Question

To observe the effect of copper on protein structure, my lab student devised an experiment wherein he added aliquots of a copper stock solution to a cuvette containing a protein in 50 mM MOPS buffer, pH 7.55 (pK_a = 7.20). He then measured the electronic absorption spectrum of the protein after the addition of each copper aliquot. However, to stabilize the copper ions in the Cu(I) oxidation state, the copper stock solution needed to contain 0.10 M HCl. If he added too much Cu(I), Joey risked exceeding the buffer capacity of the protein solution, resulting in a dramatic decrease in the pH of the solution and altered structure of the protein.

If Joey begins with 500 ul (1 ul = 1x10^-6 L) of protein solution and adds 10 ul aliquots of the Cu(I) solution, how many aliquots can Joey add before the pH drops below 7.0? (Assume that the addition of Cu(I) has a negligible effect on the total concentration and volume of the buffer.)

Answer 1

Steps to solve the proble:

Step 1: Determine the concentration ratio of salt:acid for the given buffer solution using Henderson-Hasselbalch equation.

Step 2: Determine the moles of salt and acid present in the buffer used before the addition of HCl.

Step 3: After addition of 'x' aliquots of 0.1 M HCl the salt will get neutralized and acid concentration will increase. Therefore determine new salt and acid concentrations.

Step 4:Using Henderson-Hasselbalch equation, solve for 'x'