HYDROUS CHROMIC OXIDE.
I1
BY C . F. NAGEI,, JR.
In the preceding paper1 it was pointed out that Fischer and Herz? consider that hydrous chromic oxide is peptonized and not dissolved by caustic alkali. They base this conclusion on dialysis experiments and on conductivity measurements. At the suggestion of Professor Bancroft it was decided to supplement their work by other experiments. Freshly precipitated hydrous chromic oxide was treated with an excess of caustic potash to prevent jelling and was set away in glass-stoppered bottles. &4tthe end of three weeks quite a large amount of green precipitate had formed, but the liquid was still green. On further standing, the amount of precipitate increased while the green color of the liquid became correspondingly less intense. At the end of two months the liquid was only faintly yellowish green, the bulk of the chromium oxide having precipitated. It is probable that practically all the chromium oxide would precipitate in time; but the experiment was stopped a t this point for academic reasons. Experiments were also made on ultra-filtration. Collodion filters were prepared by pouring 2 0 cc collodion solution on a clean surface of mercury in a flat dish. The films thus formed were allowed to dry until all smell of ether had disappeared, but not so long that the odor of alcohol had vanished. mThenin this state the films can easily be lifted out with the fingers and can then be allowed to dry subsequently in the air to whatever hardness is desired, the films being more impermeable the harder they are.* The films were placed in a regular Bechhold apparatus and distilled water run through to show how the filter was working. With hydrous chromic oxide and excess of caustic potash the liquid comes through the filter colorless, all the 1
films.
S a g e l : Jour. Phys. Chem., 19,352 (1915). Zeit. anorg. Chem., 31, 3 j 2 (1902). Bechhold: Zeit. phys. Chem., 60, 2 5 7 (1907); 64, 328 (1908). I a m indebted to Professor Briggs for much assistance in preparing these
C. F. iVagel, J r
57 0
chromium being left behind on the filter. While there may be some adsorption of the chromic oxide by the filter itself, this is of no importance because so much liquid was run through the filter that the chromic oxide was left in mass on the top of the filter and could be scraped off of it. When hydrous chromic oxide is heated with a chromic chloride solution, some of the chromic oxide seems to go into solution forming what is often called a basic chloride. When this apparent solution is passed through an ultra-filter, the chromic oxide is removed. The solution comes through green, of course, because the filter does not remove the dissolved chromic chloride. A so-called basic chromic chloride solution, therefore, contains hydrous chromic oxide peptonized, but not dissolved, by chromic chloride. There is no evidence that any appreciable amount of basic salt is formed. I had hoped to be able to shake out peptonized chromic oxide with benzene but this proved impossible in alkaline solutions. When precipitated chromic oxide is shaken with water and benzene, it goes into the dineric interface but addition of caustic alkali causes it to precipitate. This seems to be a general phenomenon, because zinc sulphide was precipitated from the dineric interface of kerosene and water by addition of alkali. This is in line with the experiments of Wilson' and of Twomey2 showing that alkali is adsorbed at the surface separating organic liquids from water. The general results of this paper are as follows : I . Using a collodion ultra-filter it is possible to filter out the hydrous chromic oxide peptonized by caustic alkali. 2 . On long standing nearly all of the peptonized chromic oxide precipitates from a caustic alkali solution. 3. Chromic chloride solutions peptonize hydrous chromic oxide. No appreciable amount of basic chloride is formed. 4. It is not possible to shake out hydrous chromic oxide from an alkaline solution either with benzene or kerosene. Cornell University 1
2
Jour. Chem. SOC.,I, 174 (1848). Jour. Phys. Chem., 19,360 ( 1 9 1 5 ) .
