Norman Heatley suggested transferring penicillin’s active ingredient back into water by changing the solution’s acidity.
Heatley was the junior biologist on Howard Florey’s Oxford penicillin team at the Sir William Dunn School of Pathology. The researchers first moved penicillin from an aqueous culture fluid into an organic solvent under acidic conditions. They then faced the problem of recovering the fragile antibiotic from that solvent without destroying it.
Heatley reasoned that the process should work in reverse: if penicillin moved from water into solvent when the mixture was acidic, it might move back into water when the mixture became alkaline. Adding sodium hydroxide confirmed the idea. Heatley called this “reverse extraction,” and it became a key step in concentrating and purifying penicillin.
The method was part of a larger chain of innovations. Ernst Chain used freeze-drying to remove water without boiling the unstable compound, while Heatley also devised an assay for measuring penicillin’s activity. These advances helped the Oxford group produce enough material for animal experiments and the first human treatments, although wartime shortages still made supplies extremely scarce.