Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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ferric ammonium citrate brown : Fe C6H5O7 . 2NH3 . 2H2O
nitric acid : HNO3
sodium benzoate : C6H5 COO Na
Class:
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elem=C
redox
acid/base
Summary:
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Benzoate and citrate, used together, give rise to formation of a strange
phenomenon, when heated with concentrated nitric acid. It looks as if one
large crystalline mass is created. It must be investigated, whether the
iron is essential for this or not.
Description:
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Add some solid sodium benzoate to concentrated nitric acid (appr. 60% by
weight): No visible changes, the solid does not dissolve.
Heat the liquid: The solid still does not dissolve.
Add some solid ferric ammonium citrate brown to the still hot liquid: The
solid slowly dissolves. The liquid becomes yellow/brown and a small amount
of NO or NO2 is evolved (the gas-mixture above the liquid becomes pale
brown). The sodium benzoate now also dissolves. Finally, a clear yellow/
brown liquid remains.
Cool liquid under cold tap water: The liquid solidifies and looks as a large
crystalline/glass-like mass (it is not like a jelly or extremely viscous).
The cryalline mass is yellow and turbid.
Add a large amount of water: The water becomes light yellow, but the solid
does not dissolve.
Decant the water and heat the solid again: The solid melts and becomes liquid
again (not viscous, but like water). The liquid again is clear and yellow/
brown.
Pour the yellow/brown liquid at once in a large excess amount of water (appr.
3 ml of yellow/brown liquid in appr. 150 ml of water): Liquid mixes with the
water. The water becomes yellow and clear.