch for the veracity of these sources, but may be good info. It would make sense given how difficult it has been to selectively base out THH. It also means it may be possible to separate THH/Harmaline from Harmine in a caapi extract. I may try this some time in the future.

You had my hopes up for a moment
In the metabolomics database, thats not THH (tetrahydroharmine). Its tetrahydroharmol, an acid that would be easy to isolate. Moreover the pKa is a mathematically calculated theoretical value, not a real world value.
In the chembase one, what the, thats just nonsense jibborish lol. It seems they mixed together two database entries, giving THH's synonyms to some totally unrelated compound. Its not even a beta-carboline, and again the pKa is a theoretical one.

It might take a nexian in a basement lab to finally determine the pKa. Not hard with pure THH and a good pH meter.

Facepalm....

Well, thanks for looking into it, Auxin.