Evils of the pH electrode
Sorry to exaggerate, but I wanted your attention. The pH electrode is not evil, but misuse of the glass electrode is probably the most common source of biochemical error. I hear rumors that students are told they needn't understand buffers because they'll use pH meters. This assurance would be true if pH electrodes were simple and robust. In fact they're complex and delicate. Novices infer that expensive, high precision pH meters automatically make reliable buffers. Students commonly ignore contaminated electrode solutions and blocked junctions. (If this is Greek, read up on glass electrodes).
Glass electrodes should be regularly calibrated and monitored for stability. Unfortunately those who're too impatient to understand buffers are likely too impatient to understand electrodes and consequently shouldn't be trusted to make buffers. The pH meter is least sensitive to error at buffer pKs, i.e. where DpH/Dc is minimum, Dc/DpH is maximum. (Again if this is Greek, read up on buffers.) If you can't answer this question you need help.
How do I know buffers are a serious problem? Look at biochemical catalogues. They now push buffer solutions, not buffer components - a clear sign that laboratory buffer preparation is unreliable. If you don't believe me try this test. Make a buffer; then measure its pH on meters in several different labs. If the variation doesn't shock you, I congratulate you on the quality of your institution.
Tips
Buffers made by weighing and pipeting will invariably be more reproducible than those made by adjusting with a pH meter. The conductivity meter is a robust instrument. Two preparations of the same buffer with identical conductivities should be similar. Differences in conductivity suggest error in one (or both) of the buffers.
Take the time to understand buffers. You'll discover they're not trivial, but intellectually challenging. You can often make buffers that satisfy your specific requirements better than "recipe buffers".
For information of calibrating pH meters see the following open access article:
On Calibration of pH Meters, Cheng, K.L., and Zhu, D-M. Sensors 5, 209-219 (2005)
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