First let me state that I am not a chemist - so be gentle.
I want to very accurately measure, nondestructively, the pH of paper (specifically envelopes). As a stamp collector I am very concerned about the effect that acids have on the life of low rag content paper. I purchased a Hanna HI 99171 and successfully calibrated to the 7.01 and 4.01 standards. When I place the flat glass probe to the paper with the probe dry (or slightly moist) I get drifting readings in the 5-6 range. It doesn't matter if I put it on a calcium bicarbonate (2%) paper or an old envelope. Putting the probe into the 7.01 reference water gets me to about 6.8 as does putting it in 4.01 (somewhere around 4).
Why are the values so far off and inconsistent? I'm sure it must be me.