The Chemistry of the Solid Earth - Principles of Environmental Chemistry - page 265

concept of ion activity (see also Chapter 3)

{i} ¼ g i [i] (5.29)

where
{i} ¼ activity of the ion
[i] ¼ concentration of ion (mol L 1 )
g i ¼ activity coecient.
At infinite dilution, {i} ¼ [i], i.e. g i ¼ 1. Ionic interaction increases with
concentration and the square of the ionic charge, which is described by
the ionic strength of the solution (I):

X ½ i z i

I ¼ 1
2

ð 5 : 30 Þ

where

I ¼ ionic strength
[i] ¼ molarity of each ion in soilution
z i ¼ charge on each ion in solution.
If all the ions in soil solution can be accounted for, I can be calculated,
and then g.
The relationship between the activity coecient of an ion and the ionic
strength of the solution is given by the Debye-Hu¨ ckel limiting law
Log 10 g i ¼ 0 : 5 z i

p I

ð 5 : 31 Þ
For soil solutions, the Davies equation, which is a modification of the
Debye-Hu¨ ckel equation, is commonly used, and is applicable to solu-
tions up to approximately I ¼ 0.7 mol L 1 (see also Section 3.2.1):

h 0 : 3I i

p I

= 1 þ I

p

Log 10 g i ¼ 0 : 5 z i

ð 5 : 32 Þ

Worked example 5.7 - soil solution chemistry: ionic strength and
activity coecients
The following analytical data were obtained for an isolated soil
solution:
Ca 21 , 5.05 mmol L 1 ;Mg 21 , 3.55 mmol L 1 ;K 1 , 0.68 mmol L 1 ;
Na 1 , 1.80 mmol L 1 ;NO 3 , 3.70 mmol L 1 ; HCO 3 , 1.80 mmol L 1 ;
SO 4 2 , 6.25 mmol L 1 ; and Cl , 1.40 mmol L 1 .