Cleaning of Glassware with Nitric Acid and Chromic Acid

The success of a test or assay of the Pharmacopoeia is determined to a very large extent by the state of cleanliness of the apparatus used. Glassware such as beakers, burettes, flasks, pipettes, etc. should be very clean, especially when employed for certain microbiological assays, the pyrogen test and where small volumes of liquid are measured.
Related: Calibration of Volumetric Glassware used in Pharmaceuticals
For cleaning glassware, one of the most useful agents is hot nitric acid. A very effective cleaning fluid for removing organic matter from glass without heating is by treatment with a chromic acid mixture prepared by dissolving 200 g of sodium dichromate in about 100 ml of water, cooling in an ice-bath and adding slowly to it, with stirring, 1500 ml of sulphuric acid.

The mixture should be prepared in a hard, borosilicate glass beaker and safety glasses must be worn during the addition of acid. Chromic acid mixture is extremely corrosive and hygroscopic, and should be stored in glass-stoppered bottles in a safe place. Crystalline chromic acid tends to separate from the mixture on standing and may be removed by decantation. When the mixture acquires a green color, it should be discarded under continuously flowing water.

Glass treated with the chromic acid mixture should be subjected to prolonged rinsing as glass tends to adsorb the chromic acid. It should not be used for cleaning calibrated containers used for optical measurements.

For general cleaning of glassware, synthetic detergent solutions or alkaline cleansing agent such as trisodium phosphate may be used but these also require prolonged rinsing. All glassware should be finally rinsed with purified water before use.