Hexavalent - 9-12-2012 at 07:15
In my local garden store, they've just introduced a new product designed to change the colour of hydrangeas from pink to blue. The label says
'Contains aluminium sulfate', which works because the pigment goes blue in the presence of iron or aluminium salts.
So, in a moment of inquisitivity, I bought a bottle of the stuff and dissolved some in water, which yielded a cloudly green suspension. After
magnetically stirring for an hour, not all had dissolved, so a filtration was carried out. This resulted in a clear, green solution. Hydrogen peroxide
was added which, as expected, turned the solution brown/orange (Fe2+ - e- → Fe3+).
Therefore, I now have a solution containing aluminium sulfate and an iron (III) salt. To separate the two, I thought of adding sodium hydroxide
solution, so precipitate the two metals as hydroxides, and then adding more until the aluminium hydroxide re-dissolves due to its amphoteric nature. I
could then add sodium carbonate solution to precipitate aluminium carbonate, which I could react with mineral acids to produce some nice Al salts.
Alternatively, I could add some aluminium metal to the solution to displace the iron from its salt. However, this could lead to an impure product as I
don't know which salt of iron is actually present.
ATM, I think I'll go with the first option, but any thoughts or comments would be greatly appreciated.
kristofvagyok - 9-12-2012 at 07:43
Why are you doing a hard and not so cost and chemical effective reaction series for a simple thing? Aluminium sulfate is a cheap and useless chemical,
good for nothing category.
Your producere has a few problems: 1: H2O2 is added what will make Fe(III) ions, but if you would simply add lye, then it would also end up with the
same, insoluble iron-oxide, hydroxide. So why is it added? I don't get it.
2: The excess of NaOH will pull a lot other things not just the Al, remember that Zn is also amphoteric and they add it a lot time to these like
products.
Why not simply get a kilo of Al2(SO4)3 for 10bucks and be happy with it? Or just use the Al metal/Fe(II) ion displacement? I can promise that you
won't get an AR product with this horrible method, it's just waste of time.
Hexavalent - 9-12-2012 at 07:50
Ok, thanks.
I didn't actually think about the peroxide step being unecessary, and I'd only intended for the experiment to be a) something to do, and b) an
exercise in multi-step purifications. I paid £1.99 for 250g, so the price wasn't too bad.
I'll try the displacement method and see what I get, although I do disagree with "Aluminium sulfate is a cheap and useless chemical, good for nothing
category.".
Thanks for your response
Ozone - 9-12-2012 at 08:23
I think that the color of the hydrangea is related to the chelate-form of the various anthocyanidin dyes--that is, blue is an anthocyanin-lake. The
ligand itself is red-pink. The pH of the soil determines whether Al is available for uptake. Your Al2(SO4)3 provides Al while acidifying the
soil--e.g. free Al.
REDOX is not explicitly the mechanism which changes the color of Hydrangea.
http://www.hydrangeashydrangeas.com/colorchange.html
Hexavalent - 11-12-2012 at 08:41
Bump!