DISTINGUISH BETWEEN Fe
2+ & Fe
3+ SALT USING NaOH. [1
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To equal volumes of solutions containing Fe2+ & Fe3+ ions, in separate test tubes, add sodium hydroxide, NaOH, solution drop wise until visible colour changes and precipitation occurs. The hydroxide, OH-, ions from NaOH readily combine with the iron cations and form different coloured precipitates.
Fe2+ + 2OH- = Fe(OH)2 a grass green ppt. Fe3+ + 3OH- = Fe(OH)3 a reddish-brown ppt.
On standing the test tubes, the Iron(II)hydroxide, Fe(OH)2, gets slowly oxidised to Iron(III)hydroxide Fe(OH)3, and its colour changes from grass green to reddish brown. especially on the surface of the solution. Fe2+ = Fe3+ + e- (oxidation).
The NaOH solution test, is widely used as a confirmatory test for the detection of Fe2+ & Fe3+ ions
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