Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
-----------------
sodium hydroxide : NaOH
potassium ferrocyanide : K4 [Fe(CN)6] . 3H2O
nitric acid : HNO3
Class:
------
elem=Fe
redox
Summary:
--------
Potassium ferrocyanide reacts with concentrated nitric acid. The compound
which is formed is dark brown/green. What is this compound?
Is the dark brown/green color due to formation of a Fe(NO)+ complex, well
known from the brown-ring test for nitrates?
Description:
------------
Add some solid potassium ferrocyanide to nitric acid (appr. 52% by weight):
The crystal structure disappears immediately, the crystals break apart and
a yellow powder-like solid is formed, which remains on the bottom of the
test tube. The liquid above the solid remains clear and colorless. The
color of the powder-like solid is much brighter yellow, than the color of
the original crystals of potassium ferrocyanide (which are light yellow).
When the liquid is shaken, then it becomes turbid and yellow.
Add water, such that the acid is diluted to 20 - 25%: The powder-like solid
becomes crystal/glass like almost immediately and a few seconds later the
solid has dissolved. When the solid has dissolved, the liquid is intense
yellow and clear, resemling a solution of potassium ferricyanide.
Heat the liquid until it almost starts boiling: The liquid becomes dark
brown. No (or just a very small amount of) gas is formed, the liquid does
not bubble or foam.
Let liquid cool down: Liquid remains brown, but it gets a greenish hue.
Add an excess amount of a solution of NaOH: The liquid becomes yellow/brown
with small brown/orange flakes, floating around in the liquid.