CHM110H5 Chap.5 Laboratory Skills and the Iodine Clock
Laboratory Skills and the Iodine Clock
What the first lab is really testing
The opening experiment in CHM110H5 is described in the course manual as a low-stakes skills lab. You prepare your own iodine clock, time how long the mixture stays colourless, and compare your interval with a clock prepared by a Lab Coordinator.
The chemistry is simple on purpose: the point is to handle an analytical balance, a volumetric flask, graduated and volumetric pipettes, a burette, a micropipette and a stir plate correctly, and to follow the safety protocols that every later experiment assumes you already know.
Experiment 1 is assessed as a short report worth 4% of the course mark.
Why the colour waits and then appears all at once
Three reactions run in the beaker. Hydrogen peroxide oxidises iodide ions to free iodine in acid. Ascorbic acid reduces that iodine back to iodide, and because this second reaction is faster, free iodine cannot build up while any ascorbic acid is left.
Once the ascorbic acid has been used up, iodine concentration climbs quickly, iodine combines with iodide to form triiodide, and triiodide makes a dark blue complex with starch. The manual tells you to expect a yellow or orange tint just before the dark blue.
Since the length of the delay depends on the concentration of every reagent, small measuring errors become visible as a clock that runs fast or slow compared with the reference.
How the solutions are made
You weigh 0.2500 g of ascorbic acid on the analytical balance using a spatula from your locker, record four significant digits, and move the solid into a 25 mL volumetric flask through a powder funnel, rinsing the weigh boat into the flask.
Distilled water goes in until the meniscus base rests on the mark, then the solution is stirred, capped and inverted. The reaction beaker receives 2.5 mL of that solution by graduated pipette, 1 mL of iodine tincture by volumetric pipette, starch from a preset pump dispenser, 22.5 mL of water from a burette and finally 1 mL of hydrogen peroxide by micropipette, for 30.0 mL in total.
Work happens in the fume hood, the beaker travels under a watch glass, and timing starts at the first drop of peroxide.
Rules that cost marks if ignored
WHMIS training proof must be uploaded before you can enter the teaching labs.
A pre-lab with condensed procedure notes and a table of reagents, including hazards taken from safety data sheets, is due the Friday before your lab, and arriving unprepared can mean being sent out with a zero. Clean-up should start at least 15 minutes before the session ends, because leaving glassware dirty or waste undisposed brings an immediate 20% deduction.
The post-lab asks you to design a procedure for preparing an assigned mixed solution as accurately and precisely as the glassware allows, choosing glassware from the tolerance table in the manual, writing in past tense formal prose and showing every calculation.
What this chapter covers
- 01
Three competing reactions in the iodine clock
- 02
Why starch turns dark blue only after ascorbic acid runs out
- 03
Preparing a standard solution in a volumetric flask
- 04
Dilution with C1V1 = C2V2 into a 30.0 mL mixture
- 05
Glassware tolerances from the course manual
- 06
WHMIS, pre-lab and clean-up rules
- 07
Writing the post-lab procedure
Worked example · free
Heavier weighing, longer clock
- 2Moles of ascorbic acid: n = 0.3000 g ÷ 176.12 g/mol = 1.7034 × 10⁻³ mol.
- 2Stock molarity: c = 1.7034 × 10⁻³ mol ÷ 0.02500 L = 0.06813 M.
- 2Dilution into the beaker: C2 = 0.06813 M × 2.5 mL ÷ 30.0 mL = 5.678 × 10⁻³ M.
- 2Clock time scales with the ascorbic acid that must be consumed: 40.0 s × 0.3000 ÷ 0.2500 = 48.0 s.
Key terms
- Iodine clock
- A timed reaction in which a solution stays colourless until a reducing agent is used up, then suddenly turns dark blue as triiodide forms a complex with starch.
- Volumetric flask
- Glassware calibrated to hold one exact volume when the meniscus base rests on its mark, used to make standard solutions such as the 25 mL ascorbic acid stock.
- Mohr pipette
- A graduated pipette that delivers a chosen part volume, such as 2.5 mL, with a tolerance listed in the manual as ±0.02 mL for the 5 mL size.
- Tolerance
- The calibration limit on how accurate a piece of equipment can be, quoted in the manual for each item and added to the reading error when estimating uncertainty.
- Meniscus
- The curved liquid surface in narrow glassware; for aqueous solutions the bottom of the curve is lined up with the calibration mark when filling or reading volumes.
- Table of reagents
- A pre-lab table listing each reagent's appearance, molar mass, density for pure liquids, melting or boiling point, and hazards taken from its safety data sheet.
Laboratory Skills and the Iodine Clock FAQ
Why does the iodine clock stay colourless for a while and then change suddenly?
Ascorbic acid destroys iodine faster than hydrogen peroxide makes it, so free iodine stays near zero. When the ascorbic acid is exhausted, iodine accumulates, joins iodide as triiodide and binds starch, producing the dark blue colour within moments.
Which glassware should I pick when the post-lab asks for the greatest accuracy and precision?
Use the tolerance table in the manual. Volumetric pipettes and flasks carry the tightest limits for fixed volumes, a burette or Mohr pipette handles odd volumes, and graduated cylinders, at ±0.1 or ±0.2 mL, are the weakest choice for anything quantitative.
What happens if I run out of time to clean up after the lab?
The manual applies an immediate 20% deduction if glassware is not cleaned, waste is not disposed of properly or the bench is not restored. Begin clean-up at least 15 minutes before the session ends to protect that mark.
Exam move
Write the three clock reactions from memory and explain in one sentence why the second must be faster. Recalculate the ascorbic acid stock and beaker concentrations from 0.2500 g, then repeat with a mass of your own choosing and predict whether the clock runs faster or slower. Turn the tolerance table into flashcards and practise converting each tolerance into a percent for a realistic volume.
Before writing the post-lab, draft your assigned solution's calculations first, pick glassware item by item with its tolerance, then write the procedure in past tense formal prose.
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