The residue left behind contains alkaloids. A basic aqueous solution with a pH of at least 11 is then used in the extraction process. An alkali hydroxide usually gives the best results. This process changes the non-morphine-containing base into its morphine form which can dissolve in basic solutions having pH values of 11 and above (Kilpatrick 525).
Any resultant precipitates are filtered out of the basic aqueous solution. The remaining alkaloids are then separated from the aqueous solution containing morphine by drawing out, by the use of a solvent that cannot mix with water. Such solvents include benzene, xylene, diethyl ether, chloroform, or toluene. Toluene is preferred in this process since it harbors no health hazards. During extraction while using these solvents, about eight percent of morphine gets extracted into the solvent. Recovery of this is attained by back extraction by using an additional basic solution.