[retupmoc]

When slowly adding dil NaOH to Cobalt(II)nitrate why are there a series of colou changes that occur.  In particular what are the transient blue/green then pink precipitate and why does a brown precipitate finally form?  Ive looked everywhere for the equation (equations presumably) which describes this reaction and can only come up with

Co(NO3) + 2NaOH --> Co(OH)2 + Na2(NO3)

thus presuming the oxidation state of the cobalt is still +2 and Co(OH)2 is the final brown precipitate tho im sure this is inaccurate.  Please Help

[hmx9123]

What you're most likely seeing is the color change of several complexes of cobalt.

First, you have [ Co(H₂O)₆ ] ⁺², which is the aqua complex formed when cobalt II is put into aqueous solution.  This is a pink color.  When you add OH^- to the solution, you get Co(OH)₂ at first (which is either pink or blue, depending on conditions).  This is amphoteric, which means it dissolves in either acid or base, and will dissolve if enough OH^- is added, to give the tetrahedral complex [ Co(OH)₄ ] ^-2.  This complex is a deep blue.

This information came from Cotton & Wilkinson's Advanced Inorganic Chemistry.  You might want to see if you can find a copy of it in your library someplace.

[retupmoc]

is the Co(H2O)6 the pink precipitate then? Im not fully understanding this yet, initially didnt fully understand what aqueous solution actually meant in terms of hydrated cations but gaining a better understanding

[retupmoc]

Forget that last reply, the pink precipitates the [Co(OH)2(H2O)4]2+ depending on the conditions right? and the blue/green precipitate is [Co(OH)4(H2O)2]2+? Then when this mixture is left to stand in air for a short time and finally get a brown precipitate, has the cobalt been oxidised here to Co(III)?

[hmx9123]

Actually, the [ Co(H₂O)₆ ] ⁺² and the other cobalt complex should not be precipitates; rather, they should actually be dissolved in the solution.  They simply impart their color to the solution.

Could you describe exactly the steps that you took and the colors that you see?  This would help a great deal.