Poly(2,5-dimethoxyaniline) based pH sensors Nophawan Paradee ...

93 NU Science Journal 2009; 6(S1)
Table 3 Selectivity coefficients of the PDMA-modified electrode in the presence of
various interfering ions
Interfering ions
Selectivity coefficient
None -
Zn 2+ 3.16 × 10 -2
Mg 2+ 3.16 × 10 -4
Na + 1.00 × 10 -7
Al 3+ 3.33 × 10 -3
2-
SO 3 1.58 × 10 -5
2-
SO 4 1.58 × 10 -3
-
NO 3 1.00 × 10 -9
Cl - 1.00 × 10 -7
2.3 Electrode Stability
Potentiometric response of the PDMA-modified electrode was examined
over a period of 43 days in order to study the stability of electrode. As seen in Figure
6, the calibration curves of the PDMA-modified electrodes remain linear and the
response slopes are close to the fresh electrode (1 st day) even after repeatedly using
for 20 days. A significant decrease in response slope is observed for the PDMAmodified
electrode after 28 days of the first use. It is important to note that the
PDMA film was found to peel off the supported electrode and it is a possible reason
for the change of response slope.
E (mV) vs Ag/AgCl
250
200
150
100
50
0
-50
-100
-150
Day 1
Day 4
Day 9
Day 14
Day 21
Day 28
Day 36
Day 43
0 2 4 6 8 10
pH
Figure 6 Influence of aging time on the potentiometric responses of the PDMAmodified
electrodes