Buffer Solutions – Why Does the pH Hardly Change?

What it shows

A buffer contains a weak acid and its conjugate base in similar, sizeable amounts. Added H⁺ is taken up by the conjugate base and added OH⁻ is neutralized by the weak acid, so the pH barely moves until one component is nearly used up. The simulation calculates the pH exactly from the charge balance and compares it with the Henderson–Hasselbalch equation. Pure water, with nothing to take up the ions, changes pH by several units after a few drops. The blood buffer shows how pH stays near 7.4.

How to use

Choose a Buffer system, set Total concentration and the ratio of base form to acid form. Press +1 drop HCl, +10 drops HCl, +1 drop NaOH or +10 drops NaOH, or tap the left or right half of the picture. Compare the pH meters and ΔpH, watch the magnifier and amount bars, and follow the graph of pH against drops added. Turn on Theory curves to see the whole curve and the buffer region.

Parameters you can change

• Buffer system CH₃COOH / CH₃COO⁻ (pKa = 4.76), NH₄⁺ / NH₃ (pKa = 9.25), H₂PO₄⁻ / HPO₄²⁻ (pKa = 7.21), H₂CO₃ / HCO₃⁻ – blood buffer (pKa = 6.1 at 37 °C)
• Total buffer concentration 0.01–0.5 M
• Ratio [base form]/[acid form] 0.1–20
• Show theoretical pH curves

Questions to explore

1. Why does one drop of HCl change the pH of distilled water far more than the pH of the buffer?
2. How does raising the total buffer concentration change the buffer capacity and the starting pH?
3. Does the blood buffer, with HCO₃⁻/H₂CO₃ = 20, resist added acid or added base better?