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According to my school textbook (which is notorious, for being in possession of several errors), a certain "Everitt's salt" is a complex that results from the reduction of Prussian Blue.

Everitt's salt (according to our inglorious textbook) has the chemical formula $\ce{K2[Fe(CN)6]}$... which I straightaway dismissed as nonsense, because it would mean it's got $\ce{Fe^{4+}}$ (ferryl ion). From my (very limited) knowledge of the subject, the ferryl ion doesn't look like something that would occur in any reasonably stable compound; since Everitt's salt is, well, a "salt" (and since salts can usually be crystallized into stable solids).

Google wasn't much help in this regard,

enter image description here

I've no idea how to interpret the formula that is supposedly that of Everitt's salt


Questions:

1) Is there really something called "Everitt's salt"? Is it produced by the reduction of Prussian Blue?

2) What's its chemical formula?

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    $\begingroup$ Everitt's salt appears to be of mixed valence in iron: $\ce{K2Fe[Fe(CN)6]}$ molbase.com/en/cas-15362-86-4.html $\endgroup$ – Gert Dec 7 '17 at 19:24
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    $\begingroup$ @Gert Hmm... so Everitt's salt is a ferrous ferrocyanide salt? :O $\endgroup$ – paracetamol Dec 7 '17 at 19:25
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    $\begingroup$ acc. to that source, yes. Never before heard of it. $\endgroup$ – Gert Dec 7 '17 at 19:27
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    $\begingroup$ pubs.acs.org/doi/abs/10.1021/j100323a055?journalCode=jpchax Yes it's made by reduction $\endgroup$ – Mithoron Dec 7 '17 at 19:30
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    $\begingroup$ @Mith Aha! Good find! Okay, so now there's a (fairly) reliable source that mentions Everitt's salt... but there's no mention of the formula. So I can't tell if what Gert found is the same thing :( $\endgroup$ – paracetamol Dec 7 '17 at 19:33
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According to various sources Everitt's Salt has the formula $\ce{K2Fe[Fe(CN)6]}$, where both $\ce{Fe}$ are in the $+2$ Oxidation State. Everitt described it as a yellow precipitate, resulting from the reaction:

$$\ce{2 K4Fe(CN)6(aq) + 6 H2SO4(aq) \to 6 HCN(g) + 6 KHSO4(aq) + K2Fe[Fe(CN)6](s)}$$

Acc. Everitt, the wet salt can be oxidised to Prussian Blue by the action of air (oxygen).

Source.

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