Now, it would really be useful if we could estimate to what extent minerals might change

Question

their solubilities when moving between aqueous environments of different ionic strengths.
Converting concentrations to activities (or effective concentrations) allows us to do just
that. Let’s take the fairly insoluble mineral barite as an example.
The equation for barite dissolution is:
BaSO4 = Ba2+ (aq) + SO4
2-
(aq)
From the law of mass action, the equilibrium constant = a Ba2+
.a SO4
2-
. This is also called
the solubility product constant or Ksp. Because barite is so insoluble, the actual value of
Ksp is very low, about 10-10. This means that when the product of the activities of dissolved
barium and sulphate ions exceeds 10-10 mol L-1
, barite will no longer be soluble and will
begin to precipitate out from solution.
Now we know that: (a Ba2+
. a SO4
2-
) = 10-10
= (γ Ba2+
. γ SO42-).( [Ba2+].[SO42-] )
and we can work out the activity coefficients (γ) from the ionic strengths using:
-log γ = Az2 {[I
1/2 / (1 + I
1/2)] – 0.2I} (Davies equation)

Explain it in simple English and give a example

Answer 1

In simple terms, we can use the concept of activities to estimate how minerals like barite will change their solubilities when moving between different environments with varying amounts of dissolved ions. For example, when barite dissolves in water, it forms dissolved barium and sulphate ions. The solubility product constant, or Ksp, tells us how soluble the barite is based on the product of the activities of these dissolved ions.

If the product of the activities of the ions exceeds the Ksp value, then the barite will start to precipitate out of the solution. To calculate the activities of the ions, we can use the ionic strengths of the solution. This information can help us predict when and where mineral precipitation may occur, based on the chemical composition of the aqueous environment.

For example, if the ionic strength of a solution increases, it can affect the activity coefficients of the ions, ultimately impacting the solubility of minerals like barite. This knowledge can be valuable in various fields, such as geochemistry and environmental science, where understanding mineral solubilities plays a crucial role in studying natural processes.