Question

In a lab, we were supposed to calculate the pKa of KHP by titrating with NaOH and building a curve with that data. I calibrated my pH meter via taking mV readings in buffers at pH= 2, 4, 7, and 10. I got a linear regression that had an R^2 of >.999 so I feel pretty good about that. I fixed my pH meter in the soln and made a curve. When I reached the endpoint, I found the inflection point of the curve and used. I took the pH of the point where I delivered half of the volume I had delivered at the EQ point. It was 5.06. The textbook value of pKa for KHP is 5.4. Any ideas as to the cause of the error? PLEASE, do not answer with a calcluation, I know how to do the math. The water used was DI and boiled to remove any CO2.

Answer 1

Probable reasons:

1) Purity of the chemicals used:

Check the chemical packs for the purity. Generally it is written as >98% or 99.9% . So, include them things in your calculation.

ii) Temperature of the experiment and the temperature at which .. is reported in the book.

iii) Errors that occur while seeing the burects reading or during the pipetting of solution.

ii) Temperature of the experiment and the temperature at which .. is reported in the book.

iii) Errors that occur while seeing the burects reading or during the pipetting of solution.