Basically, when a mineral like barite is dissolved in water, it can become insoluble again if the concentration of the dissolved ions (barium and sulphate) reaches a certain level. This level is determined by the solubility product constant (Ksp), which is very low for barite.
To calculate the concentration of the dissolved ions, we use activity coefficients (γ) based on the ionic strength of the solution. The Davies equation helps us determine these activity coefficients.
In simpler terms, this means that we can predict when barite will start to come out of solution and form solid particles based on the concentration of its dissolved ions in water.
Now, it would really be useful if we could estimate to what extent minerals might change
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 SO4
2-
).( [Ba2+].[SO4
2-
] )
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
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