Saturation index

[pia said]

Hello, I would like to know if the value of the saturation index indicates something. I've already know that if SI>0 the specie tends to precipitate, but I want to know what means SI(specie1)>SI(specie2)??

[Pak]

I am not sure about what I write here, read at your own risk (disclaimer).

The SI = log(IAP/Ksp). Ksp is the equilibrium constant, and IAP is the ion activity product. So in equilibrium IAP/Ksp=1, and hence SI=0. If SI=1 you have ten-fold supersaturation and there is precipitation. You can assume that your Ksp is constant, and hence all the game is on the IAP.

If the species share ions and are extremely similar in composition (e.g. CSH0.8 [=species1] and CSH1.2 [=species2]from thermochemie database), one might thing that both species will precipitate but in this competicion CSH0.8 will precipitate more. Also, both will consume ions while preciptating so IAP will become smaller.

If they do not share ions, the effect is indirect and it will be related to how the precipitation or dissolution of the minerals affect the activity which affects the IAP.

Actually, this question can be tricky, it also depends on what chemical system you have, I think. The answer I give you is not too good but maybe it helps you a little.

[dlparkhurst]

In general, I would not compare magnitudes of saturation indices. Simply consider that assuming the SIs are accurate, a mineral with an SI < 0 may dissolve, but cannot precipitate; a mineral with an SI > 0 may precipitate, but cannot dissolve; and a mineral with an SI ~ 0 may be reacting fast enough to remain near equilibrium.

Note that the statements are qualified with "may" because the SI is a statement of thermodynamic direction of reaction, but kinetics may not favor the reaction, or, obviously, a mineral not present cannot dissolve.