The propensity for iodine to catenate is well illustrated by the numerous polyiodides, which crystal — Organic Chemistry Chemistry Question

The propensity for iodine to catenate is well illustrated by the numerous polyiodides, which crystallise from solutions containing iodide ions and iodine. The stoichiometry of the crystals and the detailed geometry of the polyhalide depend very sensitively on the relative concentrations of the components and the nature of the cation.

In this experiment, you will generate and crystallise a quaternary ammonium polyiodide salt of the form Me₄N⁺In^- (n = 3, 5 or 7) and then titrate the amount of iodine in the anion using sodium thiosulphate. From the results of this analysis, you can determine which anion is present in your salt.

Experimental
Two salts, A and B, of different composition may be prepared by using different quantities of starting materials, as shown below. You can carry out the experiment for either one or both.

Salt A | Salt B
mass of NMe₄I / g: 1.00 | 0.50
mass of iodine / g: 1.26 | 1.26

Chemicals and reagents
* tetramethylammonium iodide, solid
* iodine, solid
* sodium thiosulfate, aqueous solution, c = 0.1 mol dm^-3
* dichloromethane, liquid

Procedure
a) Add the iodine to a 100 cm₃ beaker containing 25 cm₃ ethanol and a magnetic bar. Heat and stir the solution until all the iodine has dissolved, then add the tetramethylammonium iodide. Continue to stir with moderate heating until no white solid remains. Do not allow the solution to boil at any time.
b) Allow the solution to cool slowly to room temperature and finally in an ice bath over about 15 – 20 minutes.
c) Collect the product under suction (Hirsch funnel) and wash on the filter with cold ethanol (10 cm₃) followed by ether (10 cm₃) using a disposable pipette.
d) Allow the product to dry on the filter for several minutes, and then transfer the crystals onto a filter paper. Place into a desiccator and leave under vacuum to dry.

Analysis
e) Weigh approximately 0.5 g of the product onto a weighing boat using a four decimal place balance. Record the weight accurately.) Using a distilled water wash-bottle, carefully transfer all the weighed product into a 250 cm₃ bottle.
g) Add approximately 25 cm₃ of dichloromethane, replace the stopper and shake to extract the iodine into the organic layer.
h) Fill a 50 cm₃ burette with sodium thiosulfate (c = 0.100 mol dm^-3) using a small glass funnel.
i) Remove the funnel and titrate the iodine by running small quantities of the sodium thiosulfate from the burette and then replacing the stopper and shaking the bottle.
j) The end-point is very sharp and is given by the removal of all iodine colour from the dichloromethane.