In most textbooks of introductory chemistry will be found the statement that the interaction of ferric salts and alkali ferrocyanides yields Fe4[Fe(CN)6]3 or Prussian blue, whereas the interaction of ferrous salts and alkali ferricyanides yields Fe3[Fe(CN)6]2 or Turnbull's blue. Actually, the oxidation-reduction reactions which take place on mixing iron salts and alkali iron-cyanides complicate the problem to such an extent that certain investigators conclude from chemical analysis that all of the blue gels are ferrocyanides, whereas others arrive at the diametrically opposite conclusion that all of the blue gels are ferricyanides (for a survey see reference 9).In earlier investigations on complex iron-cyanides (10), it was reported that the ferrocyanide gels of copper, manganese, cobalt, and nickel prepared with varying ratios of heavy-metal sulfate and potassium ferrocyanide gave identical or almost identical x-ray diffraction patterns irrespective of the nature of the divalent cation or of the relative amounts of the salts. It was suggested that the very similar crystal structures of the several iron-cyanides resulted from such an arrangement of the large iron-cyanide ions that channels were formed in which the smaller heavy-metal cations may be grouped. At about the same time, van Sever (8) suggested a similar structure for various heavy-metal ferricyanides, but he assumed the presence of combined water molecules within the unit cell of the proposed structure. Similarly, Keggin and Miles (3) claimed that several molecules of combined water were present within the unit cell. Rigamonti (7), on the other hand, did not assume the presence of combined water in the unit cell of the complex cyanides.The state of the water in.heavy-metal ferroand ferri-cyanide gels has been the subject of a recent investigation (11), which disclosed that the water in most compounds of this type is adsorbed and no definite hydrates exist. For example, smooth dehydration isotherms were obtained for cupric ferroand ferri-cyanides and for the so-called Prussian blue and Turnbull's blue. Further support of the absence of hydrates is furnished by the observation that a given moist gel, and the same gel completely dehydrated in a vacuum, gave identical x-ray and electron diffraction patterns. In this connection, it was observed further that thg x-ray and electron diffraction patterns of the so-called Prussian blue and Turnbull's blue were identical within the limits of accuracy of the experiments. This confirms an earlier report of Levi (5) to the effect that the x-ray diffraction patterns of Prussian blue and Turnbull's blue are identical.In the light of the above survey, a comprehensive x-ray diffraction study of the 1 Presented at the Eighteenth Colloid Symposium, which was held at
Tetravalent titanium, like tin, is said to form two distinct acids: orthoor alpha titanic acid and meta-or beta titanic acid. Since the work of several investigators has shown that the stannic acids (13) are in reality hydrous stannic oxides differing essentially in the size of the primary particles, it seems not unlikely that the so-called titanic acids are also hydrous oxides whose properties are determined by the size and physical character of the particles.The product commonly called orthotitanic acid, or alpha titanic acid, is a white gelatinous precipitate formed by the addition of ammonia or alkali hydroxide to a solution of a tetravalent titanium salt. The highly hydrous gel is readily soluble in dilute acids and is easily peptized by dilute alkalies and suitable salts to give stable sols. On the other hand, the product referred to as meta titanic acid, or beta titanic acid, is a granular, difficultly soluble, and but slightly peptizable substance obtained (a) by the interaction of nitric acid and titanium, (b) by the aging under water of the precipitated gel of the so-called orthotitanic acid slowly in the cold and more rapidly in the hot, (c) by precipitation in the hot, and (d) by hydrolysis of solutions of tetravalent titanium salts.The composition of the preparations referred to above varies with the method of precipitation and with the subsequent treatment. The only evidence for the existence of definite hydrates corresponding to the formulas of the acids that earlier investigators (2, 5,, 9, 10, 11, 12) have assumed, is the analysis of the products formed and dried under arbitrary conditions, Carnelly and Walker (1) obtained a smooth temperature-composition isobar for a precipitated gel and hence concluded that no definite hydrates were formed, unless it is assumed that a large number exist, each stable over a very narrow temperature range. Nforley and Wood (8) prefer to assume that the change from the so-called alpha to beta form consists of a condensation to complex salt-like compounds, but there is no experimental justification for this point of view. More recently Gutbier and coworkers (3) obtained smooth composition-temperature isobars for several preparations, indicating the absence of definite hydrates of titania. 513
The interaction of solutions of ferric salts and soluble bases gives a highly gelatinous precipitate of hydrous ferric oxide, frequently misnamed ferric hydroxide. The composition and other properties of this brown gel depend upon the conditions of formation and the age of the sample. Heating under water (1,4,5,8,12) or dilute salt solutions (2) for a long time, or aging at room temperature (14) for a much longer time, gives an almost anhydrous, more or less brick-red product. X-ray analysis by Bohm (2) showed that boiling in the presence of water, potassium chloride, or ammonium chloride solutions gives a-Fe203, whereas aging at 150°C. in the presence of 2 M potassium hydroxide gives a-Fe203-H20. Katsuria and Watanabe (9) obtained the a-Fe203 x-ray pattern from a gel heated to 150°C. in an autoclave. Nichols, Kraemer, and Bailey (11) obtained the a-Fe203 pattern for the coagula from aged sols.Although all the evidence points to the aged gel being a-Fe203, it is generally agreed that the x-ray diffraction pattern of the freshly precipitated oxide shows no lines or bands (2,3,6,7,15). The results of a systematic study of the transformation from the brown gel to a-Fe203 are given in the following section.Aging under water and ferric chloride solutions The brown gel was precipitated at about 20°C. by the addition of 300 cc. of 15 M ammonium hydroxide to 1 liter of 0.2 M ferric chloride solution. The precipitate was washed with dilute ammonia by the use of the centrifuge until free of chloride, and finally with water until only a trace of ammonia remained. Samples of the washed brown gel were aged as indicated in table 1. X-ray analysis of the products shows that, although no lines or bands can be detected in the original gel, the lattice of a-Fe203 appears gradually upon aging in the cold, and more rapidly at 100°C. In figure 1 are given densitometer curves (10) of the x-ray negatives for the products aged at 100°C. These show clearly the progress of the transformation from the brown gel to a-Fe203.
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