Encyclopedia of Environmental Science and Engineering, Volume I and II

568 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS

The potassium electrode utilizes a chelating agent, valin-

omycin, an antibiotic and a microcyclic ether, that selectively

complexes potassium ions. Thus the potassium electrode is

composed of a 10^ ^4 to 10^ ^7 M solution of valinomycin dis-

solved in decane in the porous polymer disk (see Figure 22).

The internal solution contains a known activity of potassium

chloride; the chloride ion is for the silver/silver chloride ref-

erence electrode and the potassium ion, K^ ^ , for the reference

potassium activity. The electrode selectivity for K^ ^ over Na^ ^

is 3,800 times, an excellent specificity factor.

(d) Gas-sensing electrodes^12

A gas-sensing electrode assembly featured in Figure 23

is effective for the analysis of gases. This electrode assem-

bly is actually a cell with an indicating glass electrode, a

silver-silver chloride reference electrode and a cell contain-

ing internal solution. The gas diffuses through the membrane

into the thin layer of internal solution where a chemical reac-

tion takes place generating some soluble reaction products.

A specific product of this reaction that is proportional to the

concentration of the analyte in the sample is detected by a

selective electrode sensitive to the specific reaction product.

Two types of gas-permeable membranes are used in this

assembly, homogeneous and microporous. The former is

a thin (0.01 to 0.03 mm) silicon rubber membrane which

allows the gas to “dissolve” in the membrane and leave by

dissolving in the internal solution. A microporous membrane,

about 0.1 mm thick, is prepared from hydrophobic polymers

(polypropylene or polytetrafluoroethylene) and contains

pores about 1  m in diameter. Water, electrolytes and other

polar substances are excluded from these membranes, but

gases easily penetrate the membrane effusing through pores

into the internal solution. In this manner the gaseous analyte

in a solution or in a gas sample is separated from the sample

matrix.

An example of a gas sensing probe is the ensemble

illustrated in Figure 23 used in physiological investigations

to measure carbon dioxide, CO 2 , content of gas or water

samples. The CO 2 in the sample travels through the mem-

brane dissolving in the internal solution. The internal solu-

tion contains a relatively high concentration of bicarbonate

ion HCO 3. The reaction which takes place between the dis-

solved CO 2 and water in the internal solution is

(24)

The equilibrium (K and K are equilibrium constants) taking

place in equation 24 is written

KHCOH CO⎡⎣  32 ⎤⎦⎡⎣ ⎤⎦⁄[].

(25)

Since the [HCO 3 ] is relatively high in the internal solution,

then the equation can be written

[H^ ^ ]  K [CO 2 ] external. (26)

Measurement by the glass electrode for the hydrogen ion

activity will yield the CO 2 analyte concentration. An impor-

tant feature of this gas sensing assembly is that the analyte

never comes in contact with the selective ion electrode.

Nitrogen dioxide, NO 2 , can be detected in the presence

of the gases, namely, CO 2 , SO 2 and NH 3 , using a nitrate elec-

trode because of the following equilibrium

(27)

In Table 8 gas-sensing ensembles available commercially

are listed.^12

Reference

element

Internal filling

solution

O-ring

Bottom cap

Sensing element Membrane

Spacer

Inner body

Outer body

FIGURE 23 Construction of the gas-sensing electrode. For exam-

ple, in the CO 2 gas-sensing electrode, the filling solution contain-

ing bicarbonate ion, HCO 3 , forms a thin film between the sensing

element, a glass electrode, and the membrane. A suitable reference

electrode, e.g., SCE or Ag/AgCl can be the reference element. The

CO 2 gas diffusing into the thin film reacts chemically with the HCO 3 

forming hydrogen ion, H, that is detected by the glass electrode.

See the text and equations 24–26 for the chemical mechanism of

the CO 2 gas-sensing electrode. (Orion Research Inc.: Boston, MA

with permission.)

TABLE 8

Gas-sensing electrodes

Gas Detected ion Sensing Electrodeb

CO 2 H glass

NH 3 H glass

Diethyl amine H glass

HCN CN silver (Ag 2 S)

HF F fluoride (LaF 3 )

H 2 SS 2 silver (Ag 2 S)

NO 2 H or NO 3  glass or nitrate

SO 2 H glass

The substances in the parentheses indicate the composition

of the solid-state electrode.

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