Which base was added in Heatley's reverse-extraction method for penicillin?

Sodium hydroxide was the base added in Norman Heatley’s reverse-extraction method for penicillin.

The Oxford team first moved penicillin from an acidic aqueous solution into a solvent such as amyl acetate. That created a new problem: recovering the fragile drug from the solvent without destroying it. Heatley reasoned that changing the solution from acidic to alkaline might force penicillin back into water.

Sodium hydroxide achieved that transfer. Heatley called the technique “reverse extraction,” because it reversed the earlier movement of penicillin from water into solvent. The aqueous solution could then be freeze-dried, a process Ernst Chain used to produce a dry, brown penicillin powder.

This was not the same as adding a growth supplement to make the mould produce more antibiotic. Sugars, salts, malt, alcohol, marmite and brewing yeast were tested separately. Reverse extraction was a purification step, and its success helped make clinical testing and later mass production possible.