Encyclopedia of Environmental Science and Engineering, Volume I and II

564 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS

indicating electrode, E Mb  /Ma  , since the potential of the stan-

dard hydrogen electrode is zero. This potential is referred to

as, E h , the “h” referring to the standard hydrogen electrode.

The sign, however depends on the sign of the standard reduc-

tion potential, E^0 , and the relative concentrations of M b^ and

M a^. In the cell the sign of E h depends on the direction of the

flow of the electron current; if the flow is from the hydrogen

to the indicating electrode, the ORP is negative and flow in

the opposite direction indicates a positive value.

However, since the standard hydrogen electrode is not

practical to use, a calomel or silver-silver chloride reference

electrode is conveniently employed. The cell potential is

then designated, E c , when a calomel reference electrode is

used. The E c value is converted to the E h or ORP value using

the standard potential for the calomel reference electrode,

as follows:

EEEhcc

o.

(19)

The ORP or E h measuring cell can be standardized readily

using a saturated solution of quinhydrone. However, gold

electrodes show no concentration dependence, whereas plat-

inum electrodes show a marked dependence in the quinhy-

drone solution.^41

Hole for filling with

saturated KCl outer

filling solution

Pin-type connector

to potentiometer

Pt-Hg connection

Porous ceramic plug

(salt bridge)

Paste of Hg, Hg 2 Cl 2 ,

and KCl

Saturated KCl inner

filling solution

Pt wire connection

from paste to

pin-type connector

FIGURE 19 Commercial saturated calomel reference electrode. (Courtesy of Thomas

Scientific.)

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