Preparation of Labile Compounds under Protective Conditions. Chromium II Salts

PREPARATION OF LABILE COMPOUNDS UNDER PROTECTIVE CONDITIONS. CHROMIUM (II) SALTS 

Submitted by Maksymikian Kbanz and Anna Witkowska Checked by John T. Yoke  

Chromium(II) salts are extremely sensitive and when affected by the least traces of air are oxidized to form chro-mium(III) salts. Because of these circumstances research and preparation of such compounds is very troublesome. The difficulties arise both in the investigation of the product s purity and absolute dryness during preparation, and in the removal of the final product from the protective conditions of the reaction field. The methods of obtaining chromium (II) salts employed at present are based on a rather complicated apparatus with indirect control. Thus preparation procedure has to be fixed a priori. Our method, described below, has been adapted particularly to prepare extremely sensitive compounds such as the chro-mium(II) salts. Using our apparatus the experimenter is able to intervene at any time and to handle the contents of the box in the same manner as he would on laboratory tables. 

The air is removed (pressure about 10 mm.) from the box and is replaced by purified nitrogen. After having performed tests for the absence of traces of oxygen inside the box, 17.0 g. (0.26 mol) of zinc is placed in a 400-ml. beaker, f To this is added a solution of 10.0 g. (0.0376 mol) of chro-mium(III) chloride 6-hydrate in 11.6 ml. of water. To the mixture so formed is added 23.0 ml. (0.27 mol) of concentrated hydrochloric acid. Then the beaker is covered with a watch glass. After about 15 minutes the zinc is largely dissolved and the solution is robin s egg blue in color. Next the watch glass is rinsed, the solution filtered quickly into a Buchner funnel suction flask, and the beaker and residue of zinc rinsed off with water. This done, the filtrate is poured into another 400-ml. beaker and the suction flask rinsed briefly. To the solution of chromium-(II) chloride is added at once a slurry of 27.5 g. (0.336 

The mixture is suction filtered, the precipitate being collected on a tared, Pyrex-glass filtering crucible of medium porosity. The product is next washed several times with small portions of water until the washings are free of chloride ion, as tested with silver nitrate solution. Finally, it is washed with a little ethanol followed by a little ether. The crucible containing the product is placed in a vacuum desiccator over concentrated sulfuric acid and dried for 1 day with occasional addition of nitrogen to the desiccator, followed by reevacuation. The yield of chromium(II) acetate is 6.1 g. (95% of theory). 

---