Physicochemical Characteristics
Ketamine ( Fig.
10-20
) has a molecular weight of 238 kd, is partially water soluble, and
forms a white crystalline salt with a negative log of the acid ionization constant
(pKa
) of 7.5.[285]
[444]
It has a lipid solubility 5 to 10 times that
of thiopental.[445]
Ketamine is prepared in a slightly
acidic (pH 3.5 to 5.5) solution, and the racemic mixture comes in concentrations
of 10-, 50-, and 100-mg ketamine base per milliliter of sodium chloride solution
containing the preservative benzethonium chloride. The ketamine
molecule contains a chiral center and therefore occurs as two resolvable optical
isomers or enantiomers. The commercial racemic preparation is a mixture of both
enantiomers [S-(+) and R-(-)] in equal amounts.[444]
S-(+)-ketamine is available in 5- and 25-mg/mL concentrations.
