[bryanh]

Hey everybody,
I use solutions of Mohr's salt (ammonium iron sulphate) in some experiments at concentrations from 0.1mM to 150mM and small volumes (50ul, typically). They are prepared in an anoxic glove box along with TRIS buffer and have pH from 8.2 to 6.7. I would like to know if anyone has any thoughts on the following.

My supervisor using simulation software (something I don't fully trust) has told me that most of the iron in these "solutions" will be solid (Fe(II) and Fe(III) oxyhydroxides/sulphates) with dissolved iron being in the form of Fe2+ and Fe(III) hydroxides but at concentrations negligable compared to the concentration of Mohrs salt added. What I don't understand is if nearly all the iron is in a solid phase, why don't I see any precipitate in these samples? Is it possible that the solid particles are too small to be seen? But the solution does not look even remotely cloudy! Is it possible that the numbers from the simulation are complete nonsense? Is it possible that much more of the iron is in solution?
Thanks.

[Borek]

My supervisor using simulation software

Good

(something I don't fully trust)

Wrong

Is it possible that the solid particles are too small to be seen?

Yes. At least in some solutions. But in some IMHO you should be able to see precipitate.

Is it possible that the numbers from the simulation are complete nonsense?

Yes. But there is no reason for prejudice.

Is it possible that much more of the iron is in solution?

Thing is, iron will be present in many forms - Fe³⁺, FeOH²⁺, Fe(OH)₂⁺, Fe(OH)₃, Fe(OH)₄^-, Fe₂(OH)₂⁴⁺, Fe₃(OH)₄⁵⁺ - and these complexes are probably not all. No idea what kind of simulation software you use, but I will start looking at these numbers - at least in some cases there must be precipitate present (more concentrated solutions - 0.150M is a lot for Fe³⁺, it will start to precipitate at pH about 1.4). If there is no precipitate listed, results are dubious.

[bryanh]

Hi, thanks for the reply,
The software used is geochemists work-bench, and is based on thermodynamic data. It says the solids should be shwertmannite, ferrihydrite and/or green rust 1, with dissolved iron as Fe(OH)3 and Fe(OH)₂⁺ and Fe2+. I appreciate that it should be there but I really don't see the precipitate, I will maybe try a small pore filter and analyse the filtrate for iron, although I worry that if the particles are so small I can't see them they will go through a filter.

[Borek]

The software used is geochemists work-bench, and is based on thermodynamic data.

PHREEQC?

It says the solids should be shwertmannite, ferrihydrite and/or green rust 1, with dissolved iron as Fe(OH)3 and Fe(OH)₂⁺ and Fe2+. I appreciate that it should be there but I really don't see the precipitate, I will maybe try a small pore filter and analyse the filtrate for iron, although I worry that if the particles are so small I can't see them they will go through a filter.

Let the precipitate age - left warm solution for several hours.

No idea whether TRIS doesn't complex iron, never seen any data.

[bryanh]

PHREEQ?
REACT and SPECE8!

What effect do you expect aging to have?

[Borek]

PHREEQ?

http://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc/

REACT and SPECE8!

Never used these.

What effect do you expect aging to have?

As always - precipitate crystals get larger (some grow, other disspear), this makes filtration easier.

[bryanh]

Cheers for the link.

As always - precipitate crystals get larger (some grow, other disspear), this makes filtration easier.

thats what I thought you'd say. Surely that would mean that the proportion of Fe in the solid phase increases with time? This is interesting in itself if it occurs over just a couple of hours, but it also means if I do this I won't know how much Fe is dissolved at the time at which I normally use the solutions. 


Edit: Use [ quote] and [ /quote] to quote

[Borek]

As always - precipitate crystals get larger (some grow, other disspear), this makes filtration easier.

thats what I thought you'd say. Surely that would mean that the proportion of Fe in the solid phase increases with time?

Why? It is just change of the crystal size, formula stays the same.

[bryanh]

So are you saying that the total amount of solid will stay the same and that the crystals grow as others shrink?

[Borek]

So are you saying that the total amount of solid will stay the same and that the crystals grow as others shrink?

Exactly. Bigger crystals are just more stable due to the better surface to volume ratio.

[bryanh]

Cheers, thats excellent.