4.76
1
0.1
M
1,000
mL
0.05
mol
0.05
mol
4.76
1
0.1
M
1,000
mL
0.05
mol
0.05
mol
The Buffer pH Calculator computes the pH of a buffer solution prepared by mixing specific volumes and concentrations of a weak acid and its conjugate base. Buffer solutions resist changes in pH upon addition of small amounts of acid or base, making them essential in biological systems, laboratory experiments, industrial processes, and pharmaceutical formulations. This calculator accounts for the actual moles of acid and base mixed (not just concentrations), properly handling dilution effects when solutions of different volumes are combined. Using the Henderson-Hasselbalch equation internally, it provides the resulting buffer pH, the [A⁻]/[HA] ratio, the total buffer concentration, and the total volume. Accurate buffer preparation is critical — blood pH must stay within 7.35-7.45, enzyme assays require specific pH values, and many chemical reactions are pH-sensitive.
The calculator first determines the moles of acid and conjugate base:
moles_acid = [HA] × V_acid(L)
moles_base = [A⁻] × V_base(L)
After mixing, the total volume is V_acid + V_base, and the new concentrations are:
[HA]_final = moles_acid / V_total
[A⁻]_final = moles_base / V_total
The buffer pH is then calculated using the Henderson-Hasselbalch equation:
pH = pKa + log₁₀([A⁻]_final / [HA]_final)
Note that the ratio [A⁻]/[HA] = moles_base/moles_acid (the volume cancels out), so the pH depends on the mole ratio, not the concentration ratio. However, the total buffer concentration (which affects buffer capacity) does depend on the actual concentrations after dilution. A higher total concentration means the buffer can absorb more added acid or base before its pH changes significantly. The effective buffer range is typically pKa ± 1.
The buffer pH indicates the expected pH of your prepared solution. If it falls outside pKa ± 1, the buffer has limited capacity and may not effectively resist pH changes. The [A⁻]/[HA] ratio should ideally be between 0.1 and 10 for practical buffering. Equal moles of acid and base (ratio = 1) gives maximum buffer capacity at pH = pKa. The total buffer concentration reflects the dilution effect of mixing — higher concentrations provide greater resistance to pH perturbation.
Inputs
Results
Equal volumes of 0.1 M acetic acid and 0.1 M sodium acetate produce 1 L of buffer at pH 4.76 (= pKa), with maximum buffer capacity.
Inputs
Results
Mixing 390 mL of 0.05 M NaH₂PO₄ with 610 mL of 0.05 M Na₂HPO₄ gives a phosphate buffer near pH 7.4, suitable for biological experiments.
Buffer capacity (β) measures how much acid or base a buffer can absorb before its pH changes significantly. It depends on both the total concentration and the [A⁻]/[HA] ratio. Maximum capacity occurs at pH = pKa and increases with total buffer concentration. A 0.1 M buffer has roughly 10× the capacity of a 0.01 M buffer.
Common biological buffers include: phosphate buffer (pKa₂ = 7.2, for pH 6.2-8.2), Tris (pKa = 8.1, for pH 7.0-9.0), HEPES (pKa = 7.5, for pH 6.8-8.2), and bicarbonate (pKa = 6.4 in blood). Good's buffers (PIPES, MOPS, HEPES) are designed to minimize interference with biological systems.
The pKa of most weak acids changes with temperature, which shifts the buffer pH. Tris buffer is particularly temperature-sensitive: ΔpKa/ΔT ≈ -0.028/°C. A Tris buffer prepared at 25°C will have a different pH at 37°C. Always adjust pH at the intended working temperature.
In principle, yes — you just need a weak acid with pKa near your target pH. In practice, you are limited to the effective range of pKa ± 1. For pH 4-5, use acetate; for pH 6-8, use phosphate, MOPS, HEPES, or Tris; for pH 9-10, use borate or carbonate buffers.
The strong acid converts some conjugate base (A⁻) to acid (HA): A⁻ + H⁺ → HA. This shifts the ratio but changes pH minimally as long as neither component is depleted. Once all A⁻ is consumed, the buffer fails and pH drops sharply.
Increase the total concentration of both components while maintaining the same [A⁻]/[HA] ratio. For example, doubling both [HA] and [A⁻] doubles the buffer capacity without changing the pH (since the ratio and pKa are unchanged).
Roboculator Team
The Roboculator Team explains calculations, planning tools, and practical formulas in clear language for real-life situations.
How helpful was this calculator?
Be the first to rate!
