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Testing for iron in fruit juice
Name: Elaine
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: 1999
Question:
I am in the fifth grade. I have been conducting an experiment to
see if fruit juices contain iron. I put a teaspoonful of each
of five juices in a test tube (one tube for each type of juice.)
Then I put a teaspoonful of tea in each tube. I conducted the
experiment several times with the same result. A precipitate
formed in the tube containing pineapple juice. Since I have read
that this is a way to test for iron, I assume that iron in the
pineapple juice has reacted in some way with the tannic acid in
the tea. I would like to know more about the reaction. Can
anyone tell me the chemical equation that describes it?
In what form is the iron dissolved in the pineapple juice at
the beginning? What is the name of the substance that is precipitated?
Also, why doesn't it work with prune juice, even though
the prune juice is supposed to have more iron than the pineapple
juice?
Thanks for your help.
Replies:
Hi Elaine,
An excellent question. Glad to see that you are doing
some careful thinking about an experiment that you've done!
I am still looking into this, but I think I can give you
a little preliminary info. First of all, I think that
you are forming iron(II)sulfide, which is insoluble.
Although I don't yet know the formula of tannic acid, it
may contain sulfur....
A lot of this is actually guesswork, based on the fact that
you are making a black, insoluble iron-containing
compound.
It may not work so well in prune juice due to the fact that
prune juice is so dark that the presence of a black precipitate
may not be obvious. Another possibility is that iron sulfide
might be more soluble in prune juice than pineapple juice due
to the difference in pH. I'd expect pineapple juice to be
much more acidic than prune juice. You can test this out by
adding a little acid to the prune juice before adding the
tea...you could use a weak acid, like acetic acid (vinegar) for this.
Chemists often adjust the pH to adjust solubility, especially
if the anion of the insoluble compound could conceivably be
part of an acid-base reaction.
If I can find out more detailed info, I'll post it here!
topper
By the way, if iron(II) sulfide is what is precipitating,
I'd guess that the iron was originally in the form Fe2+...
not necesarily though, could be Fe3+, or some combination.
topper
Elaine told me offline that the precipitate is
sort of orangish, not black at all...I need to
go back and look in my old qual ID book, not sure
about that one...
topper
Whoops, more complications...Elaine did some detective
work and found that tannic acid contains no sulfur...
I was at least correct in remembering that Fe^{2+} can
be precipitated as FeS. But looking in my old text,
it seems that FeS is only insoluble if the solution is
basic...thus the role that tannic acid is obscure to me...
however FeS (and its cousin, FeS_2), is typically black...
So where the heck would the sulfur come from?
I Just Don't Know the answer to this one....
blush
topper
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