Adjust pH to 3.0 with dilute H2SO4 or KOH. Sterilise
the medium by filtration. Alternatively, autoclave separately the basal
medium (pH adjusted to 3.0) and the ferrous sulphate (40 g FeSO4
x 7 H2O in 100 ml distilled water, pH not adjusted, in a
sealed vessel under nitrogen gas atmosphere).