The possibility to use ammonium hydroxide to control the pH in fed-batch microbial fermentations would have the advantage over sodium hydroxide that it could be used as a nutrient thus avoiding the increase of the media osmolality. However, this reaction equilibrium should be analysed carefully:
NH3 + H2O = NH4+ + OH-
At 25ºC and infinite dilution:
[NH4+][ OH- ]/[ NH3] = 0.000018
And when temperature or pH rises, free ammonia could be given off, polluting the outlet gas of the bioreactor with this extremely toxic component.
NH4+ + OH- = NH3 + H2O
Due to this reason, ammonium hydroxide only could be used as pH corrector in solutions which are slightly acid, thus in the case of pH = 5, where pOH = 14-5 = 9.
[ NH4+](0.000000001)/[ NH3] = 0.000018 [NH4+]/[ NH3] =18000
In this particular case, the main quantity of the ammonium hydroxide appears to be in the form of ammonium ion which will be consume as nitrogen source and therefore it could be used to rise the pH without the production of free ammonia.
For this reason, ammonium hydroxide shouldn´t be used to adjust the pH in fed-batch microbial fermentations of Escherichia coli where pH 7 is necessary, but it would be very convenient in the case of the methylotrophic yeast Pichia pastoris which optimal growth takes places at pH 5.