While working with Ca(OH)2 (lime), I observed a behavior that seemed to go against my (limited) understanding.

While basifying an acidic solution with lime, I noticed that to bump the pH to higher levels, I had to add more of it, even though there was undissolved lime in the system already that wasn't dissolving further (meaning: the solution was saturated).

Then when I reached pH ~12, let the undissolved lime settle at the bottom and decanted the supernatant off, the latter had a lower pH, maybe 10.

It would thus seem that undissolved lime in a lime-saturated solution rises the pH. How is that possible? Or did I make an error?

Hi Jagube, my pH probe is cheap and seem to lack accuracy, I would take mine with a grain of salt in the extreme pH regions. There is also acid in the air playing with the liquids, maybe your undissolved lime was buffering that. Just guessing ...

That was what I was thinking. Once you decant away from the solid, the CO2 absorbs fairly rapidly into solution, taking two equivalents of hydroxide with it as it precipitates out as chalk. This particularly will occur during decanting.

Jagube I remember your pH situation in another context, it was the very same effect but not with lime yet with rue alkaloids.

The last basing was done with lye to get the rue freebase.
Scare of lye traces I followed this advise from the Easy Caapi Vine Extraction Guide:

I had a hard time getting that low pH especially when the rue alkaloids where settled at the bottom, they just kept pH at 9-ish, unless I decanted the washing water away from the freebase then suddenly the pH was around 7 - 8.
I remember questioning which pH value I should respect, the one when freebase was at the bottom, or the pH of the decanted wash water, it puzzled me at the time.

The alkaline alkaloids are base just as your lime is, the situation is very identical with what you describe.

I've no idea if caapi alkaloids differ in this regard to rue alkaloids due to the very different harmaline content. If I wash rue alkaloids in overkill the wash water gets yellow indicating dissolving harmaline, this might differ from caapi. Maybe the harmaline's higher pka value is a stronger (higher) pH buffer against (air acids CO2, SO2) pH drop? Is this possible?