Chemistry 11–12 · Year 11

The iodine clock: a sharp end point for measuring rate with a data logger

Module 3: Reactive Chemistry

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

A reaction whose product is masked until a fixed amount of a second reagent is consumed gives a sudden colour change, so the time to that change measures the average rate exactly.

Safety card

Medium riskLearners carry it out, with a teacher supervising

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

glacial ethanoic acid is corrosive (technician prepares B in a fume cupboard)
hydrogen peroxide irritant
iodine stains
sulfur dioxide if acid meets thiosulfate directly

Controls

learners handle only the diluted solutions A and B
never mix acid into solution A
eye protection
gloves for solution B
wastes to the sink with running water

Note

NSW Department of Education Chemical Safety in Schools (CSIS) package, 2021 Technical Update: Section 1.7 (risk assessment) and Volume 2 Appendix D (generic assessment advice and DoE chemical categories); record a RiskAssess (riskassess.com.au) risk assessment before the lesson and check the school's hazardous chemical register (CSIS Section 1.9) for local restrictions.

What you need

Solution A (per litre): 0.2 g soluble starch boiled into 100 mL water then diluted, 4.1 g sodium ethanoate, 50 g potassium iodide, 9.4 g sodium thiosulfate pentahydrate; 100 mL per run
Solution B (per litre): 30 mL glacial ethanoic acid and 500 mL 20-volume hydrogen peroxide made up with water; 100 mL per run
250 mL beakers, 100 mL measuring cylinders, two
Light sensor and data logger, or a stopwatch
Water baths for 10, 20, 30 and 40 degrees Celsius, thermometer
Deionised water for diluting solution A

How to do it

Measure 100 mL of A and 100 mL of B at room temperature; pour both into the beaker at the same instant and start the logger or clock; record the time the mixture turns blue-black.
Repeat at four temperatures, warming or cooling both solutions to the target before mixing; record the actual temperature of the mixture.
Concentration series: keep 100 mL of A and mix 80 mL of B with 20 mL water, then 60 with 40, then 40 with 60 (diluting the peroxide, not A, because A carries both the iodide and the thiosulfate that sets the delay); time as before.
Plot 1/time against temperature and against the fraction of B; compare the shape of each plot with the thiosulfate practical.
Export the light-sensor trace and mark the end point on it; report the uncertainty of each time from the trace width.

What you should see

At room temperature the mixture stays colourless for about 20 seconds (the RSC figure for 100 mL of each solution) and then turns blue-black suddenly (the thiosulfate that was consuming iodine runs out and iodine binds to starch). The delay shortens as the temperature rises and lengthens as B is diluted, with 1/time close to proportional to the fraction of B because the iodine-forming step is first order in hydrogen peroxide while the amount of thiosulfate to be used up stays the same: from about 20 s, the mixtures with 80, 60 and 40 percent of B should take about 25, 33 and 50 s. The light-sensor trace drops sharply from its plateau at the end point, and the width of that drop on the trace gives the timing uncertainty for each run.

What changes

What you change: temperature, or the dilution of solution B
What you measure: time to the colour change (s), converted to 1/time
What you keep the same: volume of A
total volume
mixing method
starch and thiosulfate amounts per litre
temperature (in the dilution series)

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

The colour change happens when the reaction starts (it marks the moment the thiosulfate is used up; the iodine-forming reaction ran the whole time).
A faster clock means a bigger reaction (the amount is fixed by the thiosulfate; only the time changes).
Blue means iodine has been made for the first time (iodine formed continuously and was removed by thiosulfate until it ran out).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document)CH11-10 CH11/12-2 CH11/12-3 CH11/12-4
Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-11-03 CH-11WS-02 CH-11WS-04
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities