Aqueous solutions and precipitates of nickel
In aqueous solutions, nickel only exists in the +2 oxidation state. From such solutions, hydrous nickel hydroxide can be precipitated. Nickel can also be brought to the +4 oxidation state, but this requires strong oxidizers. In water, a nickel (IV) compound can be prepared, but not in solution. It appears as a black precipitate. An experiment, in which the nickel (IV) compound is prepared, is given here.
Oxidation state +2
Nickel forms green hexaaqua ions in acidic and neutral media. The intensity of the green color is moderate. When a solution of sodium hydroxide is added to a solution of a nickel (II) salt, then a green precipitate of nickel (II) hydroxide is formed. The pictures below show a green solution, containing nickel (II) ions, a freshly prepared precipitate of nickel (II) hydroxide and the same precipitate, shaken.
Oxidation state +4
This oxidation state can be obtained by oxidation of nickel hydroxide. This can be achieved with bleach or with persulfate as oxidizer. The following two pictures show what happens, when some solid persulfate is added to nickel (II) hydroxide. The left picture shows the contents of the test tube, immediately after adding some sodium persulfate, the right picture shows the same test tube after some shaking.
With persulfate the change to black hydrous NiO2 is instantaneous, with bleach, it takes a few tens of seconds.
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