Do you add Nacl or HCL if your target buffer pH is 8, and the buffer is HEPES
The correct answer is to measure the pH of the "made up" buffer, then add acid or base as appropriate to reach the desired pH.
If this is a problem, look at the pka for HEPES, and add acid or base as appropriate to get to 8.
Dr. Bob222 is correct, you will need to add base to increase the pH of HEPES to 8, since the pKA of the solution is at 7.5. Remember, buffers have equal concentrations of its acid and conjugate base. Therefore, the pH of the solution is equal to the pKa.
To determine whether you need to add NaCl or HCl when the target buffer pH is 8 and the buffer is HEPES, you'll need to consider the pKa value of HEPES.
HEPES is a zwitterionic buffer commonly used in biological research. Its pKa is approximately 7.5 at room temperature. The pKa is the pH at which the buffer is in its half-neutralized form, meaning it is present in equal amounts as its acidic and basic forms.
If the target pH is 8, which is higher than the pKa of HEPES, it means that the buffer needs to be more basic. To increase the pH, an acid needs to be added, and in this case, HCl is generally used. HCl is a strong acid that dissociates completely in solution, providing hydrogen ions (H+). By adding HCl to the HEPES buffer, you will be increasing the concentration of H+ ions, which will shift the pH towards the acidic range.
Therefore, if your target buffer pH is 8 and the buffer is HEPES, you would need to add HCl to adjust the pH to the desired value.