Encyclopedia of Environmental Science and Engineering, Volume I and II

INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS 565

The ORP values can be expressed as pE. The value of pE

measures redox power, at equilibrium. Large positive values

indicate a strongly oxidizing environment and smaller or

negative values a strongly reducing environment. Varying

values for pE of 10 to 14 are found in water systems. E h 

pH limits for natural environments and E h values for sani-

tary engineering processes have been published. 42,43,44

(2) Membrane electrodes 45,46,47

Membrane electrodes are those electrodes that contain a

selective membrane that responds to the presence of specific

ions or molecules. In these electrodes a potential develops

across a selective membrane, ideally, in response to a spe-

cific ion or molecule. However, the mechanism of physical

and/or electrochemical action differs for the different types

of membrane electrodes. The membrane, the differentiat-

ing feature of these electrodes, can be crystalline, glass, or

a porous polymer. All membrane electrodes contain, inter-

nally, a reference electrode and a known standard activity

of the analyte ion (see Figure 20). Membrane electrodes are

quite different from metal electrodes in their electrochemical

and physical mechanisms and structural design. However,

as in all potentiometric measurements the membrane (indi-

cating) electrode forms a cell with the reference electrode.

Therefore, potentiometric measurements using selective

electrodes have two reference electrodes; the external refer-

ence in the sample solution and the internal reference that is

part of the selective electrode. Those substances important

in wastewater analysis and detected by these electrodes are

listed in Table 6.

Two subdivisions of membrane electrodes can be envi-

sioned: namely, ion- and molecular-selective electrodes. In this

article the electrodes are categorized as follows: glass, solid-

state, liquid membrane, gas sensing and enzyme and micro-

bial electrodes. The latter three electrodes detect molecules,

whereas the first three are sensitive to cations and anions.

Enzyme and microbial electrodes are designed to operate in

either the potentiometric or amperometric mode. For simplic-

ity both types will be treated in this, the potentiometric instru-

ments section. However the type will be clearly indicated.

(a) Glass electrodes^48

The glass electrode commonly refers to the glass mem-

brane electrode used to measure pH (or hydrogen ion activity

or concentration). A glass electrode may also be sensitive to

cations of sodium, potassium, lithium, and silver^49. This elec-

trode has a selective membrane, a thin glass bulb of specific

chemical composition, that responds most sensitively to a

specific cation. Inside of the bulb is an internal standard ana-

lyte solution of known activity and a reference electrode; the

outside surface contacts the sample solution (see Figure 20).

The mechanism of the function of the glass membrane is

quite involved and the following explanation is offered.^50 The

glass membrane has an inner dry layer, about 50  m thick,

sandwiched between 5–100 nm thick layers of hydrated

glass (silicic acid gel): that is, the hydrated layers are on the

inside and outside surfaces of the glass bulb. The hydrated

glass layer attracts specific cations via an ion exchange or

adsorption mechanism to, most likely, an ionic fixed sites.

Conductivity is due to transport through the dry layer, ide-

ally by the specific ion, but actually by a cation such as

sodium, Na^ ^ ; hydrogen ion, H^ ^ , is not transported through

the dry layer. The inside layer interfaces with a fixed standard

analyte concentration and therefore a fixed quantity of cat-

ions on its sites, while the outside layer contacts the sample

solution whose analyte activity may vary with each sample.

A membrane potential develops due to the difference of

charge between inside and outside hydrated layers.

The glass electrode must be standardized with solutions

of known cationic activity, since the glass membrane does

not respond ideally giving rise to errors due to asymmetry

potentials, non-ideal transport effects, etc. These electrodes

have very high resistances, therefore direct-reading, solid-

state instruments with large internal resistances are required:

that is, an electronic voltmeter. The instrument has an adjust-

ment to corret the meter reading during the standardization.

(b) Solid-state electrodes 46,47

Solid-state electrodes are of two kinds: The homogeneous

type is fabricated from insoluble, crystalline, metallic salts,

while the heterogeneous type is of insoluble, non- crystalline

Ion-

exchanger

reservoir

Liquid ion-

exchange

layer

Porous membrane

saturated with

Liquid membrane ion-exchanger

Ca2+ electrode

Glass

membrane

Conventional

glass pH electrode

Ag.AgCl

reference

electrode

Internal aqueous

filling solution

(HCl and CaCl 2 ,

respectively)

FIGURE 20 Comparison of a liquid membrane calcium ion electrode with a

glass electrode. (Orion Research Inc.: Boston, MA with permission.)

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