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.