I would avoid lemon for two reasons:
1) the citric acid does not boil off, but concentrates as you reduce your brew...this can, if too much is used, cause stomach issues and premature purging.
2) your brew will taste 1000x worse than a traditional brew
Use white vinegar it boils off completely and won't contaminate your brew with unnesessary acids.
As posted phosphoric acid also works amazingly well, the only issue is it is not volitile and concentrates like citric acid. It does make for a potent brew if you have good plant materials.
The amount really doesn't matter all that much. I use 4 tablespoons per liter of water for my first cook. 2 tbls for my second, and NO vinegar for my thrid cook. The acid is only an aid, but tap water itself (unless you live where the water is really basic) can pull Caapi alkaloids with no issue. When you reduce your brew, slowly reduce it all the way down, if it still smells of vinegar (and this can happen) simply add more water and reduce some more. Boiling doesn't hurt Caapi alkaloids, so this can be done without fear of hurting or weakening your brew.
As for Chali, I propose you brew it seperately and combine at the very end, or even drink seperately but back to back with your Caapi (I combine liquids before drinking but that is a personal preference). Chali only needs 30 minutes at a VERY light boil and then another 10-30minutes at below boiling temps. It does not need hours upon hours of brewing (like Caapi) to release it's magic. The amount of water is greatly reduced as well....and I only add a splash or two of vinegar...it does not need much at all.
After brewing many weak or unsucceful brews, this is the method that I have found works best for me.
The Spice extends life
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