Page 362 - Instrumentation Reference Book 3E

Electrical conductivity 345

move in opposite directions through the liquid. It is units most work is reported using volume units of
this motion of electrically charged particles which an3 since the liter is a convenient volume for labora-
constitutes the current. Liquids which conduct elec- tory use and A is usually in units of S cm2/mol.
tricity in this manner are known as electrolytes. At infinite dilution the ions of an electrolyte are
so widely separated by solvent molecules that
17.3.2 Conductivity of solutions they are completely independent and the molar
conductivity is equal to the sum of the ionic con-
The passage of current through an electrolyte gen- ductivities, A'. of the cation and anion, Le.,
erally obeys Ohm's law, and the current-carrying A, = AT +
ability of any portion of electrolyte is termed
its conductance and has the units of reciprocal The values of Xo are the values for unit charge,
resistance (l/n), siemens (S). The specific current- referred to as equivalent ionic conductivities at
carrying ability of an electrolyte is called its con- infinite dilution. The general case is
ductivity and consequently has the units of S m-'.
The conductivity of electrolytes varies greatly A, = z+n+A: + z-n-A?
vi7ith their concentration because dilution (a) where z is the charge on the ion and n the number
increases the proportion of the dissolved electro- of these ions produced by dissociation of one
lyte, which forms ions in solution, but (b) tends to molecule of the salt, e.g.,
reduce the number of these ions per unit of
volume. In order to measure the first effect alone X,(LaCI3) = 3 x 1 x A;a + 1 x 3 x A$,
another term, molar conductivity, A, is defined, Since, for example, the ionic conductivity of the
chloride ion is the same in all chloride salts, then
A (S m2/mol) = nic,
the molar conductivity at infinite dilution of any
where K is the conductivity and c is the concentra- chloride salt can be calculated if the correspond-
tion in mol m-'. Although these are the basic SI ing value for the cation is known. Values of ionic
conductivities at infinite dilution at 25°C are
Table 17.1 Limiting ionic conductivities at 25 "C given in Table 17.1.
Providing the concentration of a fully disso-
Cation x" Anion x" ciated salt is less than about lop4 mol/l, then the
s cm'tmol s cm'tmol conductivity K at 25 "C can be calculated from
H+ 349.8 OH- 199.1 K(S cm-') = zn(AP, + X:)C
Li+ 38.7 F- 55.4
Na+ 50.1 c1- 76.4 or
K' 73.5 Br- 78.1 n(p~ cm-')= zn(AO, + A:)C 10'
NH,- 73.6 I- 76.8
(CH3),NHZ 51.9 NO; 71.5 where c is the concentration in mol/l.
4 Mg*+ 53.1 ClO, 64.6 Values of limiting ionic conductivities in aque-
t ea2+ 59.5 Acetate 40.9 ous solution are highly temperature-dependent
5 cu2+ 53.6 5 so;- 80.0 and in some cases the value increases five- or
sixfold over the temperature range 0-100°C
4 Zn2+ 52.8 4 c0;- 69.3
(see Table 17.2). These changes are considered
Table172 ionic conductivities between 0 and 100°C (S cm*/mol)

Ion 0" 5" 15" 18" 25" 35 45" 55" io0
H+ 225 250.1 300.6 315 349.8 397.0 441.4 483.1 630
OH- 105 - 165.9 175.8 199.1 233.0 267.2 301.4 450
Li+ 19.4 22.7 30.2 32.8 38.7 48.0 58.0 68.7 I15
Na- 26.5 30.3 39.7 42.8 50.1 61.5 73.7 86.8 145
K+ 40.7 46.7 59.6 63.9 73.5 88.2 103.4 119.2 195
C1- 41.0 47.5 61.4 66.0 76.4 92.2 108.9 126.4 212
Br- 42.6 49.2 63.1 68.0 78.1 94.0 110.6 127.4 -
I- 41.4 48.5 62.1 66.5 76.8 92.3 108.6 125.4 -
NO? 40.0 62.3 71.5 85.4 - - 195
ClO, 36.9 58.8 67.3 - - - 185
Acetate 20.1 35 40.9 - - - -
5 Mg2- 28.9 44.9 53.0 - - - 165
t ea2+ 31.2 - 46.9 50.7 59.5 73.2 88.2 - 180
$SO, 41 68.4 80.0 - - - 260