Trying to measure the pH of a falling liquid

just background, i'm new arduino , electronics/coding in general, i've been fooling around these past few days have feel now. i'm working on idea device measure ph of liquid automatically.

so, mount device against wall of toilet or sink, use pir detect when liquid falling, dispense litmus paper goes stream, , discard paper (once it's been analyzed chemist) sink or toilet.

now, stumped on mechanism use dispensing of litmus paper, , more perplexed possible discarding/reloading mechanisms. there parts or sensors me achieve purpose? appreciate guidance on topic.

for dispensing, can on roll , use small print head thermal printer?
i think i've seen small printheads @ adafruit.com example.

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