A skeuomorphic titration apparatus — a graduated 50 mL burette on a stand over a conical flask on a magnetic stirrer. Fill the burette, open the stopcock and dispense titrant in measured aliquots (or dropwise near the end). The flask colour turns with the indicator, a real equilibrium engine computes the pH after every drop, and the titration curve builds point by point to the equivalence point. Nothing is pre-baked: strong–strong, weak-acid buffer regions via Ka, and weak-base hydrolysis are all solved live.
Presets
Flask — analyte
Burette — titrant
Bench controls
Dispense titrant
Use the 1.0 mL chip to advance quickly, then switch to 0.1 mL and the dropwise mode (~0.025 mL/drop) as the indicator starts to flash near the endpoint. Holding the stopcock streams titrant continuously.
Live readout
| Titrant added | 0.00 mL |
| Burette reading | 0.00 mL |
| pH | 7.00 |
| Flask volume | 25.0 mL |
| Moles analyte left | 2.50 mmol |
| Moles titrant added | 0.00 mmol |
Titration curve
pH (y) versus volume of titrant (x). The dashed orange line marks the indicator endpoint colour change; the green dot is the computed equivalence point (the maximum of dpH/dV).
Aim
To determine the unknown concentration of an acid or base by titrating it against a standard solution of accurately known concentration, using an acid–base indicator (or the pH curve) to locate the equivalence point, and to understand how pH evolves through the four regions of a titration.
The reaction and the equivalence point
A neutralisation reaction combines hydrogen ions with hydroxide ions to form water. For a monoprotic acid HA titrated with a strong base, the net stoichiometry is one mole of base per mole of acid. The equivalence point is the volume of titrant at which the moles of added titrant exactly equal the moles of analyte originally present — it is a property of the chemistry. The endpoint is where the chosen indicator changes colour; a well-chosen indicator places the endpoint within the steep part of the curve so the two nearly coincide.
so the unknown C_a = (C_b · V_b) / V_a
Strong acid versus strong base
Both are fully dissociated. The pH is found purely from the moles of whichever ion is in excess, divided by the total volume. Before equivalence there is excess strong acid; at equivalence the solution is just water plus a neutral salt, giving pH 7 at 25 degrees Celsius; after equivalence there is excess strong base.
At eq: pH = 7.00 (neutral salt, no hydrolysis)
After eq: [OH−] = (excess mol OH−) / V_total, pH = 14 − pOH
Weak acid versus strong base — the buffer region
A weak acid only partially dissociates, governed by its acid dissociation constant Ka (here expressed as pKa). As strong base is added, it converts HA into its conjugate base A−, creating a buffer mixture. In this region the Henderson–Hasselbalch equation applies. At the half-equivalence point the amounts of HA and A− are equal, so pH equals pKa exactly — a convenient way to read off pKa from the curve. At the equivalence point all the acid has been converted to A−, which is a weak base that hydrolyses water, so the pH is above 7.
Half-equivalence: pH = pKa
At equivalence: [OH−] = sqrt(Kw / Ka · C_salt), pH > 7
Strong acid versus weak base
By symmetry, titrating a strong acid with a weak base (such as ammonia) builds a buffer of the weak base and its conjugate acid. The equivalence point here lies below 7, because the conjugate acid hydrolyses to give an acidic solution. The engine solves this with the base dissociation constant Kb and the analogous hydrolysis expression.
How the engine computes pH
After every increment the simulator recomputes total moles of acid and base, decides which region the system is in, and applies the exact closed-form expression for that region — excess-ion for strong–strong, Henderson–Hasselbalch in the buffer zone, and the appropriate hydrolysis square-root at and beyond equivalence (with a robust quadratic for the very first and last points to avoid log singularities). The equivalence point is independently located as the maximum of the numerical derivative dpH/dV of the recorded curve.
Procedure
- Pick a preset, or set up your own: choose the analyte in the flask (acid or base), its concentration and volume, and the standard titrant in the burette.
- Choose an indicator whose transition range sits inside the steep part of the expected curve. Phenolphthalein (8.2–10.0) suits strong–strong and weak-acid titrations; methyl orange (3.1–4.4) suits strong acid with a weak base.
- Press Refill burette & reset flask. The burette fills to 0.00 mL and the flask shows the starting indicator colour for its pH.
- Open the stopcock with the dispense chips. Add titrant in 1.0 mL steps at first and watch the curve rise gently through the buffer plateau.
- As the indicator begins to flash a transient colour on each drop, switch to 0.1 mL and finally the dropwise mode. Stop at the first permanent colour change — that is your endpoint.
- Read the burette volume at the endpoint. Compare it with the green equivalence point on the curve, and read the computed unknown concentration in the readout panel.
- For the find-the-unknown task, the analyte concentration is hidden; titrate to the endpoint and report the concentration you calculate from your endpoint volume.
Good practice modelled here
- The burette is read to the bottom of the meniscus to 0.01 mL.
- Adding past the endpoint overshoots; the curve and pH show the consequence immediately.
- The endpoint (indicator) and equivalence point (chemistry) are reported separately so you can see the small titration error between them.
Five questions, marked instantly with a worked explanation.
- Skoog, West, Holler & Crouch — Fundamentals of Analytical Chemistry, 9th ed., Ch. 14–16 (neutralisation titrations and titration curves).
- Harris — Quantitative Chemical Analysis, 9th ed., Ch. 10–11 (acid–base equilibria, buffers, the Henderson–Hasselbalch equation).
- Atkins & de Paula — Physical Chemistry, equilibria and the autoprotolysis of water (Kw).
- IIT / Virtual Labs (vlab.co.in) — Acid–Base Titration experiment, the apparatus and procedure modelled here.
- IUPAC — recommended pKa values: acetic acid 4.76, ammonia conjugate acid 9.25 at 25 degrees Celsius.