bob800
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No crystals forming on cooling; miscalulation?
Today I attempted to prepare some ferric ammonium sulfate from Inorganic Preparations by Henderson Fernelius:
| Quote: |
Dissolve 100 g. of commercial ferrous sulfate (copperas)
in 400 cc. of hot water to which has been added 1 or 2 cc.
of sulfuric acid and then add an equimolecular weight
of ammonium sulfate, remembering that the formula for
ferrous sulfate is FeSO4"7HO. It is well to add a
bright iron nail and keep the solution hot for a time to
reduce any ferric sulfate which may be present. Bring
the solution to crystallization, avoiding the formation
of crystal masses. The salt crystallizes in the mono-
clinic system, forming large, clear crystals of a somewhat
lighter shade of green than the crystals of ferrous sulfate.
The salt should be carefully air dried, avoiding efflores-
cence, and its solution should give no test for ferric iron
with potassium thiocyanate.
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However, on cooling, no crystals formed! Where I may have messed up was calculating the amount of ammonium sulfate. Here are my calculations:
What I believe to be the reaction:
FeSO<sub>4</sub>*7H<sub>2</sub>O + (NH<sub>4</sub> <sub>2</sub>SO<sub>4</sub> ---> (NH<sub>4</sub><sub>2</sub>SO<sub>4</sub>*FeSO<sub>4</sub>*6H<sub>2</sub>O + H<sub>2</sub>O.
Then, to find the proportion used by the text, I divided 100g (amount of FeSO<sub>4</sub> used in text) by 278g (1 mol hydrated
FeSO<sub>4</sub> and got 0.3597. I multiplied this by 1 mol of ammonium
sulfate to get 47.53g, which should be the amount of ammonium sulfate required in the text.
Since I didn't want to work on that scale, I multiplied all the amounts by 0.25, giving me the following final amounts:
25g Iron(II) Sulfate
0.25-0.50 mL H<sub>2</sub>SO<sub>4</sub>
100 mL H<sub>2</sub>O
11.875g (NH<sub>4</sub><sub>2</sub>SO<sub>4</sub>
I can't find any error in my calculations, but maybe I'm misunderstanding "equimolar weight". Or perhaps there's an obviously stupid mistake in my
calculations  . If anyone could help find my error, I would really appreciate
it!
Thanks!
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Mumbles
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Your amounts look right, but did you heat it at all? Besides ensuring everything is Iron (II), I wouldn't be surprised if it reduces the volume. It
will make sure absolutely everything is dissolved, well mixed, and ready to co-precipitate. At room temp, about 10g should crystallize out. It may
just be super saturated, and need some time to crystallize. Try storing it in the fridge overnight, or scratching the beaker or something.
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bob800
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Quote: Originally posted by Mumbles  | | It may just be super saturated, and need some time to crystallize. Try storing it in the fridge overnight, or scratching the beaker or something.
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You're right, it just needed to sit overnight! Some crystals have finally formed on the bottom of the beaker.
Thanks!
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blogfast25
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Your ratios were correct. Always add a few of seed crystals to your finished mix, then let cool...
[Edited on 24-4-2011 by blogfast25]
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