1. SHREE MALLIKARJUN COLLEGE TYBSC
SAFETY: Refer to MSDS of cobalt chloride, cobalt nitrate, aluminum nitrate, aluminum chloride
-Dr. Mithil S. Fal Desai
Aim: Preparation of cobalt blue (azure)
Chemicals: Cobalt salt (cobalt chloride, cobalt nitrate), aluminum salt (aluminum nitrate,
aluminum chloride)
Apparatus: Beakers, conical flask, funnel, glass rod with policemen, porcelain dish, mortar and
pestle, silica crucible.
Theory:
Cobalt blue or cobalt(II) aluminate (CoAl2O4) is a blue inorganic pigment made by sintering CoO
and Al2O3 (alumina) at high temperatures. It has good thermal stability and is not affected by acid
or alkali.
Cobalt aluminate is a spinel-type of compounds that have the general formula AB2O4, where A is
a divalent cation and B is a trivalent cation having tetrahedral and octahedral coordination,
respectively.
Reaction:
CoO + Al2O3 → CoAl2O4
Procedure: 1) Weigh 0.2 g of CoCl2.6H2O and 4.1 g of Al(NO₃ )₃ ·9H₂ O in a 250 mL beaker.
2) Add 10 mL of water to dissolve.
3) Heat the beaker on the flame with constant stirring until the evolution of brown gases ceases.
4) Cool the beaker and transfer the resultant mixture in a mortar and pestle.
5) Grind the purple-blue solid and transfer it to a silica crucible.
6) Sinter the product at 800 °C for 1 hr and cool, weigh the product.
7) Report the percentage yield.
Result: Yield of cobalt blue is ____%.
Questions
1. Why high-temperature sintering is advisable to prepare cobalt blue?
2. How you will check the presence of nitrate and chloride in the product?
