IB Chemistry IA idea · Equilibrium, acids and buffers

IB Chemistry IA idea: how much iodine is locked up as triiodide?

Iodine dissolves poorly in water, but far better with iodide, because I3- forms.

Equilibrium, acids and buffers
SL ideal
HL good
Lab Extended Essay potential
Syllabus: R2.3.2, R2.3.3, R2.3.4, T1

By Photon Academy · Updated 13 November 2026

These ideas are starting points, not an IA to copy. Never copy an idea blindly: make the question, method and analysis your own, and check your plan with your teacher.

Example research question

To adapt, never to copy

How does the concentration of potassium iodide added to a fixed amount of iodine (0.01, 0.025, 0.05, 0.075 and 0.10 mol dm⁻³) affect the absorbance at about 460 nm, and the Kc of I₂ + I⁻ ⇌ I₃⁻?

Change the system, the range or the method so the question is yours. Check it against the research question checklist in our Chemistry IA guide.

Variables

You change: Concentration of potassium iodide added to a fixed amount of iodine, 0.01 to 0.10 mol dm-3 (5 or more values)
You measure: Absorbance of triiodide at about 460 nm (colorimeter, blue filter), and Kc for I2 + I- <-> I3-
You keep the same: Total iodine
Temperature
Wavelength and cuvette
Time for equilibrium

Method

Make iodine solutions with different iodide concentrations, read absorbance after equilibrium, and obtain the triiodide concentration using a calibration or the molar absorptivity from the literature.

Data analysis

Calculate equilibrium concentrations by an ICE table; compute Kc at each iodide concentration; test its constancy; compare with the literature value (about 700 dm3 mol-1 at 25 degrees C, check the source); propagate uncertainty.

How it works

You change Potassium iodide concentration (mol dm⁻³) and measure Absorbance of I₃⁻ in Iodine in KI(aq), keeping Iodine amount, Wavelength or filter, Temperature the same. Expect a graph of Absorbance at 460 nm against [KI] / mol dm⁻³: More iodide should lock up more iodine, then it levels.

Make it yours

Repeat at different temperatures for Delta H.

Extended Essay angle

Use a full spectrophotometric method with a variable path length and the effects of ionic strength.

Safety

Take care with: hazardous chemicals.

Iodine is harmful and stains; dilute solutions only.

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