Potentiometric titration using a pH meter Why is the pH of a buffer solution measured before measuring the pH of an unknown solution?
I do not have any idea about it. I had read a chapter about ionic equilibrium and electrochemistry, but have not been able to find out an answer.
In principle, you should know the pH of the buffer solution. Measuring it before an unknown sample is a straightforward way of calibrating the detector (pH detectors can get a bit funny after a while). 

If you put the detector into a pH 5.5 solution and it reads 7, you're going to have to tweak the detector (if possible) or find another. 

Measuring unknown samples with a detector that incorrectly measures a known buffer solution is a bad idea but an even worse idea would be to measure unknown samples without knowing your detector was incorrect in the first place.