Preparation, Analysis and Photolysis of Manganese II Iodide

Manganese (II) iodide is photosensitive but its solution can be prepared and studied when freshly prepared. The simplest preparation is from Mn carbonate and HI solution. However, commercial carbonate is coloured, an indication of partial oxidation. Hence the carbonate has to be prepared freshly by reacting die sulphate solution with sodium bicarbonate solution. Its use, rather than sodium carbonate, is to avoid the formation of a basic solid. 

The iodide solution is analysed for iodide by Andrews titration (Sec.8.3.3) ie titration in presence of cone. HCl and a small amount of an organic liquid to indicate the endpoints. 

When Mn I2 solution is irradiated by a low pressure mercury lamp, the iodine formed by photolysis can be determined by titration against standardised sodium thiosulphate solution. 

This is the first stage in the preparation. Commercial samples are impure. Dissolve 19.8 g MnCL.4H20 in 50 cm water. Warm the solution and add slowly with constant stirring 250 cm of a solution containing 16.8 g NaHC03. Filter quickly the precipitate under suction while pressing between filter paper. 

Dilute 20 cm HI solution (nominally 66% HI) to 100 cm and add dropwise a solution of 16.4 g Na2S03.7H20 until the colour of the contaminating I2 just disappears. 

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