QiLing Tools

🫗Titration Curve Simulator - Equivalence Points, pH Jump & Indicators

A titration curve shows how the pH of the analyte changes as titrant is added. Take 25.00 mL of 0.1000 M acetic acid titrated with 0.1000 M NaOH: the initial pH is 2.88, at half-equivalence (12.50 mL) pH = pKa = 4.75, and at the equivalence point (25.00 mL) the solution is 0.05 M sodium acetate with pH ≈ 8.72. Between −0.1% and +0.1% titration error the pH jumps from 7.74 to 9.70, so phenolphthalein works and methyl orange does not. This simulator solves the full charge-balance equation at every point using the diluted concentrations, every dissociation step and water autoionisation, instead of switching between region-by-region approximations, so the initial point, buffer region, equivalence point and excess region are all equally accurate. The analyte can be a strong acid or base, a weak monoprotic acid or base, a polyprotic acid (phosphoric, carbonic, oxalic, sulfuric, citric, phthalic and more) or one of their salts (Na₂CO₃, NaHCO₃, Na₂HPO₄, potassium hydrogen phthalate and others); the titrant can be NaOH, KOH, Ba(OH)₂, HCl, HNO₃, H₂SO₄, ammonia or acetic acid. The tool marks each equivalence point's volume and pH, the half-equivalence points and the ±0.1% pH jump, applies the c·K ≥ 10⁻⁸ and 10⁴ K-ratio rules to decide whether each step can be titrated, and compares the result with the transition ranges of 11 indicators including methyl orange, methyl red, bromothymol blue and phenolphthalein to recommend suitable ones. Hover or drag on the chart to read the pH at any volume, list pH at chosen volumes, and export the data as CSV or the chart as PNG or SVG.

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How to use Titration Curve

  1. 1Choose the analyte and titrant, enter concentrations and volume, or tap an example such as NaOH vs acetic acid or HCl vs Na₂CO₃
  2. 2Read the curve, equivalence-point pH, pH jump and recommended indicators; drag on the chart for a readout
  3. 3Tick indicator bands, enter volumes for the pH table, and export CSV, PNG or SVG

FAQ

Why is the equivalence point pH not always 7?
Only strong acid–strong base titrations give a neutral salt at equivalence (pH 7). Titrating a weak acid with a strong base leaves its conjugate base, which hydrolyses to give a basic solution: NaOH vs acetic acid reaches pH ≈ 8.72. A weak base titrated with strong acid gives an acidic equivalence point, about pH 5.28 for ammonia with HCl. That pH decides which indicator to use.
How do I choose an indicator for a titration?
The indicator's transition range should fall wholly or partly within the pH jump and ideally contain the equivalence-point pH. For 0.1 M HCl vs NaOH the jump is 4.30–9.70, so methyl orange, methyl red and phenolphthalein all work; for acetic acid it is 7.74–9.70, so only indicators changing in the basic region such as phenolphthalein are suitable. The tool lists recommended and usable indicators for every equivalence point.
When can a weak acid be titrated accurately?
The usual rule is c·Ka ≥ 10⁻⁸, which gives a break sharp enough for an indicator end point within about ±0.2%. To titrate a polyprotic acid stepwise, adjacent Ka values must also differ by at least 10⁴. Phosphoric acid meets this for its first two steps but c·Ka₃ is too small to see the third; oxalic acid's Ka₁/Ka₂ is only about 10³, so it titrates in one step to the second equivalence point.
Why does titrating sodium carbonate with HCl use two indicators?
Na₂CO₃ has two equivalence points with HCl: first NaHCO₃ forms (pH ≈ 8.3, phenolphthalein), then H₂CO₃ (pH ≈ 3.9, methyl orange). This double-indicator method is used to analyse mixtures of NaOH, Na₂CO₃ and NaHCO₃. Because Kb₁/Kb₂ is only about 10⁴, the first break is weak, and the tool flags it accordingly.