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AXD W. A. WEYL
ferrous and ferric ions. This interaction is weak, because the probability of electron transfer through the phosphate anion is low. h similar blue ferrous-ferric phosphate was obtained by G. Tammann and H. 0. von Samson-Himmelstjerna (5) when ferric phosphate \vas partially reduced. In crystals consisting of layers of oxide ions in different states of polarization, the probability of electron transfer assumes directional properties. Biotite, a mica mineral containing both ferrous and ferric ions, is described as an extreme case of a crystal which is transparent for one vibration of light and opaque for the perpendicular vibration of light. This can be attributed to the impossibility of transferring electrons across the hydroxide layer. -4 possible connection is mentioned between this type of electron transfer and the quenching action of iron in phosphors containing anions of increasing polarizability (fluoride, oxide, sulfide, selenide). REFERENCES
K., ASD KREIDL,s.J . : J. Am. Ceram. soc. 31, 105 (1948). (1) FAJASS, (2) F O R L A N D T ., , ASD WEYL,W 0. S . R. Technical Report X D , 13, Task Order 8; J. Am. Ceram. SOC. 33, 186 (1950). 13) HOFMASS, K . A . , RESESCHECK, F., ASD HOESCHELE, IIMASS, G . , A S D V D S S . A ~ f S O S - H I ~ l U E L S T J E R S AH, . 0.: z. anorg. Chem. 207, 319 (1932). (6) Weyl, W. A.: J . SOC.Glass Techn. 27,265 (1913); this paper also contains references to the earlier literature on this subject. ( i )WHITSEY,J., ASD DAVIDSOS, S . :J . Am. Chem. SOC. 69, 2076 (1047).
T H E COLOR CHASGE OF FERROUS HYDROXIDE UPON OXIDATION R . R . SHIVELY, JR.,ASD W. A . WEYL Department of Mineral Technology, School of Mzneral Industries, The Pennsylvania Slate College, State College, Pennsylvania Received April 14, 1950 INTRODUCTION
In the preceding paper, W. A. Weyl ( 2 ) developed a theory which accounts for the phenomenon that iron or, generally speaking, metal ions may give rise to intensive light absorption when present in two states of oxidation. K. .4. Hofmann (1) attributed this phenomenon to the possibility of an “oscillation of valency.” The process consists in an electron transfer between two metal ions. It was pointed out that the “looseness” of the outer electrons of the anions plays an important part in permitting electrons to move through a crystal or a
COLOR CHANGE OF FERROLY HYDROSIDE O S 0SID.ITION
513
glass. Anions of low polarizability, such as sulfate, carbonate, or fluoride, are not conducive to such an electron transfer. Ferrous and ferric ions are Jvidely distiibuted in the mineral kingdom and one observes, in general, that “acid rocks” are lighter colored than “basic rocks.” The largest part of the volume of an acid rock containing aluminum and silicon ions consists of oxide ions which are more strongly “tightened” than the oxide ions in basic rocks, which are primarily under the influence of magnesium ion\, calcium ions, etc. The minerals cryolite (ice-stone) and chiolite (snow-stone) occur with iron-bearing minerals. Their white color, however, is due to the fact that “impurities” cannot produce intensive light absorption in a crystal consisting of fluoride ions evposed to and tightened by the relatively strong force fields of aluminum ions. For a given anion, say a hydroxide ion, the electron transfer must be a function of Its state of deformation. In other words, if all other factors could be kept constant, the probability of an electron being conducted through the outer electronic shells of hydroxide ions should decrease with the increasing force fields of the central cation. A rigorous examination of this condition is impossible because the deformation of the hydroxide ions varies nith crystal symmetry, internuclear distance, and coordination number. I t is not possible to fix these parameters and, at the same time, replace calcium ion by barium ion or magnesium ion by cadmium ion, etc. Kevertheless, the influence of the central cation upon the hydroxide ions can be detected qualitatively. For experiment, coprecipitates of ferrous hydroxide and other ccjlorless hydrosides and other colorless hydrosides were oxidized by exposure to air and the color changes were observed. EXPERIhIENTS
Ferrous ammonium sulfate and aluminum chloride were dissolved in degassed nater. From this solution ferrous and aluminum hydroxides were precipitated by adding ammonia. In the absence of oxygen the precipitates are white but assume color when filtered and exposed to air. The color change of pure ferrous hydroxide is not, as one might expect, from white to tan to light brown to the deep brown of ferric hydroxide but is from white to bluish white to deep blue to dark olive to deep brown. The dark blue and olive colors can be attributed to a “hydrated magnetite,” Le., a system containing both ferrous and ferric ions with hydroxide groups rather than oxide ions as the anion. By weighing out the proper amounts of the ferrous and the aluminum salts, coprecipitates of decreasing Fez+:A13+ ratio were obtained. Table 1 summarizes the color changes of the white precipitate on exposure to air. .S rather sharp transition in the color scheme is observed between 10 and 12 mole per cent ferrous hydroxide. If the Fe2+:Als+ ratio in the coprecipitated hydroxide exceeds 1:10,the ferrous ions and the ferric ions, formed on oxidation, are sufficiently close to permit “oscillation of valency.” Systems with lower ferrous-ion concentration change from white to light tan and brown. The average distance between the iron atoms is too large for electron transfer
514
R . R. SHIVELT, J R . , .4XD
Iv,
A . WEYL
through the hydroxide ions, n hich are tightened by the relatively strong lorce fields of the aluminum ions. Similar experiments carried out with some other hydroxides allow us to determine the role of the central cation in the electronic conductivity of the hydroxide groups. Table 2 summarizes the results. There are several factors which prevent a quantitative interpretation of the results,-for example, the distribution of the ferrous ions in the coprecipitates. However, the difference in the concentration of ferrous ions which is necessary to produce the blue color center is in full accord with that expected from the previous discussion of the color of sulfate, phosphate, silicate, and borate glasses. TABLE 1 Coloi ehangc of frrrous hydiorzde-alumznuin hydroxide c o p ~ e c ~ p ~ t uupon t e s oradation ? n uzr Fe(OH)n mole
pa
ISTERYEDIATE
cent
Olive Olive-green Green Green Bluish green Blue Blue Tan Light tan White
100
80 60 40
20 16 12 10 5 1
Dark brown Dark brown Dark brown Dark brown Dark brown Dark brown Dark brown Brown Tan Light tan
TABLE 2 M i n i m u m iron coi~centrationnecessary f o r ferrous-ferric i o n interaction HYDPOaDE
I
PLPROUS10N
1
EYDBOXWE
~
PEPPOUSION
mole pn cm1
Cadmium . . . . . . . . . . . Magnesium . . . . . . . . Calcium . . . . . . . . .
0.5 1 .0
Beryllium. . . . . . . Zinc . . . . . . . . . . ' . . . . .
~
~
2.0 3.0 12.0
In cadmium hydroxide the electrons of the anions are sufficiently loose to allow electron exchange over several atomic distances, whereas the transfer is very limited in aluminum hydroxide. COSCLUSIONS
The oxidation of ferrous hydroxide to ferric hydroxide goes through deep blue intermediate products which owe their strong light absorption to an exchange of electrons oia the outer electron shell of the hydroxide ions. The distance over which such interaction is possible depends upon the state of deformation of the hydroxide ions. The distance is greater for cadmium hydroxide than for zinc hydroxide. It is greater for beryllium ion than for aluminum ion, etc.
ADSORPTIOS O F LIQUIDS BY B.IUXITE AND CLAY
5 15
The method of studying electron-transfer processes over a few atomic distances by this “dilution method” should have a more general applicability. There are several phenomena which make it desirable to learn more about electron migration in insulators. Examples are the quenching of fluorescence by iron, the effect of nickelous ions on the decay of light emission by certain phosphors, the darkening of crystals and glasses under irradiation, and-the reverse of this process-phototropy and thermoluminescence. The authors are indebted to the Geophysics Branch of the Office of Naval Research for supporting this work. REFERESCES (1) H O F M A N X , K A ,. , RESEXCHECK, F., A N D HOESCHELE, K . : Ann. 342, 364 (1905); Ber. 48, 20 (1915). (2) WEYL,W. A , : J. Phys. & Colloid Chem. 66, 507 (1951).
T H E EFFECT OF SOLVEYT O X T H E ADSORPTIOS OF LIQUIDS BY BAUXITE AS‘D ATTAPULGCS CLAY JOHN G. MILLER,’ HEINZ H E I S E M A S N , %A N D W. S. W. McCARTER Atlapulgus Clay Company and Porocel Corporation, Philadelphia, Pennsylvania Received April 88, 1950
Bauxite and Attapulgus clay (a type of fuller’s earth) are well-known, industrially important adsorbents (3, 4). These porous natural substances have large surface areas and are strong adsorbents when activated by heating. The investigation reported here was made to determine the effect of the nature of the solvent and the solute in adsorption from binary liquid solutions by these solids in activated condition. The studies were carried out with single, but representative, specimens of the fuller’s earth and bauxite, a large group of liquids being employed. The observations may be classified and discussed according to the behavior of the adsorption of the solvent. Three classes of behavior were met. In the first, the solvent adsorption mas negligible. In the second, the solvent adsorption was considerable but apparently unaffected by the adsorption of the solute. In the third, the adsorption of the solvent was high and was affected by the solute adsorption. All of these behaviors have been demonstrated previously by other workers, using different systems, but it appears that a study of the series undertaken here is novel and that the treatment of the data may be of general utility in expressing adsorption data for binary Jiquid systems. 1
2
Harrison Laboratory, University of Pennsylvania, Philadelphia, Pennsylvania. Present address : Houdry Process Corporation, Marcus Hook, Pennsylvania.
