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axial site coordination with metalloporphyrins (1:1 and 2:1 complexes).
Potentiometric anion selectivities of polymer membranes containing
(TFPP)Sn and (TPP)Sn are about the same, suggesting that there is no
Figure 20. Potentiometric response of a sensor prepared with a membrane
containing (TPP)Co(ll) to chloride a (x), salicylate (A) and nitrite (•)
anion (a), Dynamic response of electrode toward increasing nitrite
concentration and reversibility in buffer solution (b).
effect of the electron-withdrawing group (fluorine) at the para-position
of the phenyl rings.
The membrane of the sensor is prepared from 66 wt.% o-nitrophenyloctyl
ether (plasticizer), 33 wt. % PVC and 1 wt. % porphyrin and KTFPB (20 mol
% relative to porphyrin). Seven- mm diameter disks of the membrane are
mounted into Philips electrode bodies. A solution of 5.0 mM NaCl, 5 mM
HBHA in 10 mM phosphate buffer pH 7.2 is used as the internal filling
solution. Before use, the membrane sensors are conditioned overnight in a
solution having the same composition as the internal filling solution.
The sensor response to log of HBHA concentration is linear with a slope
of -73 to -76 mV/decade with a detection limit of 30 mM at pH 7.4. The
response time of the sensor for a step change in HBHA concentration (0.1
mM) is about 90 s (Figure 21).
The detection limit of the porphyrin-based sensors toward HBHA
decreased as the pH increases, ranging from
0.3 mM at pH 5. to 0.65 mM at pH 9.0. However, at physiological pH 7.4
the detection limit (30 mM) was quite
Figure 21. Typical dynamic potentiometric response of Sn TPP-based
membrane electrode upon addition of 1 mM HBHH. Reprinted with permission
from Badr, H. A. et al. Anal. Chim. Acta, 1996, 321, 14.
sufficient for the determination of HBHA in pharmaceutical formulations
and in physiological fluids. This is in contrast to Sn(IV)-porphyrin-
based sensors when used for salicylate measurements, where at
physiological pH, it is not feasible to detect lower than 1.0 mM of total
salicylate concentration, owing to the significant hydroxide ion
interference. The porphyrin-based sensor is shown to be useful for the
assay of HBHA in pharmaceutical formulation with an average recoveiy of
101% and mean standard deviations of 1.8%.
B. Potentiometric Sensor for Nickel
The determination of nickel is important due to its toxic nature and its
widespread use in catalytic processes. Nickel may be present at
relatively high amounts in red meat, cottonseed, commeal, chocolate,
unsaturated oils, milk and milk products. Nickel toxicity can cause acute
pneumonitis, dermatitis, asthma, disorders of the central nervous system
and lung cancer. 5,10,15,20-tetra(4methylpheny^porphyrins (TMPP) have
been used as electroactive materials for preparing porphyrin-based
potentiometric sensors for Ni(II).124
The sensor membrane is prepared by dissolving (w/w%) 54% PVC, 7% TMPP,
30% dibutylphthalate (DBP) and 4% tetraphenylborate (NaTPB) in 20 ml
tetrahydrofuran. NaTPB is used as an anion excluder and DBP as a solvent
mediator. The solution of various components in THF is poured into
aciylic rings placed on a smooth glass plate. After 48 hours, transparent
membranes of 0.5-mm thickness are obtained, which can be cut into disks 5
mm in diameter and glued to one end of a Pyrex glass tube. Galvanic cell
used for Ni(II) measurements consists of an internal reference electrode
(SCE) Ni(II) 0.1 M, 0.01 M ÊÑ1/ membrane/test/solution, and an external
reference electrode (SCE).
The sensor has to be equilibrated for 3-5 days in 0.5 M Ni(II)
solution. A linear response (potential vs log [Ni(II)]) is observed in
the range of nickel(II) concentration
44/Porphyrin-Based Electrochemical Sensors
5.6xl0-6-1.0 10"1 M (the slope is subherstian-30.1 mV). A response time
of the sensor is 20-25 seconds. The sensors can be used over a period of
six months without observing any significant change in response time. The
potentials measured are independent of pH in the range of
2.5 to 7.4. The selectivity coefficient pattern (median ^Ni(ii) ó = 2-5
x Þ-3 for most cations) indicates that the sensor' is moderately
selective to Ni(II) over a number of other cations (except for sodium ion
The selectivity coefficient
over sodium and Co(II) is
6.0 x 10-1 and 7.3 x 10 ~2. Thus, the sensor can tolerate Na(I)
concentrations 5 x 10"5 M over the entire working concentration range.
However, when Na(I) is present at higher concentrations, the sensor can
be used to determine Ni(II) over reduced concentration range only (10 _4-
Similarly, Co(II) ions can be tolerated at concentrations lower
than 5 x 10_5M. However, at higher Co(II) concentration the linear