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Experiment 6: Sodium hexanitrocobaltate synthesis

July 2, 2013

Introduction

Sodium hexanitrocobaltate, with the chemical formula Na3[Co(NO2)6], is a classic coordination compound where cobalt has 6 as the coordination number and a rather uncommon oxidantion state for this element, which is -3, as shown in the structure below:

116621

In analitical chemistry, it is used for the quantitative determination of K+or NH4+, which precipitate as yellow solids.

Objective

The preparation of sodium hexanitrocobaltate and the identification of Kusing the prepared sollution.

Necessary materials

Laboratory tools: 3 small beakers(cca. 50 ml), 2 big beaker(300-500ml), stirring rod

Chemical substances: Co(NO3)2 (if you don’t have it in your laboratory, follow my method of working wich uses solid CoCl2 and HNO3 sollution), solid NaNO2, concentrated CH3COOH(50-90%), KCl sollution

Method of working

DSCN5787                 DSCN5785            DSCN5788

Prepare a CoCl2 sollution by dissolving cca. 50 mg of CoCl2 into 200 ml water in one of the big beakers using the stirring rod. The put 20 ml of HNO3 sollution so as to form Co(NO3)2. In two of the small beakers then prepare a NaNO2 sollution by dissolving 10 mg of NaNO2 into 40 ml water and a CH3COOH sollution. After that, mix the contents of the three beakers into the other big beaker. The sollution formed will look like this:

DSCN5806

Here is the sodium hexanitrocobaltate. The chemical equations on which this synthesis bases on are:

CoCl2 + 2HNO3  -> Co(NO3)2 + 2HCl

Co(NO3)2 + 2NaNO2 -> Co(NO2)2+ 2NaNO3

NaNO2 + CH3COOH  -> HNO2 + CH3COONa

Co(NO2)3 + 2HNO2 -> Co(NO2)3 + NO + H2O

Co(NO2)3 + 3NaNO2 -> Na3[Co(NO2)6]

WARNING: do not stir this sollution too much as it is air-sensitive and the oxidation of cobalt might result in an unexpected and unwanted change of colour:

DSCN5803

No we will perform de identification of the potassium ion using the prepared sollution.

Prepare a KCl sollution by dissolving 10 mg of KCl crystals into 40 ml water in the other small beaker and then proceed to add it in the sodium hexanitrocobaltate sollution. The chemical equation on which this reaction bases is:

Na3[Co(NO2)6] + 2KCl -> K2Na[Co(NO2)6] + 2NaCl

K2Na[Co(NO2)6] is a yellow precipitate which will look like this:

DSCN5813

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