Buffer Simulator
Equations in this simulation
| pKa | Buffer | the pH at which the acid is half dissociated; a buffer works best within about 1 unit of it | |
|---|---|---|---|
| [A⁻] ÷ [HA] | ratio of base form to acid form | Henderson–Hasselbalch: adding acid turns A⁻ into HA, adding base the reverse, and the pH follows the log of the ratio |
With the current values:
| C_T | Total buffer concentration (mM) | acid plus base form, diluted as the additions add volume | |
|---|---|---|---|
| Kw | ion product of water | 10⁻¹⁴ at 25 °C | |
| [H⁺] | hydrogen ion concentration | solved exactly from this proton balance, so the pH stays right even far from the pKa, where Henderson–Hasselbalch fails |
With the current values:
| β | buffer capacity | moles of strong acid or base per liter needed to move the pH by 1; largest at pH = pKa and proportional to C_T |
|---|
With the current values:
| V | volume | starts at 100 mL in each beaker; the plain water has nothing to absorb the added acid, so its pH drops straight to about −log of the excess |
|---|
With the current values:
How to use the buffer simulator
- The left beaker holds a buffer, the right one the same volume of plain water. Press Add acid or Add base, or choose to keep adding, and the same amount goes into both. The liquids are tinted like universal indicator, and the pH is shown above each beaker.
- In the buffer the blue acid-form molecules turn yellow (base form) as base is added and back as acid is added, while red H₃O⁺ dots stay scarce. In the water every drop shows up as free H₃O⁺ or OH⁻ and the pH swings by several units. The chart draws the whole titration curve both ways for each beaker.
- Change the buffer, its concentration and the starting ratio of base to acid. A buffer is strongest at its pKa and when concentrated, and gives out once one form is used up, which the readout of how much more acid or base it can take shows. To make one up at a given pH, use the Buffer Calculator.
Frequently asked questions
How does a buffer work?
It holds a weak acid and its conjugate base together, such as acetic acid and acetate. Added H⁺ is mopped up by the base (A⁻ + H⁺ → HA) and added OH⁻ by the acid (HA + OH⁻ → A⁻ + H₂O). Instead of the free H⁺ changing a lot, the ratio of A⁻ to HA changes a little, and the pH follows the log of that ratio.
What is the Henderson-Hasselbalch equation?
pH = pKa + log₁₀([A⁻] ÷ [HA]). With equal amounts of acid and base forms the pH equals the pKa; ten times more base gives pKa + 1, ten times more acid pKa − 1. It is an approximation that is excellent between about pKa − 1 and pKa + 1 at ordinary concentrations, and the simulation shows it next to the exact pH.
What is buffer capacity?
How much strong acid or base a buffer can absorb per unit change in pH. It is greatest at pH = pKa, falls off on either side, and is proportional to the total buffer concentration: a 100 mM buffer resists ten times as much acid as a 10 mM one. Choose a buffer whose pKa is within 1 of the pH you need.
Why does the pH of water fall so far with one drop of acid?
Pure water has only 10⁻⁷ M of H⁺. One milliliter of 1 M HCl in 100 mL makes about 0.01 M of H⁺, a hundred thousand times more, so the pH falls from 7 to 2. In a buffer the same acid is turned into the weak acid HA and the free H⁺ barely changes.
It says WebGL is turned off.
The 3D view needs WebGL, which every current browser has. It can be switched off by hardware acceleration being disabled in the browser settings, or by a very old graphics driver. Turn hardware acceleration on, or try another browser.
Is anything uploaded?
No. The simulation is drawn by your own browser with WebGL; nothing is sent anywhere, and it keeps working offline once the page has loaded.