I am trying to measure the pH of a nanoparticle dispersion in water. The pH paper gives a pH value of around 8, while the pH meter gives a pH value of 3.00
pH paper is impregnated with dyes that change color at a certain pH range. Dyes age (degrade) over time when exposed to light. This affects the pH measurement. If the pH meter is correctly calibrated, it is better to use it.
the most common reason would be because The electrode of the meter is broken (usually because noone bothered to maintain it/put it back into the salt solution specified?
test both systems with a defined buffer and You will know.
pH is dependent on temperature and calibration if both pH meters are calibrated at the same temperature and has same type of electrode both will show same pH