[CH3:1][CH2:2][CH2:3][CH2:4][N:5]1[CH:10]([C:11]([NH:13][C:14]2[C:15]([CH3:21])=[CH:16][CH:17]=[CH:18][C:19]=2[CH3:20])=[O:12])[CH2:9][CH2:8][CH2:7][CH2:6]1.Cl.[OH-].[Na+]>O>[CH3:1][CH2:2][CH2:3][CH2:4][N:5]1[CH:10]([C:11]([NH:13][C:14]2[C:15]([CH3:21])=[CH:16][CH:17]=[CH:18][C:19]=2[CH3:20])=[O:12])[CH2:9][CH2:8][CH2:7][CH2:6]1Bupivacaine hydrochloride (Sigma-Aldrich Corporation, St. Louis, Mo.) was dissolved in de-ionized (DI) water at a concentration of 40 mg/ml (saturation). A calculated amount of sodium hydroxide (in the form of 1 N solution) was added to the solution and the pH of the final mixtures was adjusted to ten to precipitate the Bupivacaine base. The precipitated product was filtered, and further washed with DI water for at least three times. The precipitated product was dried at ca. 40° C. in vacuum for 24 hours.
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