Encyclopedia of Environmental Science and Engineering, Volume I and II

566 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS

precipitates of metallic salts. The first type is made of a slice

of a single crystal, a pressed pellet, or a casting from molten

material of the crystalline metallic salt, that comprises the

selective electrode membrane. Insoluble metallic precipi-

tates encapsulated in inert, polymer matrices constitute the

second type.

The fluoride electrode, a homogeneous solid-state elec-

trode, is composed of a 10 mm diameter disk of lanthanum

fluoride, LaF 3 , doped with europium fluoride, to increase

conductivity. The crystalline slice of LaF 3 is 1–2 mm in

thickness. Figure 21 illustrates a solid-state electrode. The

internal solution in a fluoride electrode serves two purposes:

It bathes the membrane with a known standard activity of

fluoride ions and provides the internal silver/silver chloride

reference electrode with a known activity of chloride ion.

The inside and outside layers of the LaF 3 membrane

ionize as follows:

(20)

The amount of charge on each face depends on the fluoride

concentration in the sample (outside) or the standard (inside)

solutions. The layer interfacing with the lower concentration

of fluoride ions will have a positive charge with respect to the

other face; the equilibrium will favor the ionization leaving

a surplus of positively charged LaF 2 ions in the layer. This

difference of charge mirrors the variation in fluoride con-

centration of the two solutions giving rise to the membrane

potential. The differences in the chemical potentials for the

two ion exchange equilibria for the two faces results in the

membrane potential (see equation 20). There is some similar-

ity to the mechanism encountered in the glass electrode.

The pF value is expressed in terms of the cell potential, Ev,

a constant, 0.0591, and Le, a combination of the internal and

external reference electrode potentials and junction potentials,

and an expression for the activity of the internal standard.

PF  (Le  Ev)/0.0591 (21)

The fluoride electrode, which may be used in a tempera-

ture range of 0 to 80C, detects fluoride ion concentra-

tions of saturated to 10^ ^6 M (0.02 ppm, parts per million).

Limitation of detection is controlled by the solubility of

the lanthanum fluoride. A lower solubility leads to a lower

detection limit, other factors being constant. Except for

hydroxyl ion, OH^ ^ , this electrode is uniquely selective for

fluoride ion; measurements at pH values above 8 (OH^ ^

activity  10 ^4 ) result in serious error. For 0.1 M OH^ ^ and

0.001 M F^ ^ there is, approximately, a 10% error. At pH

values below 5 (H^ ^ activity  10 ^5 ) hydrogen ions associ-

ate with fluoride ions to form the hydrogen fluoride mol-

ecule. The fluoride ion activity is, therefore, decreased.

Since the fluoride electrode does not respond to the HF

molecule, the electrode can only indicate the free fluoride

ions in solution. The error is due to this chemical equi-

librium pointing to the specificity of this electrode. The

fluoride electrode most closely fulfills the conditions of an

ideal, ion specific electrode.

Other solid-state electrodes of interest are Br^ ^ , Cd^2 ^ , Cl^ ^ ,

Cu^2 ^ , CN^ ^ , F^ ^ , I^ ^ , Pb^2 ^ , Ag^ ^ /S^2 ^ , and SCN^ ^. (See Table 7 for

a list of commercially available solid-state electrodes.) These

electrodes provide measurement of their respective ions, over-

all, in the concentration range of 1 to 10^ ^8 M. Each electrode

has its own unique interferences.^51 Some mixed, insoluble,

silver salt electrodes have multiple detection capabilities.

A membrane containing an equivalent molar content of silver

sulfide and halide (Cl^ ^ , Br^ ^ or I^ ^ ) ion will detect silver ion

(similar to a metal electrode of the first kind) and sulfide, S^2 ^ ,

ion (Similar to an electrode of the second kind). An electrode,

similar to one of the third kind, is a mixture of silver sulfide

with cadmium, lead or copper(II) sulfide yielding an electrode

sensitive to cadmium, lead or copper(II), respectively.

(c) Liquid membrane electrodes^51

Liquid membrane electrodes can detect a variety of

mono and divalent anions and cations for example, BF 4 , Ca^2 ,

C10 4 , NO 3 , K and water hardness (Ca^2 ^  Mg^2 ^ ).^51 The mem-

brane is a porous polymer disk which is filled with a water-

immiscible, organic solvent containing a dissolved organic

ion exchanger or chelating agent (neutral carrier). Figure 22

Internal

filling solution

Reference

electrode

Solid-state

ionic conductor

FIGURE 21 Cross-sectional view of

a solid-state sensor. (Orion Research

Inc.: Boston, MA with permission.)

T A B L E 6

Selective electrodes for water parameters

Metal ions Nonmetal ions

Analyte Electrode Analyte Electrode

Cd^2  Cd^2  Cl Ag/AgCl

Cu^2  Cu^2  F F

Pb^2  Pb^2 

Ca^2  Ca^2  CN CN

Hardness Divalent Br Br

ion I I

Hg^2 , RHg I S^2  S^2 

(R  allyl & aryl) Residual Chlorine I, indirect

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