Buffer pH Calculator (Henderson-Hasselbalch)
Enter the pKa of the weak acid and the concentrations of the acid and its conjugate base. You can also add a strong acid or base to see the new pH. The buffer working range is a base/acid ratio between 0.01 and 100 and a pH between 0 and 14; outside it the calculator reports an error.
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Formula
pH = pKa + log10([A-] / [HA])- Adding strong acid:
HA + n,A- - n(amounts in mmol = mol/L × mL) - Adding strong base:
HA - n,A- + n
Worked example
Acetic acid, pKa 4.76, 1000 mL of 0.1 M acid and 0.1 M acetate: 100 mmol each, pH = 4.76.
Add 20 mmol HCl: 120 mmol acid, 80 mmol base, pH = 4.76 + log10(80/120) = 4.58 (a change of -0.18).
Buffer working range
The equation is applied only for a base/acid ratio between 0.01 and 100 (the buffer working range) and a resulting pH between 0 and 14. Outside that range the calculator reports an error instead of a misleading pH.
Frequently asked questions
What is the Henderson-Hasselbalch equation?
pH = pKa + log10([A-]/[HA]). With equal acid and base the log term is 0, so pH equals pKa. For acetic acid (pKa 4.76) with 0.2 M acetate and 0.1 M acetic acid, pH = 4.76 + log10(2) = 5.06.
How do I find pH after adding strong acid?
Added H+ converts conjugate base into weak acid. From 100 mmol of each, adding 20 mmol HCl gives 120 mmol acid and 80 mmol base, so pH = 4.76 + log10(80/120) = 4.58.
How do I find pH after adding strong base?
Added OH- converts weak acid into conjugate base. From 100 mmol of each, adding 20 mmol NaOH gives 80 mmol acid and 120 mmol base, so pH = 4.76 + log10(120/80) = 4.94.
When does the buffer stop working?
The equation holds while both components remain, ideally with a ratio between 0.1 and 10 (pH within about 1 of pKa). This calculator accepts a base/acid ratio from 0.01 to 100 (the buffer working range) and a resulting pH from 0 to 14, and reports an error outside them. If the added strong acid or base is as large as the amount of conjugate base or acid, the buffer is used up and this calculator reports an error. The equation also assumes fairly dilute solutions.