The acid-base titration suffers one major flaw: in order to use it, the analyte must be non-neutral in solution - that is, it must react with water to form either the hydronium ion (H3O+) or the hydroxide ion (OH-). If it doesn't form either of these ions, it does not change the pH and therefore we cannot measure it using acid-base titration.
One way around this is to add to our analyte a species that reacts stoichiometrically in an oxidation-reduction (or redox) reaction to give a product, and then measure that (usually by means of an indicator). One very common pair for redox titrations is iodine (I2)and thiosulfate (S2O32-). When these two are added, the following half-reactions occur:
2S2O32- → S4O62- + 2e-
I2 + 2e- → 2I-
The thiosulfate is oxidised to form the tetrathionate ion, while the iodine is reduced to form the iodide ion. As an indicator we use starch, which goes a very dark purple in the presence of iodine.
Sample Experiment - Determining the Concentration of Iodine
For this experiment you will need:
NB: For a tutorial on how to operate a burette, check out DrSeudo's rather good Procedure write-up in Acid-base titration.
Pipette 10 mL of iodine solution into the flask, and fill the burette with thiosulfate. Slowly add the thiosulfate solution until the iodine is a very pale yellow (it should start off orange-yellow). Now add a couple of drops of starch solution (which will turn the solution dark blue)*. Add thiosulfate solution dropwise until the solution goes completely colourless. Note the amount of thiosulfate added. You may wish to repeat the experiment in order to smooth out any errors.
Volume of thiosulfate added: 9.8 mL or 9.8x10-3 L
Concentration of thiosulfate: 0.032 mol L-1
Amount of thiosulfate added: 0.032 * 9.8x10-3 = 3.136x10-4 mol
Thiosulfate reacts at a 1:1 ratio with iodine in this case, so the amount of iodine in solution must also be 3.136x10-4 mol.
Volume of iodine solution: 10 mL or 1x10-2
Concentration of iodine solution: 3.136x10-4
= 0.03136 mol L-1
Rounding to 2 s.f., the concentration of the iodine solution is 0.031 mol L-1.
* Why don't we just add the starch indicator at the start? Well, the reaction between starch and iodine is a rather complex one, and it so happens that if we have a high concentration of iodine when we add the starch, we get a complex that doesn't work as well as an indicator as if we leave it until most of the iodine has been tied up by the thiosulfate.