We are having some problems on preparing a concentrated ferric iron solution ( ppm) at pH 4.0. Dissolving ferric iron at low pH is easy but when we increased the pH most of the iron is precipitated. We need it at these pH value beacuse we are setting some experiments with plants and bacteria.
Any suggestions for that?
Thanks!
Graciana Willis
My iron chemistry is very rusty (pun intended) but if I recall, ferric iron is not soluble above about pH 3.5.
What about using an organic acid such as citric acid, maybe you will get iron complex and so less precipitation?
Hola Graciana,
As Matthew says, ferric iron is highly insoluble above pH 3.5. To carry out your experiments, you will need to stabilize your ferric iron in solution by using some chelating agent. A good method which is commercially available is combining sulfosalycilic acid with citrate buffer (or citric acid plus sodium citrate). By doing so, you prevent Fe(III) reprecipitation, as 5-sulfosalycilic acid chelates most of the Fe3+ ions in solution, and citrate stabilizes (buffers) the solution at the desired pH (in your case, 4.0).
The amount of reactants to be added will depend on the experimental conditions (e.g., pH, Fe(III) concentration) you want to obtain.
Have a look to this link, which includes a good description of the method (it is an old US patent from 1978):
http://www.google.com/patents/US4167214
There are many other chelating agents you can use, but the one I mention is rather convenient and easy to obtain in any lab. Some suppliers you can contact are Hach or Sigma-Aldrich, but there are many others.
Good luck !
Cheers,
Javier
Thanks for your answers. We supposed that about chelating as we have prepared original AMD from a mine in the north of Argentina at pH 4.0. Even though there is some precipitation we still have lot of iron in solution.
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