Buffer Solution Calculator
Calculate buffer solution pH using the Henderson-Hasselbalch equation. Determine the optimal ratio of conjugate base to weak acid, buffer capacity, and design buffers for specific pH values.
What is a buffer solution?
A buffer solution resists changes in pH when small amounts of acid or base are added. Buffers consist of a weak acid and its conjugate base (or a weak base and its conjugate acid) in roughly equal amounts. For example, acetic acid (CH₃COOH) and acetate ion (CH₃COO⁻) form an acetate buffer. Buffers maintain pH through equilibrium shifts.
What is the Henderson-Hasselbalch equation?
The Henderson-Hasselbalch equation relates buffer pH to pKa and the ratio of conjugate base to weak acid: pH = pKa + log([A⁻]/[HA]). This is derived from the acid dissociation equilibrium. When [A⁻] = [HA], pH = pKa. The equation allows you to calculate pH from concentrations or design a buffer of desired pH.
How do you choose a buffer for a specific pH?
Select a weak acid with pKa within ±1 of the desired pH. Buffer capacity is maximum when pH = pKa (ratio = 1:1). The effective buffering range is pKa ± 1. For example, for pH 7.4 (blood), use phosphate buffer (pKa = 7.2) or HEPES (pKa = 7.5). Outside this range, buffer capacity decreases significantly.
What is buffer capacity?
Buffer capacity (β) is the amount of acid or base a buffer can neutralize before pH changes significantly. It depends on total buffer concentration: β ≈ 2.303 × C × [Ka[H⁺]/([Ka + [H⁺])²], where C is total buffer concentration. Higher concentration means greater capacity. Maximum capacity occurs at pH = pKa.
How do buffers resist pH change?
When acid (H⁺) is added, the base component (A⁻) neutralizes it: A⁻ + H⁺ → HA. When base (OH⁻) is added, the acid component (HA) neutralizes it: HA + OH⁻ → A⁻ + H₂O. The equilibrium shifts to consume added H⁺ or OH⁻, keeping pH relatively constant. This works until one component is exhausted.
What is the relationship between pKa and Ka?
pKa = -log₁₀(Ka) and Ka = 10^(-pKa). Ka is the acid dissociation constant: Ka = [H⁺][A⁻]/[HA]. Stronger acids have larger Ka and smaller pKa. For example, acetic acid has Ka = 1.8×10⁻⁵, so pKa = -log(1.8×10⁻⁵) = 4.74. Lower pKa means stronger acid.
How do you prepare a buffer of specific pH?
Use Henderson-Hasselbalch: pH = pKa + log([A⁻]/[HA]). Rearrange to find the ratio: [A⁻]/[HA] = 10^(pH-pKa). Choose total concentration (e.g., 0.1 M), then calculate individual amounts. For pH 5.0 acetate buffer (pKa 4.74): ratio = 10^(5.0-4.74) = 1.82. Use 1.82 parts acetate : 1 part acetic acid.
What are common biological buffers?
Phosphate buffer (pKa 7.2): Good for pH 6-8, used in cells. Bicarbonate buffer (pKa 6.1): Blood pH 7.4 (with CO₂). Tris buffer (pKa 8.1): Used in biochemistry. HEPES (pKa 7.5): Minimal salt, cell culture. Acetate buffer (pKa 4.76): pH 4-6 range. Carbonate buffer (pKa 10.3): Basic solutions.
Does dilution change buffer pH?
Theoretically, no! The Henderson-Hasselbalch equation depends on the RATIO [A⁻]/[HA], not absolute concentrations. Diluting a buffer changes both concentrations proportionally, keeping the ratio constant. In practice, very dilute buffers lose effectiveness because buffer capacity decreases. The pH may shift slightly due to water ionization (Kw) becoming significant.
How does temperature affect buffer pH?
Temperature affects pKa (and thus pH) because Ka changes with temperature. Most buffers show pH decrease with increasing temperature. For example, Tris buffer changes ~0.03 pH units per °C. Phosphate buffer is more temperature-stable (~0.003 pH/°C). Always specify temperature when reporting buffer pH, especially for biological applications at 37°C vs 25°C.