phosphoric acid is a triprotic acid, which has buffer properties in the 4-5 pH range.

because it is a polyprotic acid, I wouldn't suggest its use for alkaloidal extraction. it tends to complicate things, resulting in a loss of yields and/or a mess.

pH is usually measured at room temp, 25C. consider: a pH probe is a semi-permeable glass membrane, which registers voltages (potential). to get an accurate reading, the environment should be isothermal, i.e. the temp inside the probe should be the same as the temp of the solution it is set in.