Question

Your laboratory assistant Beaker, prepared a 100 mM Tris buffer at room temp (20 degrees C) from a 1M, pH 7.4 stock solution of Tris buffer. Because the buffer was at the same pH you needed, Beaker simply diluted the buffer without checking the pH, capped and stored the solution on ice (1 degrees C). Just before using the buffer, you double checked the pH. It was not at the right pH! Predict the pH change (increase or decrease) and estimate the magnitude of the change in pH. What caused this pH change? There are two chemical reasons (there was no error in calculation or pH meter) for the unexpected pH change.

Answer 1

one reason for change in pH is the dilution. as we are diluting the stock solution of 1M to 100 mM i.e. diluting 10 times with water.

we know, pH=-log[H⁺]

given that pH of 1M stock solution is 7.4, so dilutiong solution 10 times with water will raise the pH of diluted solution i.e. 100 mM by 1 unit. Thus, the pH of 100 mM solution is 8.4

The secod reason for change in pH is the change in temperature during storage. we know that pH is dependent on the [H⁺] ions. As temperature is decreasing from room temperature i.e. 20 deg C (stock solution) to 1 deg C, the rate of ionization will decrease, which also decreases the concentraton of H⁺ ions available in solution. so the pH of the solution will increase as the pH is the negative base-10 logarithm to the number of hydrogen ions in the solution.