Keith Walters's Public Library

The traditional glassware cleaning solution is "chromic acid", and many
analytical chemistry texts detail the preparation [15,16]. Chromium (VI) is
highly toxic ( mutagenic, carcinogenic ), and disposal is expensive, as all
solutions containing more than 5 mg/l of chromium are considered hazardous
waste in the USA. Disposal of chromic acid requires a two-stage process,
involving bisulfite addition to reduce Cr(VI) to Cr(III), followed by
neutralisation of the acid. There have also been several reports of
spontaneous explosions of chromic acid cleaning solutions [17,18,19],
consequently the use of chromic acid for cleaning glassware is declining,
and several alternative glassware cleaners have recently been evaluated [20].

Sodium dichromate dihydrate is usually used to prepare chromic acid, as
potassium dichromate is less soluble in sulfuric acid. One technique is to
dissolve 140g of technical grade sodium dichromate dihydrate in approximately
100 ml of water. Add two litres of technical grade 98% sulfuric acid to a 4-5
litre glass beaker that is sitting in a cold water bath in a fume cupboard.
Carefully stir the acid gently and pour a few mls of the dichromate solution
slowly into the acid. Keep repeating the addition every few seconds - after
the previous dose has been dispersed. As long as the stirring is gentle and
continuous, little or no splattering should occur, but the solution will
become quite warm. Allow to cool before storing in a glass-stoppered reagent
bottle. Always ensure that the stopper is sufficiently loose to release any
gas pressure. Never use a screw-capped or similar types of sealed containers.

If made correctly, the chromic acid solution should have no precipitate, will
be a deep red colour, and will last for years in a glass-stoppered bottle.
Ensure the glassware to be cleaned does not have any residual organic
solvents. Chromic acid is very effective at around 80C, but an overnight soak
at ambient temperature is commonly used. If the solution develops a green
hue, it is exhausted and should be disposed of, or regenerated, using
appropriate procedures. Slowly pouring used acid down a drain with the cold
water tap fully open is no longer considered appropriate. There is a recent
report of a technique to regenerate chromic acid cleaning solution ( by
distillation of water and oleum ) that reduces disposal quantities [21].