Iodine-thiosulfate titrations (Core Practical 13b)

The titration reaction (spec 16.15)

I2(aq) + 2S2O3²-(aq) -> 2I-(aq) + S4O6²-(aq)

Ratio I2 : S2O3²- = 1 : 2. Thiosulfate goes in the burette. The brown iodine fades to pale yellow; then add a few drops of starch, giving a blue-black colour, and continue until it just turns colourless. Starch is added near the end point because at high iodine concentration the starch-iodine complex forms too strongly and does not release iodine easily, blurring the end point.

Copper(II) (spec 16.17 ii)

Excess potassium iodide is added to a copper(II) solution: 2Cu2+ + 4I- -> 2CuI + I2. A white precipitate of copper(I) iodide forms in the brown solution (it looks off-white or cream when the iodine is gone). Since 2Cu2+ gives 1 I2, which reacts with 2S2O3²-, Cu2+ : S2O3²- = 1 : 1.

Iodate(V) (standardising thiosulfate)

Potassium iodate(V) is a primary standard. With excess iodide and acid: IO3- + 5I- + 6H+ -> 3I2 + 3H2O. So IO3- : S2O3²- = 1 : 6. Other oxidising agents, such as chlorine in bleach or hydrogen peroxide, can be analysed the same way.

Worked example

6.25 g of copper(II) sulfate crystals are dissolved and made up to 250 cm³. 25.0 cm³ with excess KI needs 24.80 cm³ of 0.100 mol dm^-3 Na2S2O3. Find the percentage of copper.

1. n(S2O3²-) = 0.02480 x 0.100 = 2.48 x 10^-3 mol = n(Cu2+) in 25.0 cm³
2. In 250 cm³: 0.0248 mol; mass Cu = 0.0248 x 63.5 = 1.575 g
3. 1.575 / 6.25 x 100

Answer: 25.2% copper

Check yourself

1. 25.0 cm³ of iodine solution needs 18.40 cm³ of 0.100 mol dm^-3 sodium thiosulfate. Calculate the concentration of iodine.
   Show answer

   0.0368 mol dm^-3. n(S2O3²-) = 1.84 x 10^-3; n(I2) = 9.20 x 10^-4; c = 9.20 x 10^-4 / 0.0250 = 0.0368 mol dm^-3.

2. A solution containing Cu2+ ions is treated with excess KI. The iodine released needs 20.00 cm³ of 0.0500 mol dm^-3 sodium thiosulfate. How many moles of Cu2+ were present?
   Show answer

   0.00100 mol. Cu2+ : S2O3²- = 1 : 1, so n(Cu2+) = 0.02000 x 0.0500 = 1.00 x 10^-3 mol.

3. 25.0 cm³ of 0.0100 mol dm^-3 KIO3 is added to excess acidified KI. The iodine formed needs 24.20 cm³ of sodium thiosulfate. Calculate the thiosulfate concentration.
   Show answer

   0.0620 mol dm^-3. n(IO3-) = 2.50 x 10^-4; n(S2O3²-) = 6 x 2.50 x 10^-4 = 1.50 x 10^-3; c = 1.50 x 10^-3 / 0.02420 = 0.0620 mol dm^-3.

4. Why is starch added only when the iodine colour has faded to pale yellow?
   • Starch reacts with thiosulfate
   • At high iodine concentration the starch-iodine complex does not release iodine easily, so the end point is unclear
   • Starch is expensive
   • Starch turns iodine colourless
   Show answer

   At high iodine concentration the starch-iodine complex does not release iodine easily, so the end point is unclear. Adding it late gives a sharp blue-black to colourless change.

5. What is the colour change at the end point of an iodine-thiosulfate titration with starch?
   • Colourless to pale pink
   • Orange to green
   • Yellow to blue-black
   • Blue-black to colourless
   Show answer

   Blue-black to colourless. The last iodine reacts, so the blue-black starch complex disappears.

6. What is the white precipitate formed when potassium iodide is added to copper(II) sulfate solution?
   • Copper(II) iodide
   • Copper(I) iodide
   • Copper metal
   • Potassium sulfate
   Show answer

   Copper(I) iodide. 2Cu2+ + 4I- -> 2CuI + I2: copper is reduced to +1.

7. Using the worked example method: 6.25 g of copper(II) salt in 250 cm³; 25.0 cm³ needs 24.80 cm³ of 0.100 mol dm^-3 thiosulfate. What mass of copper is in the 6.25 g sample? (Cu = 63.5)
   Show answer

   1.57 g. n(Cu) = 2.48 x 10^-3 x 10 = 0.0248 mol; mass = 0.0248 x 63.5 = 1.575 g.

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Sources

• Pearson Edexcel IAL Chemistry (2018) specification
• Pearson Edexcel IAL Chemistry (2018) scheme of work
• Pearson IAL Chemistry teacher practical guide

Last updated 7 November 2026. Spotted a mistake? Tell us.