Preparation of sodium hexanitrocobaltate (III)

Chemistry 261

Practical Report

Practical 2

Thermos Thurman

211526893

PRACTICAL 2: Preparation of sodium hexanitrocobaltate (III)
Date: 12 March 2014

INTRODUCTION
Sodium hexanitrocobaltate (III) is a coordination complex which has the formula Na3[Co(NO2)6]. This compound is yellow in colour. It consists of a central Co3+ ion surrounded by six nitro ligands [1]. This compound is used as a qualitative test for potassium and ammonium ions (so long as certain other cations are not present) [1]. Potassium and ammonium are insoluble in water, while sodium is [1]. Hence, the potassium and ammonium salts are precipitated as a yellow solid.

AIM
To prepare sodium hexanitrocobaltate (III) from pure potassium-free sodium nitrite, cobalt nitrate hexahydrate, 50% acetic acid and 95% ethanol.

THEORY
In this practical, the nitrite ions acts as an oxidant and a ligand [2]. Co2+ (aq) is more stable than Co3+ (aq), however when cobalt attaches to nitrogen containing ligands, Co3+ becomes the more stable ion [2]. Initially, the hexanitrocbaltate reduces the excess nitrite. The reaction produces nitrogen oxide gas, which reacts with air to form nitrogen dioxide [2]. These gases are removed when the solution is introduced to a steady stream of air [3].
Reaction Equation: Co(NO2)3 + 5NaNO3 + 2NO3 → Na3[Co(NO2)6] + 2NaNO3 + NO + H2O

STRUCTURE

MATERIAL 12 grams pure potassium-free sodium nitrite 12 ml cold water 4 grams cobalt nitrate hexahydrate 4 ml 50% acetic acid ethanol ice glass rod dropper hotplate beakers measuring cylinder weighing boat test tube filter paper funnel filter flask aeration tube scale vacuum pump

METHOD
12.01 grams of pure potassium-free sodium nitrite was weighed and added to 12 ml of cold water in a 100 ml beaker. This mixture was then heated on the hotplate until the solid was completely dissolved. The resulting solution was cooled to 50 0C. 4.07 grams of colbalt nitrate hexahydrate was weighed and dissolved in the cooled solution. Thereafter, 4 ml of 50% acetic acid was added dropwise to the solution while being stirred continuously. The solution obtained was transferred into a test tube, placed into a filter flask and attached to an aeration tube. The vacuum pump was opened slowly to allow a steady stream of air to flow through the solution. This was done for approximately 20 minutes. After 20 minutes, the solution was placed in an ice bath, where 20 ml of 95% ethanol was added slowly while stirring. The mixture was left to stand for 20 minutes in the ice bath to crystallize. The product was filtered and washed three times with 10 ml of ethanol. The dried product was weighed to determine the percentage yield obtained.

RESULTS Weight (g)
NaNO3 (pure potassium-free sodium nitrite) 12.01
Co(NO2)3 (cobalt nitrate hexahydrate) 4.07
Na3[Co(NO2)6] (sodium hexanitrocobaltate (III)) 9.11
Table showing data obtained from the experiment

Calculations: Molar Mass (g/mol)
NaNO3 (pure potassium-free sodium nitrite) 196.96
Co(NO2)3 (cobalt nitrate hexahydrate) 85.00
Na3[Co(NO2)6] (sodium hexanitrocobaltate (III)) 403.96
Table showing the molar masses of the different compounds

Number of moles of Co(NO2)3 = mass/molar mass = 4.07/196.96 = 0.0207 mols

Stoichiometric ratio of Co(NO2)3: Na3[Co(NO2)6] = 1 : 1
∴ Number of moles of Na3[Co(NO2)6] = 0.0207 mols
Mass of product = Molar mass × number of moles = 403.96 × 0.0207 = 8.36 g

→ Percentage yield obtained = (Actual Yield)/(Theoretical Yield)×100 = 9.11/8.36×100 = 108.95 %

The limiting reactant was determined from the following calculation:
Co(NO2)6 and 5NaNO3
0.0207 mols → x mols (number of moles available for reaction)
1 → 5 (stoichiometry) x = 0.1035 mols… number of moles of NaNO3 needed for the reaction
However, actual number of moles of NaNO3 available for the reaction is 0.1413 mols. Therefore, NaNO3 will be in excess, while Co(NO2)3 is the limiting reactant.

OBSERVATIONS When the cobalt nitrate hexahydrate was dissolved in the solution of pure potassium-free sodium nitrite, the mixture turned reddish brown in colour. As the 50% acetic acid was added to the solution, its colour changed to a yellow and became thicker in consistency. As the solution cooled, a layer of brownish solution was observed at the top of the product. The final product of sodium hexanitrocobaltate (III) was seen to be a yellowish powder.

DISCUSSION
A yield of over 100% was obtained. This result obtained has some errors that can be due to:

Not filtering and drying the product sufficiently, leaving the product with excess water. Human error while measuring. Some product could remain in the beaker. Product could have spilled while transferring the product from the beaker to the filter funnel. While removing the filter paper, product could have dropped or could remain in the funnel.

CONCLUSION
The aim of the experiment was to prepare sodium hexanitrocobaltate (III), which was achieved. A yield of 108.95% was obtained, which was said to be as a result of errors while conducting the practical.

REFERENCES

[1] Anon, Available from: http://www.rsc.org/learn-chemistry/wiki/Substance:Sodium_cobaltinitrite [Accessed on 15/03/13 at 12:53]
[2] Anon, “Experiment 9: Sodium Hexanitrocobaltate(III)”. Available from: http://www.chem.mun.ca/courseinfo/c2210/lab/W2013Exp9NaHexanitrocobaltate.pdf [Accessed on 15/03/13 at 13:00]
[3] University of KwaZulu-Natal, Howard College, School of Chemistry and Physics, “Applied Inorganic Chemistry for Chemical Engineers Laboratory Manual”, 2013

References: [1] Anon, Available from: http://www.rsc.org/learn-chemistry/wiki/Substance:Sodium_cobaltinitrite [Accessed on 15/03/13 at 12:53] [2] Anon, “Experiment 9: Sodium Hexanitrocobaltate(III)”. Available from: http://www.chem.mun.ca/courseinfo/c2210/lab/W2013Exp9NaHexanitrocobaltate.pdf [Accessed on 15/03/13 at 13:00] [3] University of KwaZulu-Natal, Howard College, School of Chemistry and Physics, “Applied Inorganic Chemistry for Chemical Engineers Laboratory Manual”, 2013