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salt19 "Prussian blue" another iron-III salt
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Hydrochloric acid is corrosive! Use goggles, work in a trash,
ask an adult to join you.
Potassium hexacyanoferrate(II)
is another sensible indicator for iron(III) salts beside ammonium thiocyanate.
The blue colour formed during
this test is called "Prussian blue).
* Once
again make some iron(III) solution by adding 1 ml of hydrochloric acid
(20%) to a small piece of a rusty Coal can in a vial.
* Photo 1: Take four other
vials with 0.9 ml water. Stepwise dilute the iron(III) salt solution in the
left vial: by transfer 0.1 ml of the yellow solution into vial 2, mix,
do the same with 0.1 ml of this solution etc.
* Add 1 drop of the indicator
to each of the 5 vials.
Observation
Photo right: Blue colours
appear even after a dilution of 1 to 1000.
Explanation: Negative hexacyanoferrate(II) ions and iron(III)
ions react to form the blue iron salt
potassium iron(II) hexacyanoferrate KFe-III[Fe-II(CN)6].