Brown Iron (II) sulfate solution

[lazaruz]

I'm trying to understand why my solution of Iron (II) sulfate dissolved in water turns brown.

 

What I'm attempting is recrystallization of Iron (II) sulfate moss killer to purify it, but I have to deal with a lot of brown sludge forming on the bottom.

There are nice green crystals forming but it always has this brown on them.

 

Is this caused by oxidation and is it Iron oxide that is formed?

Is it because I heat the solution too much? It seems to get worse with heat.

Is there any way to prevent this?

 

I'm very new to this so any help would be greatly appreciated.

[CharonY]

It almost certainly is. Heat and pH lead to formation of different iron (hydr)oxides (which can range from red to almost black). Fe(II) oxidizes very readily and there is little to prevent that unless you add reducing agents or lower the pH significantly. At pH < 4 your Fe(II) solution should stabilize somewhat. Starting around 5 you will have a significant amount of Fe(OH)2 which oxidizes rapidly. If the water is near neutral or basic Fe(III) precipitates in form of iron hydroxides almost instantly.