You need a buffer at pH 7.5 for use in purifying a protein at 4°C. You have chosen Tris, pK 8.08, = H° = 50 kJ ∙ mol-1. You carefully make up 0.01 M Tris buffer, pH 7.5 at 25°C, and store it in the cold box 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?