Page 369 - Instrumentation Reference Book 3E

352 Chemical analysis: electrochemical techniques

oleum, hot concentrated sodium hydroxide, and Table17.6 Pure water, conductivity from 0 to1OO"C
slurries. In addition, this technique is ideal for
applications in concentrated electrolytes (not Temperature Conductivity Resistivity
necessarily aggressive) such as estuarine or sea ("C) (PS a-') (" C)
waters where polarization errors would be con- 0 0.0116 86.0
siderable in a conventional cell. Temperature 5 0.0167 60.0
compensation is normally incorporated. 10 0.023 1 43.3
15 0.0314 31.9
17.3.4 Applications of conductivity measurement 20 0.0418 23.9
25 0.0548 18.2
The measurement of electrical conductivity is the 30 0.0714 14.0
simplest and probably the most sensitive method 35 0.0903 11.1
of providing a non-specific indication of the dis- 40 0.1133 8.82
solved solids, or more correctly the ionic content 45 0.1407 7.11
0.1733
5.77
50
of a solution. If the number of ionic species in 60 0.252 3.97
solution are few then it may be possible to use 70 0.346 2.89
conductivity as a measure of the concentration of 80 0.467 2.14
a particular component. Undoubtedly the robust 90 0.603 1.66
nature of conductivity measurements has led to 100 0.788 1.27
their use in circumstances where their non-specific
response gives rise to errors in interpretation of
concentration. Consequently, any successful species giving rise to a conductivity greater than
instrumental application of conductivity as a con- that of pure water are of no interest but it is useful
centration sensor has to ensure that the species of to note how little impurity is required to raise the
interest is the dominating ion or the only ion conductivity. For example, at 25°C only about
(together with its counter-ion of opposite charge) 1Opgl-' of sodium (as sodium chloride) are
whose concentration is changing. With these required to increase the conductivity to twice that
restrictions it can be appreciated that determin- of pure water.
ations of concentrations by conductivity measure-
ments are often supported by additional analyses
or preceded by a physical or chemical separation 17.3.4.2 Condensate analyzer
of the species of interest. The purity of the water used in the steam-water
circuit of power stations is particularly important
17.3.4.1 Conductivity and water purity for the prevention of corrosion. An essential com-
ponent of such a circuit is the condenser wherein
Water of the highest purity is increasingly being the steam from the turbines is condensed before
used for industrial purposes, for example, the returning to the boiler. On one side of the con-
manufacture of electronic components and the denser tubes is the highly pure steam and water
preparation of drugs. Other examples of large- from the turbines and on the other is cooling
scale uses include process steam and feedwater water chosen for its abundance (e.g., river water
for high pressure boilers. In all these cases con- or estuarine water) rather than its chemical pur-
ductivity provides the most reliable measurement ity. Any leakage of this cooling water through the
of water purity in circumstances where contam- condenser tubes leads to the ingress of unwanted
ination from non-conducting impurities is consid- impurities into the boiler and therefore must be
ered to be absent. The conductivity of pure water immediately detected. Direct measurement of
is highly temperature-dependent due to the conductivity would detect significant ingress of,
increase in the dissociation of water molecules say, sodium chloride from estuarine water, but it
into hydrogen and hydroxyl ions of water, K,, would not be capable of detecting small leakages
with temperature. The extent of this can be seen since the conductivity of the condensate would be
in Table 17.6. dominated by the alkaline additives carried over
The conductivity of pure water can be calcu- in the steam from the boiler. A better method of
lated at any temperature provided values of X &, detection of leakage is to pass the condensate
As, K,, the dissociation constant of water, and through a cation exchange column in the Hf
the density of water d are known at the appro- form, then measure the conductivity. Using this
priate temperature. procedure, all cations in the condensate are
n(pScm-') = (A;; + ,/K~ . io3 exchanged for hydrogen ions and the solution
leaving the column will be weakly acidic if any
In the application under consideration here (Le., salts have entered through the condenser. Other-
the use of pure water) the exact nature of the ionic wise, the effluent from the column will ideally be