Is Your Henderson-Hasselbalch Calculation of Buffer pH Correct?

A survey of 21 free online buffer calculators and apps found only 2 that correctly calculated pH under conditions where [H+] and [OH-] cannot be ignored in the ratio term of the Henderson-Hasselbalch equation. Traditional guidelines for when use of formal concentrations is acceptable were found to be flawed. For acidic (pK(a) <= 7) buffers, the use of formal concentrations is valid when F-A-/K-a > 20. For alkaline (pKa = 7) buffers, the approximation is valid when F-BH(+)/K-b > 20. Robust equations for predicting pH of acidic and alkaline buffers are presented, which could be used with programmable calculators and should be used for online buffer calculators and apps.

Automated summary

A survey of 21 free online buffer calculators and apps found that only 2 accurately calculated pH under conditions where [H+] and [OH-] cannot be ignored in the Henderson-Hasselbalch equation. Traditional guidelines for when the use of formal concentrations is acceptable were deemed flawed. Robust equations for predicting pH of acidic and alkaline buffers were proposed, which can be utilized with programmable calculators and should be adopted by online buffer calculators and apps.