Potassium tetrafluoborate

Preparation of Boron Tribromide. (Perform the experiment in a fume cupboard ) Assemble a part of the apparatus shown in Fig. 112 beginning from vessel 3 without a gas-discharge tube. Put 54 g of aluminium tribromide and 13 g of potassium tetrafluoborate into reaction vessel 3 and close it. Carefully heat the vessel with the flame of a burner. Prepare and purify the boron tribromide in the same way as boron trichloride. 

Large quantities of boron chloride are best prepared by the chlorination of a mixture of boron oxide (or borax) and carbon. Smaller amounts may be obtained (1) by the chlorination of amorphous boron or metallic borides or (2) by the reaction between boron oxide and phosphorus (V) chloride in a sealed tube. A more convenient laboratory method for the preparation of boron chloride makes use of the reaction between boron fluoride and aluminum chloride. For the preparation of boron bromide, the reaction of boron fluoride and aluminum bromide is recommended as much less troublesome than other methods. Potassium tetrafluoborate may be used in place of the boron fluoride in each of these syntheses. 

Modification. One hundred thirty-three grams (1 mol) of aluminum chloride (or 267 g. of aluminum bromide) and 62 g. (0.5 mol) of potassium tetrafluoborate are placed in the apparatus described above (or in an ordinary distilling flask) and heated by means of an oil bath for 4 hours, with the temperature slowly increasing from 150 to 175. The product is condensed by liquid nitrogen and then fractionated. Yield BCU, 30 g. (0.26 mol) (52 per cent based on KBF4) BBrs, 37.6 g. (30 per cent). 

Br8 Boron bromide, 3 27, 29 BCls Boron chloride, 3 27, 28, 29 BFs Boron fluoride, 1 21, 23 BFiH Tetrafluoborie aeid, 1 25 BF K Potassium tetrafluoborate, 1 24... 

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