OTTO, BARON V.D. PFORDTEN.--The author makes use of a solution of
chromous chloride, which he prepares as follows:
He first heats chromic acid with concentrated hydrochloric acid, so
as to obtain a strong green solution of chromic chloride free from
chlorine. This is then reduced with zinc and hydrochloric acid. The blue
chromous chloride solution thus obtained is poured into a saturated
solution of sodium acetate in an atmosphere of carbonic acid. A
red precipitate of chromous acetate is formed, which is washed by
decantation in water containing carbonic acid. This salt is relatively
stable, and can be preserved for an indefinite time in a moist condition
in stoppered bottles filled with carbonic acid.
In this process the following precautions are to be observed:
Spongy flocks always separate from the zinc used in the reduction, which
float about in the acid liquid for a long time and give off minute gas
bubbles. If poured into the solution of sodium acetate, they would
contaminate the precipitate; and when dissolved in hydrochloric acid,
would occasion a slight escape of hydrogen. 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. Each
two apertures receive the glass tubes used in gas washing bottles, while
the third holds a dropping funnel. It is filled with dilute hydrochloric
acid, and after the expulsion of the air by a current of gas, plentiful
quantities of chromous acetate are passed into the bottles. When the
current of gas has been passed in for some time, the hydrochloric acid
is let enter, which dissolves the chromous acetate, and thus, in the
absence of air, produces a solution of blue chromous chloride. It is
advisable to use an excess of chromous acetate or an insufficient
quantity of hydrochloric acid, so that there may be no free hydrochloric
acid in the liquid. To keep back any free acetic acid which might be
swept over by the current of gas, there is introduced after the washing
apparatus another washing bottle with sodium carbonate. Also solid
potassium carbonate may be used instead of calcium chloride for drying
the gas. If the two apertures of the washing apparatus are fitted with
small pinch cocks, it is ready for use, and merely requires to be
connected with the gas apparatus in action in order to free the gas
generated from oxygen. As but little chromous salt is decomposed by the
oxygen such a washing apparatus may serve for many experiments.
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