點樣分iron(II)同iron(III) sulphate呀?

在水溶液狀態，Fe2+是綠色，Fe3+是黃色，氨水 ammonia 是鹼性，有反應發生

Fe2+ + 2OH- --> Fe(OH)2
Fe3+ + 3OH- --> Fe(OH)3

Fe(OH)2 是綠色，Fe(OH)3是啡色，如果問Fe(OH)3為什麼不是黃色，可能是濃度的影響，在固體狀態，Fe3+的濃度比在水溶液中高很多，可能導至顏色變深，另一個更大的可能性是 Fe3+(aq)和Fe(OH)3根本應該被視為是不同的化合物，不同色是合理的。

acidified potassium permanganate solution 是氧化劑，可以把 Fe2+轉化為Fe3+，所以 acidified potassium permanganate solution 和 Fe2+是有反應的， acidified potassium permanganate solution 會由紫色變為無色，Fe3+則沒有反應而 acidified potassium permanganate solution 不會變色。

答你問題之先,要你知道ammonia(aq)和acidified potassium manganate(VII) solution的特性

For the aqueous ammonia, there is a dynamic equilibrium,
NH3 (g) + H2O(l) → NH4+(aq) + OH-(aq) ( reversible reaction )

KMnO4 is a strong oxidizing agent.

Why can use aqueous ammonia distinguish iron(II) & iron(III) ions?
Since aqueous ammonia have hydroxide ions, iron(II) ions and iron(III) ions can react with hydroxide ions to give green color iron(II) hydroxide and brown color iron(III) hydroxide.
Fe2+ (aq) + OH- (aq) → Fe(OH)2 (s)

Fe3+ (aq) + OH- (aq) → Fe(OH)3 (s)

They are water insoluble solids.
Hence, mix aqueous ammonia with iron(II) & iron(III) ion, we can distinguish them easily!


Why can use acidified potassium manganate(VII) solution distinguish iron(II) & iron(III) ions?
It is obvious that KMnO4(aq) can oxidize iron(II) ions to iro(III) ions.
The colour change of the solution is from pale green to yellow.
The reaction is showed below,
MnO4- (aq) + 5Fe2+ (aq) + 8H+ (aq) → Mn2+ (aq) + 5Fe3+ (aq) + 4H2O(l)

But mix KMnO4 (aq) with iron(III) ions have no reaction.

The last question,
iron(III) solution is yellow colour due to the complex [Fe(H2O)6]2+ ions.
The brown color prepicipate is iron(III) hydroxide. It is a brown solid.
They are different compounds.