Assay
pH9.3Buffer
Dissolve 23.8g of sodium borate and 402mg of boric acid in sufficient water to make 1500mLof solution,and mix.
Standard preparation
Dissolve a suitable quantity of
USP Cyanocobalamin RS,accurately weighed,in
pH9.3Bufferand dilute quantitatively,and stepwise if necessary,to obtain a solution having a known concentration of about 30µg per mL.
Assay preparation
Transfer an accurately measured volume of Injection,equivalent to about 5mg of hydroxocobalamin,to a 50-mLvolumetric flask containing about 25mLof pH9.3Buffer.Add 5.0mLof potassium cyanide solution (1in 10,000),allow to stand at room temperature for 30minutes,dilute with pH9.3Bufferto volume,and mix.Transfer 15.0mLof this solution to a second 50-mLvolumetric flask,dilute with pH9.3Bufferto volume,and mix.
Procedure
Concomitantly determine the absorbances of the solutions in 1-cm cells at the wavelength of maximum absorbance at about 361nm,with a suitable spectrophotometer,using
pH9.3Bufferas the blank.Calculate the quantity,in mg,of hydroxocobalamin (C
62H
89CoN
13O
15P)in each mLof the Injection taken by the formula:
(1346.36/1355.37)(0.1667C/V)(AU/AS),
in which 1346.36and 1355.37are the molecular weights of hydroxocobalamin and cyanocobalamin,respectively;
Cis the concentration,in µg per mL,of
USP Cyanocobalamin RSin the
Standard preparation;Vis the volume,in mL,of Injection taken;and
AUand
ASare the absorbances of the
Assay preparationand the
Standard preparation,respectively.