Question

what is the volume of one drop of alcohol

Answer 1

In metric system, one drop is 0.05 ml, so 20 drop per ml.
Drop's volume changes depending of liquid viscosity, shape and surface of instrument used, gravitational pull actuated. Alcohol should give smaller drops due to its lesser viscosity compared to water, yet again...

Answer 2

if a drop is a measure of volume should that not remain constant, given that the same T and P are the same, regardless of the liquid?

it seems unscientific (to me) that one would use a "drop" as a valid quantifiable unit, why not use a micropipette if a small volume is needed?

Answer 3

In the beginnings of pharmacology any strong compound was diluted form a drop, which at the time was precise enough.