Clean glass wares are the most important requirement of lab. Even carefully done work
may have the wrong results if dirty glass wares are used. After receiving and before starting a
work, the glass wares must be chemically cleaned so that there are no chemical deposits on its
surface.
Dirty glass wares should be cleaned up first by removing grease with a rag soaked in
benzene or chloroform, followed by soaking overnight in chromic acid. Chromic acid is widely
used as a cleansing agent for glass wares. It is a mixture of sodium chromate and concentrated
sulphuric acid. It has powerful oxidising and dissolving properties.
If it is not effective, a mixture of concentrated nitric acid and sulphuric acid can be applied.
All the traces of cleanser are then removed by repeated rinsing in the tap water followed by distilled
water.
Sometimes, new glass wares may contain some bacterial and fungal spores that come with
packaging materials. The glass wares from factory also contain some amount of alkalies, hence for
removal of alkali, 2-3% Hydrochloric acid is applied for 24 hours, for the process of neutralization.
Preparation of chromic acid
Weigh 5 gm of sodium/potassium dichromate (Na2Cr2O7/K2Cr2O7) and dissolve in 5 ml
distilled water in a beaker (250 ml). Add 100 ml concentrated sulphuric acid slowly and stir it
constantly. The mixture is allowed to cool to about 40°C and then stored in a dry glass stoppered
bottle. The solution should be handled with care. Preferably, rubber gloves and apron should be
used while using it. If clothes are spilled with the solution, wash them immediately followed by
neutralization of the acid with sodium carbonate solution. This in turn is washed off with water.
Cleaning of pipettes
Place the used pipettes in 3% Lysol. If necessary, keep overnight in detergent or dichromate
or sulphuric acid cleaning solution. Wash with tap water followed by deionized water. If required,
the tip end of the pipette may be plugged with cotton wool.
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