Which acid was added during the Oxford team's penicillin extraction process?

Phosphoric acid was added during the Oxford team’s penicillin extraction process. It acidified the filtered culture liquid, allowing penicillin to be transferred into an organic solvent during the next stage of purification.

After mould growth, the broth was filtered through parachute silk to remove mycelium, spores and other solid material. The clarified solution was then acidified with phosphoric acid and cooled. At low temperature, the team could extract penicillin while slowing its chemical breakdown.

The chemistry was delicate because penicillin was stable only within a limited pH range. The process therefore required careful control of acidity and temperature rather than simply adding an acid indiscriminately. Once acidified, the penicillin could be drawn into a solvent, first ether and later the safer amyl acetate.

A frequent mix-up is to confuse phosphoric acid with the solvent. Phosphoric acid adjusted the aqueous phase; it did not carry the penicillin away. The solvent performed that separation after acidification.