Buffer Preparation Calculators

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Buffer preparation involves dissolving a weak acid and its conjugate base (or dissolving the acid/base and adjusting pH with NaOH or HCl) to create a solution that resists pH changes. The Henderson-Hasselbalch equation guides ratio selection: pH = pKa + log([A⁻]/[HA]). Effective buffering occurs within pKa ± 1 pH unit. Proper buffer preparation requires: choosing the appropriate buffer (pKa close to target pH); making stock solutions at correct concentration; measuring pH at the actual experimental temperature; and verifying stability under experimental conditions.

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Buffer Selection by pH Range

  • Acetate buffer (pKa 4.75): pH 3.8–5.8; cell culture, enzyme assays at acidic pH
  • MES (pKa 6.15): pH 5.5–6.7; protein work, electrophoresis
  • PIPES (pKa 6.76): pH 6.1–7.5; cell biology
  • HEPES (pKa 7.48): pH 6.8–8.2; most common for biological assays at physiological pH
  • Tris-HCl (pKa 8.06): pH 7.0–9.0; molecular biology, protein work; ΔpKa/°C = −0.028 (temperature-sensitive)
  • PBS (phosphate buffer saline): pH 7.2–7.4; osmolarity balanced for cell work; not ideal for reactions with Mg²⁺ or Ca²⁺

Henderson-Hasselbalch for Buffer Design

pH = pKa + log([A⁻]/[HA]). To prepare a buffer at pH 7.0 using Tris (pKa 8.06): log([Tris]/[TrisHCl]) = 7.0 − 8.06 = −1.06; [Tris]/[TrisHCl] = 10^(−1.06) = 0.087; ratio = 1:11.5. Mix 1 part Tris free base with 11.5 parts TrisHCl (or start with Tris base and add HCl until pH meter reads 7.0).

Preparing PBS (10×)

137 mM NaCl, 2.7 mM KCl, 10 mM Na₂HPO₄, 1.8 mM KH₂PO₄. Dissolve in 900 mL water; adjust pH to 7.4 with HCl/NaOH; bring to 1 L; autoclave or filter-sterilize.

Practical Tips

Always adjust pH at the temperature of use (Tris especially). Use calibrated pH meter. Add NaOH/HCl slowly near target pH. Verify osmolarity for cell biology applications.

Glossary

Henderson-Hasselbalch Equation
pH = pKa + log([A⁻]/[HA]); used to design buffers (select pKa within ±1 of target pH); calculate conjugate base:acid ratio for desired pH; rearrange to find [A⁻]/[HA] = 10^(pH−pKa).
PBS (Phosphate-Buffered Saline)
Isotonic buffer at pH 7.4: 137 mM NaCl, 2.7 mM KCl, 10 mM Na₂HPO₄, 1.8 mM KH₂PO₄; ~290 mOsm/kg; used for cell washing, dilutions, and maintaining cell viability.
Tris Buffer Temperature Sensitivity
Tris pKa changes −0.028 pH units per °C; buffers prepared at 25°C shift significantly at 4°C or 37°C; always prepare and measure Tris buffers at the temperature of use.

Frequently Asked Questions

Steps: (1) Choose buffer with pKa within ±1 pH unit of target. (2) Calculate [A⁻]/[HA] ratio: pH = pKa + log([A⁻]/[HA]); rearrange: [A⁻]/[HA] = 10^(pH−pKa). (3) Prepare solutions of the acid and conjugate base at the desired concentration. (4) Mix in the calculated ratio. (5) Check and fine-tune pH with a calibrated pH meter at the experimental temperature; adjust with HCl or NaOH. Example: prepare pH 7.4 phosphate buffer (pKa₂ = 7.20): [HPO₄²⁻]/[H₂PO₄⁻] = 10^(7.4−7.2) = 10^0.2 = 1.585. Mix Na₂HPO₄ : NaH₂PO₄ in ratio 1.585:1.

HEPES (pKa 7.48, 25°C): low temperature sensitivity (ΔpKa/°C ≈ −0.014); compatible with most biological molecules and metals; does not affect enzyme activity; suitable for physiological pH work (6.8–8.2); commonly used in cell culture media and protein biochemistry. Tris (pKa 8.06, 25°C): strong temperature sensitivity (ΔpKa/°C = −0.028) — a Tris buffer prepared at pH 7.5 at 25°C will be pH 8.0 at 4°C and pH 7.2 at 37°C; can interfere with certain enzymatic assays; standard for molecular biology (Tris-EDTA, Tris-acetate-EDTA electrophoresis buffers, Tris-glycine for protein gels). Always prepare Tris buffers at experimental temperature or correct for temperature shift.

Phosphate-buffered saline (1×): 137 mM NaCl (8.0 g/L); 2.7 mM KCl (0.2 g/L); 10 mM Na₂HPO₄ (1.42 g/L of anhydrous; 1.78 g/L of dihydrate Na₂HPO₄·2H₂O); 1.8 mM KH₂PO₄ (0.24 g/L). Dissolve in ~900 mL distilled water; adjust to pH 7.4 with HCl or NaOH; bring volume to 1 L; sterilize by autoclaving or filtration. 10× stock can be made and diluted. PBS is isotonic (~290 mOsm/kg) — suitable for cell washing and diluting cell suspensions. Note: standard PBS does not contain Mg²⁺ or Ca²⁺ (use PBS + 1 mM MgCl₂ + 0.1 mM CaCl₂ when these are needed for cell adhesion assays).

Buffer pKa changes with temperature: acid/base equilibria are temperature-dependent. Most biological buffers have negative ΔpKa/°C — pKa decreases as temperature increases. Tris has the largest effect: ΔpKa/°C = −0.028. A Tris buffer at pH 7.5 (25°C) shifts to: pH ≈ 7.5 + 0.028 × (25−4) = 8.1 at 4°C; pH ≈ 7.5 + 0.028 × (25−37) = 7.2 at 37°C. HEPES shifts less: ΔpKa/°C ≈ −0.014. Practical rule: always check and adjust buffer pH at the temperature it will be used. For experiments at 37°C, prepare buffer and adjust pH in a 37°C water bath before use. Phosphate buffers have low temperature dependence (ΔpKa ≈ −0.003 per °C) — suitable when temperature stability is critical.