I. Tembwe et al. / Electrochimica Acta 55 (2010) 4314–4318
4317
sitivity of 0.02 ppm/mV and a detection limit of 0.008 ppm (8 ppb).
Based on the Botswana guidelines for SO2 pollution, the maximum
allowable limit in the environment is 300 g/m3 of air (0.3 ppb)
[45] and 500 g/m3 for EEU [46]. The guideline limit for a gaseous
pollutant is defined as ‘the amount measured over three consec-
utive hours at locations representative of air quality over at least
100 km2 or an entire zone or agglomeration, whichever is smaller’
[46]. The implication of these guideline values is that the developed
technique could be applied to determine elevated SO2 in industrial
gas emissions.
4. Conclusions
It has been demonstrated that catecholate complexes react with
SO2 exhibiting substantial differences in the redox potentials (ꢀE)
of the pre- and post-SO2 reaction with the complexes, using square
wave voltammetry at a glassy carbon rotating disk electrode in
TBATFB/CH2Cl2, buffer/solvent system. The E1/2 values obtained
for the nickel catecholate complexes reflected the electron donat-
ing/withdrawing abilities of the –C(CH3)3, –CH3 and –F groups
bonded to the catecholate ligand, relative to the –H substituent. The
two oxidation peak potentials at +300 and +1000 mV were assigned
to the semiquinone (sq) and quinone (q), respectively, while the
reduction peak potentials at −400 and −800 mV were assigned to
sq and cat, respectively (where sq = semiquinone, q = quinone and
cat = catecholate). Investigation of analytical response parameters
of nickel catecholate complexes to SO2, showed detection limit of
0.008 ppm, linear range of 0.01–20 ppm with method sensitivity of
0.0025 A/L (0.02 ppm/mV). The peak currents recorded showed the
peak intensity to decrease over small potential range. We suggest
that the catecholate complexes are suitable as amperometric gas
sensors for elevated SO2 industrial emissions.
Fig. 6. CVs (0 to + 600 mV; 50 mV s−1) showing shifts (dotted arrows) in peak
potentials obtained upon successive addition of SO2 in complex 2 solution 0.1 M
TBATFB/CH2Cl2. Scan 1 = is for the ligand alone while scan n = the last scan after
addition of 380 L SO2.
ibration curve because the current values are not measured at a
constant applied potential (Eapplied) as is the case for amperometric
sensor but rather at slightly varying potential values (310–320 mV)
for anodic peaks. The plot is therefore a close estimate of the effect
of increasing the amount of SO2 on the current intensity. It is how-
ever important to note that, the magnitude of the peak potential
shifts was not significant and that Fig. 7 could actually represent
a calibration curve at a constant applied potential of 315 mV (this
value being the average for peak potentials ranging 310–320 mV).
The curve shows the current intensity to decrease gradually as the
amount of SO2 is increased up to a limit (350 L). Beyond this value,
further addition (10 L) of SO2 resulted in a drastic drop in the cur-
rent intensity from a value of approximately 1.8 A to about 0.0 A.
that an electrochemically inactive complex is formed after adding
slightly higher (360 L) amount of SO2 beyond 350 L SO2 which
result in nearly zero current (∼0 A) being recorded as shown in
Fig. 7 for SO2 volumes > 350 L.
Acknowledgement
Ms Tembwe, wishes to thank the University of Botswana and the
Research & Publication Committee for partial grant for undertaking
MPhil degree.
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