I have a stock solution of iron (III) chloride (10% Fe) and it's a fully dissolved solution. To get down to 1% Fe, I mixed 5mL of the stock solution with 495mL of DIH2O. However, after a while I get a precipitate (probably iron(III) oxide-hydroxide). Why does this solution precipitate, but it doesn't in the stock solution? The SDS states that its composition is iron(III) chloride [50%>x>=30%] and lists nothing else.
A couple of ideas come to mind for you to check on: Is the stock solution acidified at all? FeCl3 might be soluble in water, but to keep it in solution could the vendor be adding a bit of acid to keep it in solution? Is your DI water neutral or does it tend alkaline at all? I wonder if the pH might have something to do with it, and laboratory DI water is fine until you do something that is highly sensitive and then you discover the filtration system was last changed "a couple of years ago". If it absorbs CO2 from the air, perhaps that is causing the precipitate?
Also, your calculation seems at least a factor of 10 off: the stock is 10% FeCl3 , you make a 1% solution of that by diluting 5ml in 500ml. Final concentration is 0.1% FeCl3, of which only ~0.034% is Fe metal. Is that what you intended?
Matthew Wheal It's 10% Fe not 10% FeCl3 and I mistyped I meant to say 0.1% Fe, apologies; the math is pretty weird and confusing. (this kind of chemistry is definitely not my strong suit)
I did a test with some pH paper and it came out at about 1 so it's pretty acidic. We plan on trying to contact the vendor soon and ask them more questions; I figured I'd also ask on here to try and get a better understanding.
You may want to check: https://www.researchgate.net/post/What_iron_species_are_precipitating_after_autoclaving_ferric_chloride_water_solution
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