Page 361 - Instrumentation Reference Book 3E

344 Chemical analysis: electrochemical techniques
ionic strength is low, the error introduced by [H+] x [OH-]
neglecting the difference between the actual values [HzOl =K
of the activity coefficients of the ions,f~+ andfB-,
and unity is small (less than 5 percent). Hence for In pure water, too, because there is only very
weak electrolytes, the constants obtained by using slight dissociation into ions, the concentration
the simpler equation K = [A+][B-]/[AB] are suffi- of the undissociated water [H20] may also be
ciently precise for the purposes of calculation in considered constant and the equation becomes
quantitative analysis. Strong electrolytes are [H+] x [OH-] = K,. The constant K, is known
assumed to be completely dissociated, and no cor- as the ionic product of water.
rection for activity coefficients needs to be made Strictly speaking, the assumptions that the
for dilute solutions. activity coefficient of water is constant and that
However, the concept of activity is important the activity coefficients of the ions are unity are
in potentiometric techniques of analysis only correct for pure water and for very dilute
(described later). The activity coefficient varies solutions where the ionic strength is less than
with concentration, and for ions it varies with 0.01. In more concentrated solutions the ionic
the charge and is the same for all dilute solutions product for water will not be constant but, as
having the same ionic strength. The activity coef- activity coefficients are generally difficult to
ficient depends upon the total ionic strength of a determine, it is coinmon usage to use K,.
solution (a measure of the electrical field existing The ionic product of water, K,, varies with
in the solution) and for ion-selective work it is temperature and is given by the equation
often necessary to be able to calculate this. The loglo K, = 14.00 - 0.033(t - 25)
ionic strength I is given by
+ 0.00017(t - 25)'
I = 0.5CCiZ;
where t is the temperature in "C.
where Ci is the ionic concentration in moles per Conductivity measurements show that, at 25 "C,
liter of solution and Zi is the charge of the ion the concentration of hydrogen ions in water is
concerned. Thus, the ionic strength of 0.1 M nitric mol liter-'. The concentration of hydro-
1 x
acid solution (HNO3) containing 0.2 M barium xyl ions equals that of the hydrogen ions, therefore,
nitrate [Ba(N03)*] is given by
K, = [H+] x [OH-] = 10-14. If the product of
0.5[0.l(for H') + O.l(for NO;) [H'] and [OH-] in aqueous solution momentarily
exceeds this value, the excess ions will immediately
+0.2 x 22 (for Ba++) recombine to form water. Similarly, if the product
+0.4 x 1 (for NO;) of the two ionic concentrations is momentarily less
= 0.5[1.4] = 0.7 than more water molecules will dissociate
until the equilibrium value is obtained. Since the
concentrations of hydrogen and hydroxyl ions are
equal in pure water it is an exactly neutral solu-
17.2 Ionization of water tion. In aqueous solutions where the hydrogen ion
concentration is greater than the solution is
As even the purest water possesses a small but acid; if the hydrogen ion concentration is less than
definite electrical conductivity, water itself must the solution is alkaline.
ionize to a very slight extent into hydrogen and
hydroxyl ions:
17.3 E I ec t r i ca I conductivity
H20 H+ + OH-
This means that at any given temperature 17.3.1 Electrical conduction in liquids
As early as 1833, Faraday realized that there are
two classes of substances which conduct electri-
city. In the first class are the metals and alloys,
where a,,[x] and f, refer to the activity, concen- and certain non-metals such as graphite, which
tration, and activity coefficient of the species X, conduct electricity without undergoing any chem-
and K is a constant. ical change. The flow of the current is due to the
As water is only slightly ionized, the ionic con- motion of electrons within the conductor, and the
centrations are small and the activity coefficients conduction is described as metallic, or electronic.
of the ions can therefore be regarded as unity. In the second class are salts, acids, and bases
The activity coefficient of the unionized molecule which, when fused or dissolved in water, conduct
H20 may also be taken as unity, and the above electricity owing to the fact that particles, known as
expression therefore reduces to ions, carrying positive or negative electric charges