Ferrous Chloride solution

[nickykinz]

I’m looking for tips on making a 0.1M solution of FeCl2. It is only for precipitation reactions so accuracy isn’t too important but would like to have it in the right ballpark. It is labelled up FeCl2XH20 MW 126.75g + aq. I think last time I made it I assumed 4 H2O as that is the most common (apparently) so will probably do that again unless anyone has better ideas. But it also wasn’t a great solution and settles out a lot. Is this normal or do I need to add something other than water? I seem to remember hearing H2SO4 at some point but cannot find anything written down anywhere. Any help/advice appreciated.

[Jazz]

Iron ii sulfate solution has bluish greenish colour but oxidise easily to iron 111 sulfate which has brownish rusty colour. For the PPT test you need iron ii; best prepared fresh with splash of 2M H2SO4

[Jazz]

Iron ii chloride 7H2O
dissolve 13.5 gr of Iron ii chloride in 500 mL of water add 2 mL conc HCl.
4 H2O
126*.5*.1 =6.3 gr

[Ian]

I have a little bottle of FeCl₂ x H₂O from Chem Supply and a large bottle of FeCl₂.4H₂O from Unilab. The FeCl₂ x H₂O is labeled as 126.75 g/mol which is the same answer that you get when you add the weighs of one Fe plus 2 Chlorines together. The FeCl₂.4H₂O is labeled as 198.81 g/mol. I guess the first one (FeCl₂ x H₂O) must be anhydrous and the x H₂O must mean NO H₂O.

In that basis, I would take the molecular weight as being 126.75 and mix my solution accordingly.

Cheers
Ian

[Graham Kemp]

Yes.

Anhydrous FeCl₂ is 126.7516 ± 0.0003 g/mole. To prepare 500mL of 0.100 M, take 6.34g, disolve in ~450mL, add ~2mL conc. H₂SO₄, then dilute to the final volume of 500mL. (Don't add the acid after you make to volume!)

Tetrahydrous FeCl₂•4H₂O is 198.8129 ± 0.0003 g/mole. So use 9.91g of that, as above.



Adjust weights and measures to suit volumetric flasks being used.

[nickykinz]

Thanks guys. Got some help from the labbies forum too so think I am sorted. Don't have an issue with making the solution just which Molecular weight to use. I'm pretty sure that the X means they don't know how many H2O (i.e. its a mixture) rather than none or it would just say anhydrous. As I say doesn't need to be too accurate so will use the tetrahydrate MW.
Thanks for all your help.

[DavidPeterson]

When I need Fe2+ at any time, I normally use ferrous ammonium sulphate - it doesn't oxidise to the ferric species in solution.

[lada]

Great tip!
I found that by the time teacher uses the solution, it has oxidised. I will use ferrous amm. sulfate from now on.
Lada