Sodium borohydride (5.58g 95%, 0.14 mole, 16.6% excess) and 300mL of THF were introduced in a 500mL oven-dried RB flask, fitted with a rubber cap on the side arm, a magnetic stirring bar, and a reflux condenser connected to a mercury bubbler. To the slurry solution 100mL of THF solution of zinc chloride (8.1774g, 0.6 mole) was added slowly with stirring by means of a hypodermic syringe. The solution was stirred for 3 days, and then allowed to stand at room temperature to permit the sodium chloride precipitate to settle down. Clear chloride-free supernatant solution was visualized after 2-3 h and an aliquot was removed by a syringe and analyzed for hydride gasometrically.
The slight excess (16.6%) of sodium borohydride greatly faciliates the quantitative precipitation of chloride as sodium chloride. The hydride concentration of the solution obtained was 1.37 M, which is larger by 14% than calculated value, 1.2 M based on zinc chloride added. This means that excess amount of sodium borohydride was dissolved in this hydride solution as Na[Zn(BH4)3] whereas sodium borohydride alone hardly dissolve in THF.
The solution was stored under positive nitrogen pressure without filtering out the precipitate and did not show any measerable change of the concentration over 30 days.
Reference: Journal of the Korean Chemical Society, Vol. 20, No. 1, 1976