Description of experiment
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Needed compounds:
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hydrochloric acid : HCl
potassium hexa nitrito cobaltate (III) : K3 [Co(NO2)6] . H2O
Class:
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elem=Co
redox
Summary:
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Cobalt yellow contains the hexanitrito cobaltate (III) complex ion. This
ion is remarkably stable, when it is locked up in the insoluble potassium
salt. With concentrated hydrochloric acid, however, the complex ion is
broken down slowly.
Description:
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Add some solid K3Co(NO2)6.H2O to concentrated hydrochloric acid (30% HCl by
weight): The liquid at first becomes yellow and turbid, due to fine dispersion
of the solid in the liquid. Very slowly, the color of the liquid changes from
turbid yellow to turbid and green. It also becomes somewhat darker. A smell
of chlorine can be observed.
Heat the liquid: The liquid becomes clear and dark blue. The blue color
differs from the deep royal blue of the [CoCl4]2- complex. The color is less
intense and there is a green hue. There is no visible brown NO2 above the
liquid, however, there is a greenish/yellow gas (mixture) above the liquid.
This may be Cl2, but it may also be due to formation of NOCl. The green tinge
may also be due to formation of NOCl, which, when mixed with blue [CoCl4]2-,
gives a green color (NOCl is yellow/orange in concentrated HCl).
Dilute with a lot of water: A small amount of NO/NO2 appears to be formed,
when a small amount of water is added. On addition of much more water, the
liquid becomes pale pink. This pink color is due to formation of Co2+(aq).