Biochem:

You need a buffer at pH 7.5 for use in purifying a
protein at 4 °C. You have chosen Tris, pK 8.08, deltaH= 50 kJ/mol.
You carefully make up 0.01 M Tris buffer, pH 7.5 at 25° C, and
store it in the cold to equilibrate it to the temperature of the
purification. When you measure the pH of the
temperature-equilibrated buffer it has increased to 8.1. What is
the explanation for this increase? How can you avoid this
problem?