Liquid Membrane Electrode
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Transcript of Liquid Membrane Electrode
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- employs as the membrane a water-immiscible liquid
which will selectively bond the ion being determined.
- it is similar to the glass pH electrode in that it contains
an internal silver-silver chloride electrode and an
internal reference solution or fixed composition.
- has a hydrophobic membrane impregnated with a
hydrophobic ion exchanger (called an ionophore) that
is selective for analyte ion.- develops across the interface between the solution
containing the analyte and a liquid-ion exchanger that
selectively bonds with the analyte ion.
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CHARACTERISTICS OF SELECTED LIQUID-MEMBRANE
ELECTRODES
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Analyte ion Active membrane
material
Concentration range
(M)
Recommended pH
range
Selectively coefficients
NH4+ Nonactin/monactin 10-1 10-5 5 8 K+ = 0.12 ; Na+ = 0.002; Li+ =
0.0042; Mg2+ = 2 x 10-4
Ca2+ Calcium di(n-decyl)-
phosphate
100 10-6 6 10 Zn2+ = 3.2; Fe2+ = 0.8; Mg2+ =
0.14; Na+ = 0.003
Ca
2+
+ Mg
2+
(divalent
cation)
S
imilar to that for Ca2+
electrode10
-2
10-5
6 10 Zn2+
= 3.5; Fe2=
= 3.5; Cu2+
=3.1; Na+ = 0.015
K+ Valinomcin 100 10-6 3 10 NH4+ = 0.013; H+ = 0.01; Na+
= 6 x 10-5; Li+ = 2 x 10-4
Cl- Dimethyldioctadecyl-ammonium chloride
100 10-5 3 10 I- = 17; NO3- = 4.2; Br- = 1.6;HCO3
- = 0.19; F- = 0.10; SO4-
= 0.14
ClO4- Tris(substituted 1, 10-
phenanthroline)iron(II)
perchlorate
100 10-5 4 10 I- = 0.012; NO3- = 0.0015; Br-
= 0.00056; Cl- = 0.00022;
OH- = 1
O3- Tridodecylhexadecyl-
ammonium nitrate
100 10-5 3 I- = 17; Br- = 0.1; NO2- =
0.07; Cl- = 0.005; SO4- < 10-5;ClO4
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The membrane consists of liquid calcium di(n-decyl) phosphate,
[(CH3(CH2)8CH2O)2PO2]2Ca, immobilized in a thin disk of polyvinyl chloride that
cannot be penetrated by water. At each membrane surface, the calcium compoundestablishes an equlibrium with its ions:
[(RO)2PO2]2Ca 2(RO)2PO2- + Ca2+
where r is the n-decyl hydrocarbon chain. It is important to recognize that the
didecylphosphate anion is fixed part of the nonaqueous liquid membrane. Since
the concentration of calcium ions in the solutions on each of the membrane
surface may be different also, giving rise to a potential described by the equation:
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Collecting the constants into a single term as was done gives:
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A nitrate-ion electrode in 1.64 x 10-4 MKNO3has an electrode potential of 0.017 V. Enough
potassium nitrite is added to the solution tomake its concentration 4.76 x 10-2 Mwithout
changing the volume. The new electrode
potential is -0.049 V. Calculate the selectivity of
the electrode for nitrate ions.
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the value ofQ can be calculated from this equation using the electrode
potential measured with no nitrate present:
or
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A perchlorate-ion electrode in 2.56 x 10-4 M
NaClO4 has an electrode of 0.023 V. Enough
sodium perchlorate is added to the solution to
make its concentration 5.03 x 10-2
Mwithoutchanging the volume. The new electrode
potential is -0.037 V. Calculate the selectivity of
the electrode for perchlorate over chlorate ions.
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the value ofQ can be calculated from this equation using the
electrode potential measured with no perchlorate present:
or
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A nitrate-ion electrode in 7.98 x 10-4 MAgNO3has an electrode potential of 0.056 V. Enough
silver nitrate is added to the solution without
changing the volume. The new electrodepotential is -0.024 V. Calculate the concentration
of the nitrite ion after the addition of silver
nitrate. The selectivity of nitrate over nitrite ions
is 85.
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the value ofQ can be calculated from this equation using the
electrode potential measured with no nitrate present:
or
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