Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium hydroxide : NaOH
EDTA tetra sodium : (NaOOC CH2)2 N CH2 CH2 N (CH2 COONa)2
sulphuric acid : H2SO4
potassium metabisulfite : K2S2O5
potassium permanganate : KMnO4
Class:
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elem=Mn
redox
coordination
Summary:
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Manganese builds a coordination complex with EDTA, which appears to be
pink. What oxidation state does the manganese have with this complex? It
appears to have oxidation state 2, unless EDTA is easily oxidized by
manganese, having a higher oxidation state.
Description:
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Prepare a solution with Mn2+ (aq) from potassium permanganate and potassium
metabisulfite in dilute sulphuric acid and drive off excess sulphur dioxide
from the metabisulfite by boiling gently for a while: A colorless liquid
is obtained, with Mn2+ (aq) and an excess amount of acid in it.
Add some solid EDTA-Na4 to this solution: The solid dissolves, the liquid
remains colorless.
Add an excess amount of a solution of NaOH: The liquid becomes turbid and
brown. Where the liquid is in contact with air, it becomes dark brown. A
precipitate is formed.
Add dilute sulphuric acid (appr. 2 mol/l) dropwise, until the liquid becomes
clear: The liquid becomes pink, much more intense than the very faint pink
color of plain Mn2+ in solution. The intensity of the pink color is
comparable to that of a Co2+ solution of similar concentration.
Add more dilute sulphuric acid (2 mol/l): The liquid becomes colorless. This
change does not occur instantaneously, but it takes appr. 10 seconds. The
color fades as follows: pink -> brown/pink -> light dirty pink -> colorless.