Page 252 - MODERN ELECTROCHEMISTRY

188 CHAPTER 2

This decrease in compressibility with increase in pressure explains the overlarge
decrease in volume (44%) due to electrostriction calculated earlier, assuming to be
independent of pressure. It also supports Passynski’s approximation
In Fig. 2.75 the decrease of is shown as a function of log X, where X is the
electric field in the hydration shell. For an ion of radius 100 pm, this is 49

If one compares this with the Conway relation of local pressure to field, one sees that
this field is equivalent in electrostrictional pressure to about at which (see
above) the compressibility is about 22% of the value at the standard pressure.
Thus in Fig. 2.75(a), the decrease of the molecular volume of water in the
hydration sheath of an ion, the radius of which is 100 pm, is about or
about 11% (the first approximation was 44%).
For instance,

at and from Eq. (2.165),

49
The Système International of electrical units includes most of the common electrical units, such as volt
(V), ampere (A), ohm watt (W), and coulomb (C). In this system, Coulomb’s law reads

where Useful conversions to remember are 1 J = 1 N m and 1 J/C = 1
V, In the cgs system (in which the basic units are the centimeter, gram, and second), the constant k´ is
defined to be unity, without unit. In this system the unit of electric charge is the statctmlomb or esu
(electrostatic unit), which is still used in many scientific publications. The conversion factor is, to four
significant figures.