hydrated chromium oxide

1. SHREE MALLIKARJUN COLLEGE TYBSC
SAFETY: Refer to MSDS of potassium dichromate, boric acid
-Dr. Mithil S. Fal Desai
Aim: Preparation of Guignets green ((hydrated chromium oxide)
Chemicals: Potassium dichromate, boric acid
Apparatus: Beakers, conical flask, funnel, glass rod with policemen, porcelain dish, mortar and
pestle, silica crucible.
Theory:
Chromic oxide (Cr2O3) is commercially used as a green pigment for various purposes, as it is one
the only green pigment which can withstand high temperatures and is almost inert in acidic and
alkaline media.
Guignet's green is hydrated chromic oxide Cr2O3.2H2O. In its preparation, boric acid and
potassium dichromate mixture is heated to dull redness. The resultant yellow-green solid mass
swells due to the evolution of oxygen. It is then boiled with water whereby the borate of chromium
decomposes into boric acid and hydrated chromium oxide. The potassium borate generated is
soluble in water and prepared oxide can be filtered, washed with warm water.
Reactions:
6K2Cr2O7 + 16H3BO3 → 12CrBO3 + 4K3BO3 + 24H2O + 9O2
2CrBO3 + 3H2O → Cr2O3.2H2O + 2H3BO3
Procedure: 1) Weigh 1.0 g of K2Cr2O7 and 3.0 g of boric acid.
2) Mix it thoroughly and transfer it to a silica crucible.
3) Heat the crucible on the flame for one hour. (800 °C for 2 h in furnace is desirable for good
yield)
4) Cool the crucible and transfer the resultant mixture in a 100 mL beaker.
5) Add 20 mL water, boil with constant stirring for 2 min, and filter it hot.
6) Wash the precipitate with hot water and dry the product at 80 °C in an oven.
7) Weigh the product as Cr2O3.2H2O and report the percentage yield.
Result: Yield of Guignets green is ____g
Questions
1. List any four inorganic pigments.
2. How you will check the presence of potassium in the compound?