liquid membrane electrode

8/6/2019 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:


8/18

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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Thank you for

listening. Till

next time

liquid membrane electrode

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