Sciencemadness Discussion Board

Preparation of HBF4...

Inorganic - 2-5-2010 at 07:16


I want to obtain tetrafluoroboric acid. I have a 40% HF and H3BO3... I'm afraid to obtain a mixture of acids HBF3OH and HBF4... Anyone know about this reaction, conditions and selectivity ?(how to obtain HBF4 with high selectivity?)

Thanks in advance.

not_important - 2-5-2010 at 08:08

SFAIK HBF4 is not stable as an isolated compound, only in solution.

According to Inorganic Synthesis volume 1, under the prep for KBF4, the common contaminates of fluoroborates are fuorides. In that prep they first make a solution of HBF4 by slowly adding B(OH)3 to aqueous HF in a platinum dish. Volume 2 has 2 methods for preparing NH4BF4.

In the Organic Syntheses entry for fluorobenzene (Coll. Vol. 2, p.295), they just make an aqueous solution from HF aq and B(OH)3 and add that to the diazonium solution with no isolation.

kmno4 - 2-5-2010 at 08:18

See also Brauer.

(.... 70-90% hydrofluoric acid. The reaction is highly exothermic. After the reaction is completed, the excess H3BO3 is allowed to settle out and the pure HBF4 is decanted.
Fluoroboric acid is stored in glass containers.)

Inorganic - 2-5-2010 at 09:17

Thanks for the reply

I forgot to add that I want to obtain a solution... not a free acid 100% HBF4...

Yes 70% - 90% HF should be a good but I have only 40% and this is a my problem...

When I add H3BO3 to 40% HF solution in slight excess(about 2% mol) would be obtain pure solution of HBF4 without HBF3OH ?

not_important - 2-5-2010 at 09:40

The Inorganic Sythesis uses 47% HF.

What interferes with your intended use? Ammonium bifluoride and H2SO4 could be used to make fairly concentrated HF containg some NH4 and SO4., would work for diazo reaction at the cost of needing some extra nitrite.

DJF90 - 3-5-2010 at 09:30

HBF4 is sold commercially as a 50% solution, probably due to stability issues that not_important brought up.

entropy51 - 5-5-2010 at 12:42

Quote: Originally posted by Inorganic  

I forgot to add that I want to obtain a solution... not a free acid 100% HBF4...

Yes 70% - 90% HF should be a good but I have only 40% and this is a my problem...

Using 40% HF is not a problem. Use 10 g of boric acid, added in small portions, to 32.5 g 40% HF in a paraffin coated beaker cooled in ice. This is relatively exciting and quite dangerous.

For preparation of flurobenzenes via the Balz-Schiemann reaction of diazoniums, however, it is much safer to use NaBF4 prepared from boric acid, HCl and NaF. The method is in Volume 5 of Organic Reactions, recently posted by Da Boss.

[Edited on 5-5-2010 by entropy51]

Fluoroboric acid

Inorganic - 20-5-2010 at 14:36

I wanted to confirm that the synthesis HBF4 with 40% hydrofluoric acid and H3BO3 was successful. Obtained by this method Fluoroboric acid should be concentration of 33.5%...

Thanks for help.

Paddywhacker - 20-5-2010 at 21:47

I worked in a place that did this reaction many years ago. Adding powdered boric acid to HF solution gives a violently exothermic reaction. We did it in a big steel bucket with lots of ammonia scattered around to neutralize the fumes.

Without care someone could easily die during this procedure, so take care.

I always thought that fusing the boric acid into glassy lumps would slow the reaction down a bit, but we never tried that. Only senior members of our firm took part in the procedure, on weekends, when nobody else was around.