What is the color of a FeS4O6 solution [closed]








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What is the colour of FeS4O6 solution?!
[![Question](https://i.stack.imgur.com/Fn7jx.jpg)](https://i.stack.imgur.com/Fn7jx.jpg)




This is a relatively advanced analytical chemistry question and it requires that you remember the color of all common transition metal ions and their very common precipitates. You have to remember the colors of halogens in solution as well.


Instead of worrying about the color of iron thiosulfate (which is bluish green), think about the first step. KI reacts with one the given choices, which renders the solution brown AND there is no precipitate. Iodine in aqueous solution gives a brown color. 


From the choices, see which ion can oxidize iodide (colorless) to free iodine (brown solution)? Only copper (II). Now understand that sodium thiosulfate is a powerful reducing agent. Which ion when reduced forms a colorless solution AND which ion which forms a precipitate with iodide? 


Now you have to double check mercury and see if this a possible choice or not.
[Tornado reaction](http://chemistry.elmhurst.edu/demos/tornadoreaction2.htm)


 2

The correct answer is, indeed, in my opinion choice (B) as is noted. 


The action of KI on CuSO4 creates, in situ, the very unstable CuI2 (think of the corresponding salt with iron, Ferric iodide, whose very existence is doubted).


Per the [web from Wikipedia](https://www.google.com/search?q=wikipedia+on+CuI2&rlz=1CAHKDC_enUS750US750&oq=wikipedia+on+CuI2&aqs=chrome..69i57.9535j0j8&sourceid=chrome&ie=UTF-8):



> 
> The CuI2 immediately decomposes to iodine and insoluble copper(I) iodide, releasing I2.
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> 
> 


This explains the brown coloration. The formed salt [Cuprous iodide per Wikipedia](https://en.wikipedia.org/wiki/Copper(I)_iodide):



> 
> Pure copper(I) iodide is white..
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> 


With respect to the question on ferrous tetrathionate, from an [old chemistry journal](http://iron.atomistry.com/ferrous_tetrathionate.html), it is unstable on creation and readily decomposes with the release of sulfur dioxide and elemental sulfur.


 1

In aqueous solution, the substance $FeS\_4O\_6$ is dissociated into a mixture of the ion $Fe^{2+}$ and of $S\_4O\_6^{2-}$. The ion $Fe^{2+}$ is pale green, but so pale that its solutions are considered as colorless. The ion $S\_4O\_6^{2-}$ is colorless.


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