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The porphyrin handbook - Kadish K.M.

Kadish K.M. The porphyrin handbook - Academic press, 2000. - 368 p.
Download (direct link): kadishsmishgulilard2000.djvu
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anodic
ek
P-PNi+3-P-
cathodic
(a)
Preconcentration
Detection
(b)
pH
Figure 5. Schematic diagram of the porphyrinic sensor for measurement of
nickel (a), pH dependence of demetalation of polymeric (TMHPP)Ni11 (b),
Film thickness, 20 equivalent monolayers; time, 60 s; and Ni(ll)
concentration, 1 .
slowly and only forms a relatively thin and mechanically unstable film
that incorporates Ni(II) poorly. The metalated species, (TMHPP)Ni,
quickly forms the thick, stable films desired. However, this film is
saturated with metal and shows no affinity for binding additional Ni(II).
Demetalation can be accomplished by immersion of the polymeric film
electrode in acid (0.1 M HC1). Once demetalated, the polymeric TMHPP film
can be expected to readily incorporate Ni(II) from solution (eq 1) and be
useful for chemical preconcentration. This makes (TMHPP)Ni a suitable
material for the fabrication of a sensor when the polymerization process
is followed by subsequent demetalation.
The Ni(II) demetalation process from polymeric TMHPP of Ni(II) occurs
at a pH lower than 6 and is highest at pH 2, while optimum Ni(II)
metalation is observed in the range of pH 7-9 (Figure 5b). At a high pH,
Ni(II) can react with OH- to form insoluble Ni(OH)2 (Ksp = 1.6 x 10"14)
which will decrease the concentration of nickel cation and inhibit the
metalation process. The chemical preconcentration (metalation of
polymeric TMHPP) depends on both the preconcentration time and the
initial concentration of Ni(II) in solution. The rate of incorporation of
nickel into the polymeric film is fast during the initial time period
(10-20
seconds) and that the rate is dependent on the initial concentration of
Ni(II). This is due to fast incorporation of nickel in the outermost
monolayer of the film. Further incorporation is controlled by propagation
(diffusion) into the film that will be controlled by the gradient of
Ni(II) concentration in the film. The film-concentration gradient would
in turn be expected to be affected by the concentration at the film-
solution interface.
With fully water-soluble porphyrins, the metal-incor-poration rate has
been found to be first order in free base, metal, and anion. However, it
has been suggested that during incorporation of metals in poorly soluble
porphyrins, which would correspond to the situation of polymeric
porphyrins, intermediate ("sitting-atop" complexes) can be formed.29
Incorporation kinetics were suggested to be second order in metal and
first order in porphyrin. The number of monolayers of polymeric
porphyrins occupied by Ni(II) increases with its initial concentration.
At trace concentration levels, film thickness is not important because
only the first few layers are occupied, and the analyte signal does not
depend on film thickness. The concentration levels (10 ~7-10" 5 M) and
preconcentration times (40-100 seconds) are not sufficient to exceed the
film capacity of monolayer film to incorporate Ni(II). However, for high
Ni(II) concentra-
237
238
Malinski
Table 1. Optimum Conditions for Preparation of Porphyrinic Sensors for
Nickel and Nickel (II)
Process Materials Parameters
Sensor fabrication Supporting material GCE or carbon
fiber
Supporting electrolyte 0.1 MNaOH
Monomeric NiTMHPP (1-5) x 10"4M
Voltage range 0.00-1.00 V
Scan rate 0.1 Vs-1
Number of scans 20-40
Demetalation Solution 0.1 MHCL
Time 60s
Stirring 600-800 rpm
Metalation (preconcentration) From sample solution
pH 8.5-9.0
Time 60 s
Stirring 600-800 rpm
From isolated biological cell
pH 7.4
Time 600 s
Stirring None
Ni(II) determination Supporting electrolyte 0.1 MNaOH
Method DPV
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