David L. Zellmer
January 12, 1999

In a former web site (Advanced Acid/Base pH Calculations) I showed how to do a messy pH calculation (strong acid plus polyprotic weak acid) using charge balance and Goal Seek.

Goal Seek can also be used for the classic monoprotic pH calculations for pure weak acids, buffers, and pure weak bases by using the Extended Henderson-Hasselbalch Equation and Goal Seek.

You may find the above formulation useful when doing textbook problems that call for the solution to buffer problems that have very low analytical concentrations, or where the pH makes [H⁺] or [OH^-] large enough to affect the outcome of the calculation. In the example above, the regular Henderson-Hasselbalch equation would have given you pH=1.43. Using the extended form and Goal Seek, we get pH=1.82, a significant difference.

Another feature of the Extended Henderson-Hasselbalch Equation is that it works for pure weak acids and pure weak bases as well. For pure weak acids, just set CNaA to zero; for pure weak bases set CHA to zero. Then proceed with the calculation as above.

Remember to make this method work you have to "think like a chemist" just long enough to put in a reasonable guess for the pH into cell A9. If your pH guess is too far away from the right answer, you may get an error message from Excel. If this happens, think a bit, then put in a more reasonable guess.