COLLOIDAL, FERRIC OXIDE BY G . D. KRATZ
L. T. Wright’ has given a rather surprising method of preparing colloidal solutions of ferric oxide. “Ferric chloride solution was added slowly to excess of liquid ammonia, with constant stirring. This operation was effected in a porcelain evaporating dish, which was then placed in a water bath, and its contents evaporated to dryness. By this means, a dirty red-brown mass was obtained, showing here and there patches of ammonium chloride. This mass, on being treated with water, fell mostly into an impalpable powder, a large portion of which, on attempted filtration, passed through the filter paper and even through many successive filters. The filtrates were of bright red color, and when dilute, had in certain lights a purple appearance, suggestive of the purple color of ignited ferric oxide. Each filtrate, on being boiled and refiltered, left on the paper a small quantity of red hydrated ferric oxide, but the filtrate was still red. Portions of these red filtrates, which all had an acid reaction, on treatment with a little ammonia to alkaline reaction and boiling, partially coagulated and left decided quantities of bright red powder on the filter, but the filtrates were still red, not having the appearance of solutions, being muddy and opaque, with what appeared t o be an unfilterable precipitate. This is probably similar to the material called ‘‘ colloidal ferric hydrate,” ‘‘ dialyzed iron,” or “fer Bravais.” Magnier de la Source2 i n . describing a similar condition of ferric hydrate, or compound of ferric hydrate and ferric chloride, supposes that under certain conditions ferric hydrate is soluble in water; but I cannot think that this is a case of true solution, but rather of “pseudo” solution; for the filtrates I obtained had all the appearance of holding in suspension an intensely impalpable powder. They deposited Jour. Chem. SOC.,43, 156 (1883). Comptes rendus, go, 1352 (1880).
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small quantities of red powder on standing for some weeks, and in cases where they had been treated with ammonia and boiled to neutrality, 'they exhibited after long standing an acid reaction. Now, however, a drop of liquid ammonia and the application of a little heat, caused the pseudo solution to coagulate in light red flocks, which speedily settling, left the supernatant liquor perfectly bright and clear. I have made some further observations which may explain the presence of basic salt in the ferric hydrate prepared in presence of ammonium chloride, and on the difficulty of preparing ferric hydrate free from basic salt. '' Some ferric hydrate washed, dried a t IOO', rewashed and redried a t IOO', lost on ignition (I) (2)
I I 4 9 percent. . . . . . . . . . .Fe,O,H,O 11.32 percent.. . . . . . . . . . I O . I I percent.
"One gram of this hydrated ferric oxide boiled with about IOO cc of pure water in a retort, gave a distillate quite neutral to litmus. '' Five grams "$1 in IOO cc pure water on boiling gave a weak acid distillate. On pouring the ammonium chloride solution into the retort containing the hydrated ferric oxide, and distilling to dryness, free ammonia equal to 0.005 gram was obtained. On repeating this experiment many times, I always obtained an alkaline distillate containing an appreciable quantity of ammonia, the action Fe,CI,
+ 6NH, + 6H,O
=
Fe,H,O,
+ 6NH,C1
being apparently reversed Fe,H,O,
+ 6NH,C1 = Fe,CI, + 6NH, + 6H,O.
" Having prepared numerous samples of hydrated ferric oxide by means of Fe,Cl, and NH,, I have noticed that they all possess different shades of color, no two samples having exactly the same shade of red. They range in color from dirty brown t o bright br'ck-red. D. Tomrnasil divides ferric hydrate into two classes: red, obtained by precipitating a
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Bull. SOC. chim. Paris, [ z ] 38, 1 5 2 (1882).
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G. D . Kratz
ferric salt with alkalis, and yellow, by oxidation of ferrous hydrate, ferrosoferric hydrate, or ferrous carbonate; but‘ he has found that ferric hydrate kept under water for a year is converted t o the extent of 0.3 percent into a soluble modification identical with Graham’s “ colloidal hydrate.” I am inclined to think that this ferric hydrate was slightly impure, and that the color differences and the formation of a small quantity of “colloidal hydrate” are due to contained basic salt.” As it was not at all clear how this method of making colloidal ferric oxide was to be classified, Professor Bancroft suggested that I repeat Wright’s work so as t o find out just what happened. The first attempts resulted in complete failures. I added I O cc of a I O percent FeC1, solution t o 300 cc concentrated “,OH and evaporated t o dryness in a porcelain dish on the water bath, and I repeated this adding the ferric chloride solution to 2 0 0 cc concentrated “,OH, so as not to have so large an excess of ammonia. The residue answered Wright’s description of being (‘a dirty red-brown mass, showing here and there patches of ammonium chloride.’’ When treated with water, the mass did not fall t o an impalpable powder and did not pass through the filter paper. In fact the substance appeared t o be quite insoluble in water and was so hard that it could not readily be ground to a powder. These experiments showed that Wright had not given full and clear directions as to his process. It was at first thought that the evaporation had been carried too far and that the ferric oxide had thereby been made insoluble. To test this hypothesis I went to the other extreme, adding 5 cc of a I O percent FeCl, solution to 300 cc concentrated “,OH and evaporating to about IOO cc. The precipitate thus obtained was not soluble in water. After puzzling over the matter for some time, it seemed probable that Wright had evaporated to dryness on a water Bull. SOC.chim. Paris, [ z ] 37, 196 (1882)
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bath and had then washed all the ammonium chloride out of the precipitate on a filter with boiling water. As the last traces of the ammonium chloride disappeared, the ferric oxide might peptonize and run through the filter. This guess proved t o be correct. I added I O cc of a I O percent J?eCl, solution to 2 0 0 cc concentrated ammonia in a porcelain dish and evaporated to dryness on a water bath. The resultant dark brown mass was placed on a filter and washed with boiling water to remove the ammonium chloride. As the ammonium chloride disappeared, the ferric oxide ran through the filter as a dark or bright red suspension, just as Wright describes, though none of my suspensions were as bright red as I should judge Wright’s to have been. There is another source of error t o which Wright makes no reference. I n one case the ferric chloride solution was run into the ammonia solution as usual; but the mixture was not evaporated a t once. It was poured into a beaker and forgotten for about a week. At the end of this time, the contents of the beaker were poured into a porcelain dish and evaporated as usual. When the ammonium chloride was washed out, no colloidal ferric oxide was formed. This might be due to the solution having stood for so long or it might be a result of some of the ammonia having evaporated. In order t o distinguish between these alternatives, I made up two solutions as before and placed one in a stoppered Erlenmeyer flask and the other in an unstoppered flask. Both flasks were set aside for a week and then the contents were evaporated and treated in the usual manner. No ferric oxide ran through the filter in either case even though I washed each precipitate with 500 cc boiling water. This proves t h a t standing will cause ferric oxide to become insoluble, presumably as a result of agglomeration. The evaporation of the ammonia does have an effect on the quality of the evaporated residue. From the flask that had been uncorked I obtained a gritty, lumpy, residue which did not stick to the filter. From the flask that had stood corked,
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I obtained a fine, soft powder which tended t o stick to the filter paper. Debray’ pointed out that colloidal ferric oxide is precipitated by sodium chloride and that it redissolves if the salt is washed out a t once. I n Wright’s experiments we have ammonium chloride as the precipitating agent and the ferric oxide peptonizes when the concentration of the coagulating substance is made sufficiently small. The method of Wright is not to be recommended as a means of getting a concentrated solution; but it eliminates dialysis and the necessity of washing a gelatinous precipitate. It is evident that Wright had profited by Chautard’s statement2 ten years earlier that when a gelatinous precipitate is to be washed, much time may be saved by adding an excess of the precipitant, and evaporating the whole until a dry powder is obtained, which can then be readily washed on the filter. This work was suggested by Professor Bancroft and has been carried on under the supervision of Mr. T. R. Brig@. The general results of the paper are as follows : ( I ) Wright’s method of making colloidal ferric oxide is based on the washing out of the coagulating agent, ammonium chloride. ( 2 ) If the mixed solutions of ferric chloride and ammonia are allowed t o stand for a week before being evaporated, no colloidal ferric oxide is obtained. This is the result of spontaneous changes in the gel. (3) Wright’s method does not give a very concentrated solution but i t avoids the necessity of dialysis or of washing a gelatinous precipitate. Cornell Univevsity Comptes rendus, 68, 913 (1869). Chautard: Zeit. anal. Chem., 11, 299 (1873).
