The solution of chromous
chloride must therefore be freed from the zinc by filtration in the
absence of air. For this purpose the reduction is carried on in a flask
fitted up like a washing bottle. The long tube is bent down outside the
flask, and is here provided with a small bulb tube containing glass wool
or asbestos. The hydrogen gas liberated during reduction is at first let
escape through this tube; afterward its outer end is closed, and it is
pressed down into the liquid. The hydrogen must now pass through the
shorter tube (the mouthpiece of the washing bottle), which has an India
rubber valve. When the reduction is complete, the blue liquid is driven
up in the long tube by introducing carbonic acid through the short tube,
so that it filters through the asbestos into the solution of sodium
acetate into which the reopened end of the long tube dips. When washing
out the red precipitate, at first a little acetic acid is added to
dissolve any basic zinc carbonate which has been deposited. In this
manner a chromous acetate is obtained perfectly free from zinc.
For the absorption of oxygen the compound just described is decomposed
with hydrochloric acid in the following simple washing apparatus: Upon
a shelf there are fixed side by side two ordinary preparation glasses,
closed with caoutchouc stoppers, each having three perforations.
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