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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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size. Sapphyrin with a core size of roughly
5.5 A diameter is well suited for the binding of the central moiety about
the size of fluoride with strong hydrogen-bonding interactions. The
response order by the sapphyrin-based electrode is F- > Br" > Cl-
indicating that the sapphyrin-based electrode is more selective for
fluoride than chloride and bromide. The potentiometric discriminations of
cis/trans geometrical (maleate over fumarate) and positional (phthalate
over isophtalate and terephtalate)
isomers of dicarboxylate by the expanded porphyrin-based electrodes was
observed. The liquid-membrane electrodes based on sapphyrin, rubyrin and
triphenylrosarin respond to chloroacetates. The selectivity sequences for
the three electrodes are the same: trichloroacetates > dichloroace-tates
> chloroacetates.
Generally, the protonation of expanded porphyrins at the surface of
the electrode membrane is a prerequisite for accommodating the guest
anions and yielding their potential response. The electrodes strongly
respond to benzoates, but rather less to inorganic anions and saturated
aliphatic organic carboxylates. The selectivity sequences for the
sapphyrin-based electrodes were found in some cases to deviate from the
Hofmeister series. Fluoride preference over chloride and bromide is
particularly noted.
A. PREPARATION OF PORPHYRIN-BASED POTENTIOMETRIC SENSORS FOR
ANIONS
The solvent polymeric membranes used for EMF measurements typically
consisted of 1 wt.% metalloporphyrin, 33 wt. % poly-(vinyl chloride)
(PVC) and 66 wt % plasticizer (dibutylsebacate). If the porphyrin complex
central metal oxidation state was (+ 2) or (+ 4), addition of lipophilic
cationic sites or anionic sites respectively was required to achieve
optimal anion response and selectivity. Tridodecyl-methylammonium cation
(TDMAC) or tetrakis[bis (3,5-trifluoromethyl) phenyl] anion (at a
concentration 10-20% mol relative to the metalloporphyrin) are used for
this purpose. The total weight of the components varies according to the
desired membrane thickness (typically 150-250 jxm) and diameter (7-10
mm).
The mixture of metalloporphyrin, PVC and plasticizer is dissolved in
tetrahydrofuran (THF) and transferred into a glass ring placed on a glass
plate. The ring is covered by a second glass plate, leaving a small space
between the ring and the plate. Thus, a slow evaporation of
tetrahydrofuran is achieved, which takes 24-48 hours. Disks are cut out
of the parent membrane and mounted into Phillips electrode bodies.
The inner filling solution for the sensors is usually 0.01 M NaCl,
which is also used to condition the potentiometric sensors.
Electrochemical potential is measured with the following galvanic cell:
Ag/AgCl/bridge electrolyte/sample solution/ion-selective membrane/inner
filling solution/ AgCl/Ag. A high impedance pH-mV meter is used to
measure the electrochemical potential. Selectivity coefficients are
evaluated by the matched potential method (also known as method of mixed
solutions), or via the separate solution method.
In the matched potential method, the potentiometric selectivity
coefficient K$>y is defined as the ratio of the concentrations of the
primary (x) and interfering ions (y), which give the same potential
change under the same condition, that is, under a fixed concentration of
the primary ion as a background. In the separate solution method,
selectivity coefficients K?y are calculated using the cell EMF values
obtained in 0.01 M solutions of both interfering anions (y) and the
primary ion (x).
44 / Porphyrin-Based Electrochemical Sensors
253
The potentiometric sensors are calibrated by measuring the potential
at various concentrations (c) of analyte. The slope of the plotted
potential vs log should have been 59.2mV/per decade for Nerstian
response of the sensor. However, some of the porphyrin-based
potentiometric sensors show sub-Nerstian response or super-Nerstian
response (slope lower or higher than -59.2 mV/per decade, respectively).
It was suggested that the porphyrin may have insufficient lipophilicity
to produce long-lived, stable sensors.96 So, the potential drift of
membranes prepared with metalloporphyrins has to be evaluated over
several days. During the first 10 minutes a relatively large drift of
potential at a rate of 13-36 mV/h is usually observed. This was followed
by a drift in the opposite direction of - 3 to -5 mV/h for 1 -6 hours.
After this initial period, electrodes exhibited a more modest drift of +
0.2 mV/h. The potential drift for porphyrin sensors is about five- to ten
fold higher than valinomycin-based membranes. This indicated that
porphyrin-based potentiometric sensors require more frequent calibration.
The source of the drift is not clear, although no evidence for leaching
of the metalloporphyrin from PVC membranes is observed in aqueous media.
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