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Ibuprofen crystals under a microscope
Ibuprofen is a widely used non-steroidal anti-inflammatory and analgesic drug, sold in a variety of forms.

There are a number of ways of synthesising ibuprofen, which used to be sold as a racemic mixture. However the (S)-isomer is about 100 times more active than the (R)-isomer.

(S)-Ibuprofen
The separation is achieved by using an enzyme to catalyse the hydrolysis of an ester of ibuprofen. The enzyme acts on only one of the ester enantiomers.
The enzyme used is lipase from candida rugosa, a fungus.
The enzyme is entrapped in the pores of a hollow fibre membrane and the ester is delivered in the organic phase to the outside of the membrane.

The membrane gives a high surface area of contact between aqueous and organic phases without allowing them to mix.
In a second membrane system ibuprofen is extracted into the aqueous phase at higher pH. The unreacted ester remains in the organic phase and is recycled.

The half-life of the lipase is about 30 days.

Question

Can you think of a reason why the concentration of ibuprofen is kept low in the reactor (by running it at low pH)?

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