FLUOROBORIC ACID AND FLUOROBORATES

Properties. Fluoroboric acid is stable in concentrated solutions, and hydroly2es slowly in aqueous solution to hydroxyduoroborates. For the stabihty of the duoroborate species, see Reference 3. The equiUbrium quotients (4,5) in 1 molal NaCl at 25°C show the strong affinity of boron for duo ride  

The hydrolysis of BF occurs stepwise to BF OH, BF2 (OH)2, and BF(OH)3. By conductivity measurements the reaction of boric acid and HF was found to form H[BF2(OH)] [15433-40-6] rapidly subsequentiy HBF formed much more slowly from HBF OH. These studies demonstrate that BF is 

Kinetic results (5) and F nmr experiments (6) illustrate cleady that the hydroxyduoroborates are in rapid equiUbrium and easily exchange duoride. 

The duoroborate ion has traditionally been referred to as a noncoordinating anion. It has shown Httie tendency to form a coordinate—covalent bond with transition metals as do nitrates and sulfates. A few exceptional cases have been reported (13) in which a coordinated BF was detected by infrared or visible spectroscopy. 

Manufacture, Shipping, and Waste Treatment. Fluoroboric acid (48%) is made commercially by direct reaction of 70% hydroduoric acid and boric acid, H BO (see Boron compounds). The reaction is exothermic and must be controlled by cooling. 

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Compounds containing fluorine and chlorine are also donors to BF3. Aqueous fluoroboric acid and the tetrafluoroborates of metals, nonmetals, and organic radicals represent a large class of compounds in which the fluoride ion is coordinating with trifluoroborane. Representative examples of these compounds are given in Table 5. Coordination compounds of boron trifluoride with the chlorides of sodium, aluminum, iron, copper, 2inc, tin, and lead have been indicated (53) they are probably chlorotrifluoroborates. 

Fluoroboric acid and some fluoroborate solutions are shipped as corrosive material, generally in polyethylene-lined steel pads and dmms or in rigid nonretumable polyethylene containers. Acid spills should be neutralized with lime or soda ash. 

Manufacture. Fluoroborate salts are prepared commercially by several different combinations of boric acid and 70% hydrofluoric acid with oxides, hydroxides, carbonates, bicarbonates, fluorides, and bifluorides. Fluoroborate salts are substantially less corrosive than fluoroboric acid but the possible presence of HF or free fluorides cannot be overlooked. Glass vessels and equipment should not be used. 

Potassium Fluoroborate. Potassium fluoroborate is produced as a gelatinous precipitate by mixing fluoroboric acid and KOH or K CO. Alternatively, fluorosihcic acid is treated with H BO in a 2 1 molar ratio to give HBF OH, which reacts with HF and KCl to yield 98% of KBF in 98.5% purity (52). Commercial KBF normally contains less than 1% KBF OH. 

Nickel Fluoroborate. Fluoroboric acid and nickel carbonate form nickel fluoroborate [14708-14-6] Ni(BF 2 6H20. Upon crystallization, the high purity product is obtained (47). Nickel fluoroborate is used as the electrolyte ia specialty high speed nickel plating. It is available commercially as a concentrated solution. 

Boric acid and fluoride ion react to form a series of fluroborates where OH is displaced by (see Fluorine compounds, inorganic—Boron, FLUOROBORic acid). Stepwise formation of the ions fluorotrihydroxyborate [32554-53-3] BF(OH) 3, difluorodihydroxyborate [32554-52-2] BF2(OH) 2 trifluorotrihydroxyborate [18953-00-9] BF3(OH) 3, proceeds rapidly in acidic solutions, but tetrafluoroborate [14874-70-5], forms slowly (56). A... 

A further spectrophotometric method [3, 4] for water soluble boron in soil, boron is extracted from soil with boiling water. Borate in the extract is converted to fluoroborate by the action of orthophosphoric acid and sodium fluoride. The concentration of fluoroborate is measured spectrophotometrically as the blue complex formed with methylene blue and which is extracted into 1, 2-dichloroethane. Nitrates and nitrites interfere they are removed by reduction with zinc powder and orthophosphoric acid. 

A boron analog - sodium borohydride - was prepared by reaction of sodium hydride with trimethyl borate [84 or with sodium fluoroborate and hydrogen [55], and gives, on treatment with boron trifluoride or aluminum chloride, borane (diborane) [86. Borane is a strong Lewis acid and forms complexes with many Lewis bases. Some of them, such as complexes with dimethyl sulfide, trimethyl amine and others, are sufficiently stable to have been made commercially available. Some others should be handled with precautions. A spontaneous explosion of a molar solution of borane in tetrahydrofuran stored at less than 15° out of direct sunlight has been reported [87]. 

The impure metal dissolves easily in mineral acids and in fluoroboric, sulfamic am trifluoromethylsulfonic acids to give Cr2+ solutions, but oxidation of Cr2+ by hydrogen ion (equation 6), °(Cr3+, Cr2+) = —0.41 V) even in an inert atmosphere is catalyzed by thi impurities and various ions.71 Indefinitely stable chromium(II) solutions can be obtained fron the pure (electrolytic) metal (99.5% or better), although the reaction with acid may need to b< initiated by heat and the inclusion of some metal previously attacked by acid. The use of ai excess of metal, which can be filtered off, ensures that little acid remains. In near neutra solution the hydrogen potential is lowered and the Cr2+ ion is stable. In alkaline condition brown Cr(OH)2, which slowly reduces water, precipitates.73,73... 

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