Redox and redox equilibria
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I have 2 questions: 1) Why is it that potassium manganate(VII) solution is chosen to oxidise Fe(II) in the redox titration? 2) Why is iron(II) ammonium sulphate a relatively stable iron (II)compound? What is a double salt?
Corrie writes ...
1. Potassium manganate(VII) solution is chosen because manganate(VII) ions in acidic solution are a strong enough oxidising agent to oxidise Fe(II) to Fe(III) and the reaction is quick and quantitative.Secondly, because of the change of colour, from deep purple to almost colourless, when manganate(VII) forms Mn2+(aq), the titration is self-indicating. No indicator needs to be added as the (almost) colourless mixture turns pink at the end-point with the addition of the final drop of manganate(VII).
Potassium manganate(VII) is a solid that can be obtained reasonably pure - pure enough for work at school level. For very accurate analyses, however, it is not used as a primary standard and solutions need to be standardised, using e.g. oxalic acid.
2. As you probably know, lowering the pH of aqueous solutions of iron(II) sulphate by acidifying with sulphuric acid prevents oxidation by air to iron(III). This is believed to be a kinetic effect, that is to say it slows the oxidation reaction down. Although Mohr's salt (iron(II) ammonium sulphate) is very well known for its stability to oxidation, I haven't been able to find a satisfactory explanation for this. Ammonium ions are weakly acidic, so this may have something to do with it, even in the solid,
3. A double salt is a subtance containing two salts that have crystallised together in a fixed ratio to form a new substance. The double salt can contain two different cations (+ve) or anions (-ve), and when dissolved, behaves as a mixtureof the two salts.
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updated: 14 March 2007
