Ammonium bearing standards or samples in 0.1 M H2SO4
(5.00 mL) were transferred into the GCU by a transfer pipette.
The H2SO4absorber drop was manually dispensed and allowed to
stop at the conductivity probe. The 4 M NaOH solution (5 mL) was
then manually delivered to the GCU. The solution needs to be
strongly alkaline for ammonia to be fully released and this is
accomplished by the significant amount of NaOH added. Next the
MFC was activated to begin purging of the GCU; this led to both
mixing of the solution and transport of the evolved NH3out of the
GCU. As NH3 reacted with H
þ
in the drop, the conductivity
decreased; this was recorded for 200 s. The rate of decrease of
the conductivity (RDC) was calculated. The RDC for a blank was
similarly measured. The net RDC for various NH
þ
4
samples were
plotted against the known concentrations. The indophenol blue
method was used for comparative measurements[28]; the absorbance due to the blue product was measured at 630 nm spectrophotometrically