Preparation of Boron Tribromide

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. 

Preparation of Boron Trifluoride Ammine. Assemble an apparatus as shown in Pig. 113. Place flask 1 into a bath with ice. To produce ammonia, spill solid granulated sodium or potassium hydroxide into bottle 2. Pour a concentrated ammonia solution into the dropping funnel. 

To prepare boron trifluoride, mix 50 g of potassium fluoborate and 9 g of boric acid anhydride in flask 3, and pour 25 ml of anhydrous sulphuric acid into the mixture. Prepare the anhydrous sulphuric acid by adding the relevant amount of 65 % oleum to a 96 % sulphuric acid solution carefully, wear eye protection ). 

Before the synthesis, displace air from the apparatus with a strong stream of ammonia. Next pass the ammonia and boron trifluoride through the apparatus at the same rate. (If excess boron trifluoride is introduced, it evolves in the form of white smoke from discharge opening 5.) 

Boron trifluoride ammine deposits on the walls of flask 1 in the form of a bulky, loose powder. To prevent clogging of the tubes feeding in the gases, they must be at least 7-8 mm in diameter. Gather half of the prepared substance in a drawn out test tube and seal it. 

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