hodges
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Why Double Iron Salts?
I see a lot of references to double salts, especially those of iron. Things like ferrous ammonium sulfate, ferric ammonium citrate, etc.
My question is this - if someone wants an iron salt why not just go for the pure salt (ferrous sulfate or ferric citrate in this example)? Is there
some advantage to using double salts in reactions? Why are double salts salts (particularly of iron) sold commercially? After all, you don't
ever hear of other double salts like "sodium potassium sulfate", though they would I'm sure exist just like the double iron salts.
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JohnWW
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It has to do with the crystal structure of the double salt, which with a suitable co-cation can result in greater thermal stability, better
crystallinity, lesser hygroscopicity, or crystallization with less molecules of water of crystallization.
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darkflame89
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Yes so I wondered too. Pertaining to my other post, ferric ammonium citrate was first found to be light sensitive by William Herschel.Afterwards, they
found ferric oxalate works too. But why wouldn't just ferric citrate work?
Ignis ubique latet, naturam amplectitur omnem.
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unionised
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Ferrous ammonium sulphate is used as a secondary standard for redox titrations. It is better than ferrous sulphate because
1 its degree of hydration is more stable,
2 its less readilly oxidised by air and
3 it has a higher molecular weight (since analyses are refered to weights in the end this makes for a more accurated determination.
Ferric ammonium citrate is near neutral pH, soluble and, because the Fe is present as a complex, it's compatible with other materials that, as an
early pharmacist, you might want to compound it with.
(for example, you could prepare a mixture of magnesium hydroxide and ferric ammonium sulphate for a patient with indigestion and anemia, ferric
chloride or sulphate would just ppt out.)
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