Preparation of Ammonium Tetrafluoroborate

When boric acid is dissolved in sulphuric acid solution and the solution heated with ammonium fluoride, the ammonium tetrafluoroborate is formed. 

Add carefully and slowly 15 cm cone, sulphuric acid to 25 cm of water stirred in a beaker cooled by iced water. Remove the coolant and add to the acid solution 6 g powdered boric acid and heat to dissolve it. Transfer to the fume cupboard and place on a hot plate/magnetic stirrer (Teflon-covered magnet). Add to the hot liquid, while stirring 15 g ammonium fluoride in small amounts (to avoid any loss of HF), continue heating below 100 C for half an hour. Allow to cool and then place in an ice/water bath to complete precipitation. When crystallisation seems complete, filter under suction. Dry by continued suction while pressing between filter paper. Transfer the crystalline product into a beaker containing 15 cm acetone. Shake well and refilter and finally weigh your product to the nearest eg and calculate the % yield based on ammonium fluoride. 

Recrystallise by dissolving the crude into a small volume of hot water to which 1 cm of cone, ammonia has been added. Boil to precipitate any silica impurity. Decant the clear liquid or filter into a porcelain basin. Concentrate on a hot water bath and allow to ciystallise by cooling. Collect the final product by filtration as above. 

---