FeCl2 replacement reaction

[Oz]

I recently read a note in Wikipedia “When crude platinum is dissolved in aqua regia, gold is removed from the solution as a precipitate by treatment with iron(II) chloride (FeCl2)”.

I was wondering why this would not drop the platinum as well. The reactivity series of metals would suggest both should drop.

 

[Lino1406]

Why, Pt is more reactive then gold?

 

[Oz]

Yes, Pt is more reactive than gold, in fact it should even replace gold in solution according to the chart I was looking at although it may not do it well. I would imagine that if you knew exactly how much Pt was in solution you could add just the right amount of FeCl3 to drop only platinum. If you read above it also suggests they are removing the Au from the mixed solution first with FeCl2. I have not worked with a mixed Au/Pt solution so just thought I would ask as the day will come. I usually hear of iron being used as a last drop precipitant or for dropping everything so I was just checking the accuracy of what I had read.

 

[eagle2]

OZ, Ferric (Chloride, Sulfate,) is selective for gold. The potential of the Ferric ion to move to Ferrous is just enough to reduce Gold. Platinum (also Palladium) requires a higher potential to become reduced to metal, so it stays in solution

Sodium Nitrite is also selective for Gold.

Al

 

[Oz]

Thanks Eagle,

Would that also hold true for an excess of FeCl2? And I am imagining that after dropping the gold you could still use an iron bar (elemental) to drop the platinum group then?

 

[Lino1406]

Formic acid. As was said here, specificity
or selectivity is the name of the game here
rather than reactivity - what makes it even
more beautiful

 

[eagle2]

Welcome OZ.

I do have to make a correction. The basic idea is still the same, but its The FERROUS ion that moves to FERRIC. It becomes more oxidized as it picks up Cl from the Gold Trichloride. An excess of Ferrous or any precipitant is usually a good idea. A drawback with Ferrous is it co-precips with Gold as Ferric Hydoxide and has to be washed out.

Ferrrous Chloride is FeCl2 or Ferrous Sulfate (Copperas) is FeSO4. This does the work of precipitation. Ferric ion (FeCl3) is the oxidized result and is now useless and in the way.

Iron metal for the PT group should work, but I`m not sure of the efficiency of the reaction.

Really, try to get some Zinc dust. That will surely do a good job of cleaning out the PM`s from the solution. It is what I use exclusively from heavy base metal solutions, and have good results with it. I also collect the traces of PM`s from AP solutions with it. Your solution must not be too acid, around Ph 1-2, and use a good excess of Zinc with a lot of stirring.

The was a guy on E-bay that sells it, cheap. Zincdustman, I think.

Al

 

[Oz]

Thanks to both of you for taking the time to help clarify this for me.

Oz