How to verify the accuracy of a pH meter One suggestion I received is that I prepare a solution of a known activity and probe the solution with the pH meter. The problem I see with this is that any error in solution preparation can lead us to conclude that the pH meter is faulty. 

Is there any other way to test the accuracy of a pH meter? We need to make extremely accurate measurements (we are studying the activities of ionic solutes). 

Also, what solution has an activity of exactly 1.0?
Your school should have a purchased buffer solutions that have been calibrated against a NIST standard.  If they don't, have them purchase several in order to span the entire pH range, here's one from Sigma-Aldrich.  For your calibration, pick a standard solution that is close to the range you plan to work in.  Here's a couple of links with some more "tips" on how best to actually perform the calibration.


tips #1
tips #2