Encyclopedia of Environmental Science and Engineering, Volume I and II

INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS 567

depicts the general structure of a liquid membrane electrode.

Another rendition of the membrane is a hydrophobic poly-

mer (e.g. polyvinyl chloride) film formed with the organic

ion exchanger or chelating agent as an integral part of the

membrane ensemble disk.

As seen in Figure 22, the membrane mediates between

the standard analyte solution and the sample solution. The

ion exchanger or chelating agent dissolved in the immiscible

organic solvent layer (a straight chain alcohol) permits cat-

ions or anions to diffuse between the two aqueous phases,

sample and standard. After some period of time, equilibrium

will be established to provide a stable, membrane potential,

meter reading. A calcium ion, Ca^ ^ , electrode utilizes the ion

exchanger, bis(2-ethylhexyl) phosphoric acid, [(RO) 2 POOH],

where R  2-ethylhexyl.

At each interface the following equilibrium is established.

(22)

The pCa value is expressed in terms of the cell potential, E v , and

two constants, 0.0591 and Le (see equation 21) as follows:

pCa  2(Ev  Le)/0.0591. (23)

TABLE 7

Commercial solid-state electrodes a

Analyte Ion Concentration Range, M Interferencesa

Br 100 to 5  10 ^6 mr: 8  10 ^5 CN; 2  10 ^4 I; 2 NH 3 ;

400 Cl; 3  104 OH. mba: S^2 

Cd^2  10 ^1 to 10^7 Fe^2   Pb^2  may interfere.mba: Hg^2 ,

Ag, Cu^2 

Cl 100 to 5  10 ^5 mr: 2  10 ^7 CN; 5  10 ^7 I; 3  10 ^3

Br; 10^2 S 2 O 32 ; 0.12 NH 3 ; 80 OH.

mba: S^2 

Cu^2  10 ^1 to 10^8 high levels Fe^2 , Cd^2 , Br, Cl. mba:

Hg^2 , Ag, Cu

CN 10 ^2 to 10^6 mr: 10^1 I; 5  103 Br; 10^6 Cl, mba: S^2 

F sat’d to 10^6 0.1M OH gives 
10% interferenc when

[F]  10 ^3 M

I 100 to 5  10 ^8 mr: 0.4 CN; 5  103 Br; 10^5 S 2 O 3 ;

106 Cl

Pb^2  10 ^1 to 10^6 mba: Hg^2 , Ag, Cu^2 

Ag/S^2  100 to 10^7 Ag Hg^2  must be less than 10^7 M

100 to 10^7 S^2 

SCN 100 to 5  10 ^6 mr: 10^6 I; 3  10 ^3 Br; 7  10 ^3 CN;

0.13 S 2 O 32 ; 20 Cl; 100 OH. mba: S^2 

a From Ref. (51). (Orion Research Inc.: Boston, MA with permission.)

b mr: maximum ratio M

M

interference

analyte

⎛

⎝⎜

⎞

⎠⎟

for no interference. mba: must be absent.

Ion-exchanger

reservoir

Liquid ion-

exchange layer

within porous

membrane

Porous

membrane

Ag.AgCl

reference

electrode

Internal aqueous

filling solution

FIGURE 22 Construction of liquid ion-exchange electrode. (Orion

Research Inc.: Boston, MA with permission.)

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