Fe2+ can also be determined by UV-Vis with 2,2'-Bipyridine, while Fe total can easily be determined by AAS

Speciation of iron (and it's compounds) is often described by the Pourbaix diagrams (also known as Eh-pH diagrams) so you might want to start with those. The Pourbaix diagrams are related to equilibrium which means they might not be valid in kinetic studies. You can, however, find several papers on the speciation of iron compounds.

Thanks for your response.If you don't mind me asking, how do you use the pourabix diagram?I ned to know run an experiment at different pH levels but i'm having a problem with the Ferrous ion

My perception is, that the Pourbaix diagram is not necessarily useful for chemical engineers. It comes in handy if you're a corrosion/material scientist. But you need to judge that for yourself. The use of the diagram is fairly simple: you measure the pH and the redox potential of your solution and then read the diagram to determine the stable phases. Again, I want to remind you that the diagram is valid for equilibrium. If you wish to determine the concentration of the ferrous ion, I suggest titration.

Titrating with potassium dichromate, with sulphanate indicator to a violet end point (or you could use inflection point using a pH probe) is the method that I am familiar with to determine Fe2+. the method has limitations when sulfides are present or metallic Fe...check out ISO 9035, meant for solid ores. Already in solution, should be slightly simpler but be careful for other metal species (such as Mn and Cu) that may also oxidise with dichromate and bias your result.

Thanks Victoria and Riina

Fe2+ can also be determined by UV-Vis with 2,2'-Bipyridine, while Fe total can easily be determined by AAS

we can also computer the species in different pH and ions existing solution by a computer program named EQ3 ；

or you can determin species by laser sound speturem

What's laser sound speturem? What have you used it for in the past?

look up the Pourbaix diagrams

http://www.wou.edu/las/physci/ch412/pourbaix.htm

You can use voltammetry ( Anodic stripping voltametry) for Fe +2 and Fe +3

pH is not a sensitive technique to detect specie at low concentration. you should use the polarographic technique.

A sample with ionised Iron species , can be detected by ICP quantitatively

If you are analysing samples with ICPs you have to remember these techniques are not suitable for speciation, at least if they are not coupled to a chromatrography system. Besides that if your sample was solid and you've performed some digestion probably there was also a change on the valence iron. There are some spectrometric methods for the determination of Fe2+/Fe3+ and total Fe, however you have always to take care of the interferences.

First, you must specify which iron species? Fe2+, Fe3+? The best fashion is with an E-pH diagram which represents for pH 0 to 14 all the species including the hydroxides Fe(OH)2 and Fe(OH)3 with respect to the potential applied. But you have to know all the constant values (E° of each couple and solubility products Ks of the hydroxides.

No worry! Just do it!

It should be easy to seek with Google. If you could not, someone would do it for you soon, if necessary. All with great pleasure!

See: for example [PDF] Atlas of Eh-pH diagrams www.gsj.jp/GDB/openfile/files/no0419/openfile419e.pdf

Pourbaix diagram - Wikipedia, the free encyclopedia en.wikipedia.org/wiki/Pourbaix_diagram

http://corrosion-doctors.org/Corrosion-Thermodynamics/Potential-pH-diagram-iron.htm

and in French: http://ressources.univ-lemans.fr/AccesLibre/UM/Pedago/chimie/01/deug/CHIM204B/pdf/fer7.pdf

Cyclic voltagramm may be usable to find the oxidation state of Fe. Because the CV spectra run based on the electrons in the system

As stated above its soil specific. Here's title of one for mine impacted soil

Speciation and solubility control of Al and Fe in minesoil solutions

C. Monterroso, E. Alvarez, F. Macías

I'm writing currently about Anodic stripping voltametry and would suggest the technique as it allows in situ measurements for you to get Fe speciation results in the soil solution specific to your need. Your problem will be finding collaborators. Try people on these papers Buffle seems to be father of the technology..

Buffle J., Tercier-Waeber M.L. (2005) Voltammetric environmental trace-metal analysis and speciation: from laboratory to in situ measurements. Trends in Analytical Chemistry 24:172-191. DOI: 10.1016/j.trac.2004.11.013.

Cigala R.M., Crea F., Stefano C.D., Lando G., Milea D., Sammartano S. (2010) Electrochemical Study on the Stability of Phytate Complexes with Cu2+, Pb2+, Zn2+, and Ni2+: A Comparison of Different Techniques†. Journal of Chemical & Engineering Data 55:4757-4767. DOI: 10.1021/je100384f.

Bostick B.C., Hansel C.M., La Force M.J., Fendorf S. (2001) Seasonal fluctuations in zinc speciation within a contaminated wetland. Environmental science & technology 35:3823-3829. DOI: 10.1021/es010549d.

yes due to HPLC

If you wish to calculate the speciation then I would recommend Hyss2009 which is a free hyperquad program that will allow you to calculate speciation as a function of pH.

http://www.hyperquad.co.uk/hyss.htm

If you wish to be more accurate and bring into the equation Eh and pH as utilized in a Pourbaix diagram you will need geochemist workbench etc.

If you think the hydrolytic complexes, there is a program that can calculate.

Visual Minteq

Dear Lawal

The best way is to determine Eh and pH and plot them on their respective Eh-pH diagrams. You may buy software called as HSC Chemistry. 

Truly yours

karbassi

As well specified by the Iranian colleague,  E-pH diagrams are the best tools but be aware to not forget Fe(OH)3, Fe(OH)2 and all other hydroxy-complexes of both Fe(ii) and Fe(iii). In presence of other ligands, the speciation will be hard to represent. Take a look on the excellent book of A. Ringbom 'analytical chemistry '. Otherwise,  the dosage of iron(iii) could be done spectrophotometrically with thiocyanate ions