Buffer pH Calculator

About Buffer pH Calculator

The Henderson-Hasselbalch equation calculates the pH of a buffer solution. Buffers resist pH changes and work best when pH ≈ pKa.

How to use this calculator

1. Enter the buffer's pK_a or the acid's K_a value if the calculator asks for it.
2. Type the concentration of the conjugate base, \([A^-]\), and the weak acid, \([HA]\).
3. Check that both concentrations use the same units, such as mol/L.
4. Read the calculated pH and compare it with the pK_a to see whether the buffer is more acidic or more basic.

The formula explained

The Henderson-Hasselbalch equation, \(pH = pK_a + \log\frac{[A^-]}{[HA]}\), computes the pH of a buffer from the weak acid's \(pK_a\) and the ratio of conjugate base to weak acid. When \([A^-] = [HA]\), the pH equals \(pK_a\).

• pH = the acidity of the buffer solution
• \(pK_a\) = the negative base-10 logarithm of the acid dissociation constant
• \([A^-]\) = the concentration of the conjugate base
• [HA] = the concentration of the weak acid

Step by step method

1. Find the buffer's \(pK_a\), either from the acid data or from \(pK_a = -\log K_a\).
2. Compute the ratio \([A^-]/[HA]\) using matching concentration units.
3. Take \(\log\) of that ratio, then add it to \(pK_a\).
4. Interpret the result, a larger \([A^-]/[HA]\) ratio gives a higher pH, and a larger \[HA]\) ratio gives a lower pH.

Worked example

Problem. Calculate the pH of a buffer with \(pK_a = 4.76\), \([A^-] = 0.20\) mol/L, and \([HA] = 0.10\) mol/L.

1. Use the formula \(pH = pK_a + \log\frac{[A^-]}{[HA]}\).
2. Substitute the values, \(pH = 4.76 + \log\frac{0.20}{0.10}\). Since \(\frac{0.20}{0.10} = 2\), this becomes \(pH = 4.76 + \log(2)\).
3. Because \(\log(2) \approx 0.301\), the pH is \(4.76 + 0.301 = 5.061\).

Tips and common mistakes

• Use concentrations, not moles, unless both volumes are the same and cancel out in the ratio.
• If \([A^-]\) and \([HA]\) are equal, the pH is exactly \(pK_a\), which is a helpful checkpoint.

Frequently asked questions