IODINE/ THIOSULPHATE TITRATIONS

Iodine, I₂, is sparingly soluble in water but dissolves in potassium iodide solution, KI(aq), in which it forms a complex ion, KI₃ , potassium iodide.

I₂ (s) + I^- (aq) I₃^- (aq)

This dissociates so readily that it can be regarded simply as a solution of iodine in potassium iodide. It is possible to use the colour of the iodine solution as an indicator, but this becomes very faint towards the end point. It is better to use a little starch, which forms an intensely blue compound with free iodine. The colour disappears suddenly when the last trace of iodine has been reduced. If the starch is added too early iodine is strongly adsorbed onto it and accuracy is reduced.

Reduction half-equation

I₂ (aq) + 2e^- 2I^- (aq) ( I₂ - oxidising agent)

Oxidising half-equation

2S₂O₃ ^2- (aq) S₄O₆ ^2- (aq) + 2e^- ( S₂O₃ ^2- - reducing agent)

thiosulphate tetrathionate

ion ion

Adding the two half-equations

I₂ (aq) + 2S₂O₃ ^2- (aq) 2I^- (aq) + S₄O₆ ^2- (aq)

EXPERIMENT

To determine the concentration of a solution of sodium thiosulphate using a standard solution of iodine (0.05 mol dm^-3 )

Method

· Pipette 25cm³ of iodine solution into a conical flask.

· Titrate the iodine against the thiosulphate solution adding a little starch near the end point - when the solution is a pale yellow colour. A blue colour forms. The end point is indicated by the sudden disappearance of the blue colour.

Results

Burette solution	Sodium thiosulphate
Pipette solution	25cm^3 0.05M iodine solution
Indicator	Starch
Titration	Trial	1	2	3
Final reading cm^3
Initial reading cm^3
Volume added cm^3
Mean titre cm^3

Calculations

I₂ (aq) + 2S₂O₃ ^2- (aq) 2I^- (aq) + S₄O₆ ^2- (aq)

25cm³ of 0.05 mol dm^-3 iodine oxidised cm³ of sodium thiosulphate solution.

1. Calculate the number of moles of iodine used in the titration.

2. Calculate the number of moles of thiosulphate needed for reaction.

3. Calculate the concentration of the thiosulphate solution.