In order to monitor the repeatability and reversibility of the
sensor membranes, buffered solutions of pH 4.0, 7.0 and 12.0 were
subsequently pumped through the flow cell. Fig. 8 shows the results
obtained for a period of 1.5 h. The relative standard deviations
(RSDs) of results are less than 0.4%. In order to investigate the lifetime
of the sensor membranes, they were maintained in water/ethanol
(50:50, v/v) for 3 months at 3–8 C. No significant variation in
sensor response was observed during this period. On the other
hand, due to the pH indicator (methyl orange) is covalently bonded
to the triacetylcellulose membrane, there was no dye leaching during
use