TBR Selection by Precipitation Question

[justadream]

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In the "selection by precipitation" section (part of the Equilibrium chapter in TBR GC - it's page 195 for 2010 copyright edition):

Figure 3-6 shows a precipitation flow chart for separation of a bunch of ions. As I understand it, you should separate out whatever is LEAST SOLUBLE first (then second to least....third to least...etc).

Since the cations are +2 cations, the Ksp values can be directly compared. Since you need to separate out the least soluble ion first, the Ksp values should start at the smallest and become bigger. The example doesn't seem to follow that trend:

In order of separation:
1) PbCO3 Ksp = 1.4*10^-15
2) SrSO4 Ksp = 7.9*10^-10
3) CaF2 Ksp = 4.0*10^-11

Am I misunderstanding the chart (or just misunderstanding the overall concept)?

 

[sillyjoe]

If you look at the flow chart in every step you want to utilize an ion that will selectively precipitate out only one ion of Ba, Ca, Pb, and Sr as a salt. In order to do so you look at an ion that will yield salts with a large variance between the least soluble salt and the next least soluble salt.

In this case the best ion to use is CO3 because you can see the Ksp for the following salts:

1. BaCO3 = 1.6*10^-9
2. CaCO3= 6.1*10^-9
3. PbCO3= 1.4*10^-15
4. SrCO3 = 8.4*10^-10

As you can see, the difference between the Ksp's is 5 orders of magnitude and as such you can utilize CO3- to selectively precipitate PbCO3 out of solution.

If you follow the same thought process you can continue in this flow chart to selectively precipitate each ion.

Note that in the next step there is a small amount of BaSO4 precipitated out. This is due to the fact that SrSO4 and BaSO4 have Ksp's that are ~half an order of magnitude off and therefore they are close enough to yield some precipitate of BaSO4 (the more soluble salt) even though SrSO4 is in the majority due to the lower Ksp.

Also note that you can only do this comparison with Ksp as opposed to molar solubility because each salt exists in an MX form.

 

[justadream]

Excellent explanation. It makes a lot more sense to me now. Thanks!