Encyclopedia of Environmental Science and Engineering, Volume I and II

578 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS

FIGURE 32 Three types of conductance cells, A: Precision conductance cell. B: Conductometric

titration cell. C: Concentration dip cell. (From Gray D. Christian and James E. O’Reilly, Instrumental

Analysis, 2nd edition. Copyright © 1986 by Allyn and Bacon. Reprinted with permission.)

ABC

FIGURE 31 Basic Wheatstone ac bridge circuit for

measuring conductance. (From Gary D. Christian and

James E. O’Reilly, Instrumental Analysis, 2nd edition.

Copyright © 1986 by Allyn and Bacon. Reprinted with

permission.)

60 –

1000 Hz

Oscillator

Cell

b

a

R 3

RC

R 2

R 1

Null

Detector

electrodes, respectively. The cell constant is k c  d/ar cm^ ^1.

In analytical determinations of solution conductivities, C sp is

usually calculated as micromhos per centimeter (  mhos/cm)

or in SI units of millisemens per meter (mS/m).

Temperature control during measurements is necessary,

since the temperature coefficient for conductance measure-

ments is about 0.5 to 3% per C. Conductance data is usu-

ally obtained at 25C. Temperature corrections can be made,

however the further the data temperature is from 25C, the

greater the uncertainty.

The measurement of solution resistance requires a

conductance cell to contain the solution and a Wheatstone

bridge for measurement. The Wheatstone bridge, shown in

Figure 31, has an alternating current source of 6 to 10 volts

with a frequency of 60 to 1000 Hz. R 2 and R 3 are fixed resis-

tors of known values, R 1 is a variable resistor with up to 4

decades of resistance, and R c is the resistance of the analyte

solution. The null detector may be a headphone used with

1000 Hz, a cathode-ray tube or a micrometer. A variable

capacitor is connected across R 1 and adjusted to balance out

any phase shift in the a.c. signal caused by the capacitance

of the electrode surfaces. This adjustment is made to pro-

vide the sharpest minimum in the null signal.

A conductivity cell essentially consists of two square

plates of platinum of the same area, ar and platinized with

platinum black to prevent polarization (see Figure 32).

The plates are arranged parallel to each other at a fixed

distance, d. (Other durable metals such as stainless steel

and nickel are used in field and continuous monitoring

operations.) The cell constant is not easily nor accurately

obtained by measurement of the area and distance. A stan-

dard procedure is to fill the cell with a potassium chlo-

ride solution of known molarity and specific conductivity,

C sp. The measured resistance, R m , along with C sp , when

substituted in equation 40, will give the cell constant, k c.

Standard Methods gives an excellently, detailed descrip-

tion of the measurement of conductivity.^2

The measurement of specific conductances has a number

of uses in water analysis: i) Ascertaining the mineral content

of water in order to determine the effect of total ionic content

on corrosion rates, physiological effects on animals and plants

and the effects on chemical equilibria. ii) Appraising daily and

seasonal variations in the mineral content in raw waste and nat-

ural waters. iii) Estimating the mineral content of high purity

(distilled and deionized) water. iv) Checking the results of

chemical analysis and estimating sample size. v) Determining

the endpoint in conductometric titrations. vi) Estimating the

total dissolved solids content by multiplying the specific

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