APRIL, 1948
COLOR AND THE TRANSITION ELEMENTS IAN X i . PARSONS London, England
GENERAL CONSIDERATIONS O F COLORED COMPOUNDS
The term "transition elements" was originally applied by Mendeleev to the elements Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt, occupying Group VIII of the periodic table, but the following elements are now commonly included in the term:
These transition elements are characterized by very variable valence and the associated properties of catalytic activity, magnetism, and the formation of colored compounds; it is believed that variable valence is associated with the possibility of electron exchange between the outer electron ring and the next, and it is significant that these elements occur in parts of the periodic table where the outermost group but one is being built up. It is also significant that rare earth compounds are frequently colored too, although in their case it is the outermost electronic group but two which is in process of completion.
The incidence of colored compounds is clearly shown if we consider the curve formed by plotting the atomic volumes of the elements against their atomic weights, as shown in Figure 1. It mill be seen that there are several minima, that, in general, colored compounds are formed by transition elements located at or near the minima, and that elements not on these minima rarely form colored compounds. Compounds of the elements titanium, vanadium, chromium, manganese, iron, nickel, cobalt, and copper lying on one minimum are for the most part relatively abundant and therefore cheap; compounds derived from elements occupying positions on other minima are generally rare and expensive. For the present we may restrict our attention to colored compounds of the common elements mentioned above. It is suggest,ed that a study and comparison of the color-producing properties of these transition members, as manifested in their compounds, mould illustrate a number of theoretical relationships in a striking manner, and that it would have considerable educational value. Suggested Scheme for Study. For a general consideration of these colored compounds the following scheme
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might well be followed; the procedure for a single comEducational Aspects. With regard t o the educational pound, say cobaltous sulfate, xi11 first be outlined, and advantages to be derived from a study of such preparathen a list of the various compounds a-hich might be tions, the following points are advanced. Approximate investigated with advantage will be advanced. ideas both of solubility and color intensity would be I t is desired t o prepare solutions containing 1 mol of learned, since we may say very roughly, as a first apthe salt per liter, one-half mol per liter, one-quarter proximation, that solubility would be indicated by the mol per liter, and so on, until a solution is obtained with number of fractions in each series stronger than molar, a color indistinguishable from that of an equal thickness while color intensity would be shown by the number of of pure water. It is also necessary to produce solutions fractions weaker than molar. containing 2 , 4 , 8 , etc., mols per liter up to the maximum A number of particular questions would also be anconcentration permitted by solubility. The complete swered. Thus, if we assume the cupric ion to be the series is shown in the follo~vingscheme: coloring agent in cupric chloride, cupric nitrate, cupric sulfate, and cupric ammonium sulfate, we should expect Concentrations solutions of these compounds of the same molar strength Solubility limit +8 4 M, 2 M, M, M/2, M/4, M/S Disappearance of color to have the same colors; valuable instruction would be gained by consideration of the reasons causing divergI t is clear that the number of fractions provided by a ence from this rule in certain cases. Again, the effect given compound will he dependent both on bolubility of valence on stability would be illustrated by the staand also on color intensity. - Thus, potassium perman- bility, instability or the non-existence of compounds of ganate, although not a very soluble salt, would provide the various elements. The effect of double salt formaabout 15 fractions if made up according to this system. tion on solubility, as indicated by copper ammonium A point regarding the preparation of a standard series sulfate and copper sulfate would be demonstrated, and may be made here. Having decided the desired volume other similar points will come to mind. of each fraction, say 50 ml., the entire series may he prepared quite easily from twice this volume of the DEMONSTRATIONS OF PARTICULAR THEORETICAL strongest solution in the series, by division and dilution, RELATIONSHIPS irrespective of the total number of fractions required to We now proceed to consider a number of special reach a "colorless" end fraction. arrangements designed to illustrate particular theoretiConsidering the elements Ti, V, Cr, Mn, Fe, Xi, Co, In general, for these preparations cal relationships. and Cu as constituents responsible for color, it is sugeach individual series would be made in the standard gested that preparation of a series for each of the folmethod already described. lowing compounds given by these elements should be Color and Complex Formation. The basic series in made up whenever stability permits: this collection would be a series of cobalt chloride fracChlorides RC4 tions in water ranging through the standard molar conNitrates R(NOd2 centrations, 2 M , M , M / 2 , M / 4 , M / 8 , and so on down Sulfates RSOI to colorless. Three further series would be prepared, Double sulfates RSOA.(NHMOI the solutions ranging through the same molar concenSulfates RdSOdr Double sulfates R9(SO4h.(NHMO, trations as the first series, except as modified by soluFormulas of the anhydrous compounds are given above. bility and color (extinction) considerations, but using as solvents 3 N, 6 N, and 12 N hydrochloric acid. The colors of comparable fractions in the four series would not be identical because of molecular rearrangement, probably due to the formation of complex ions. Further series might be prepared with advantage using mixtures of fuming hydrochloric acid and alcohol as solvents, to illustrate the principle in an even more striking manner. Color and pH. The basic series in this collection would be a standard series of cupric chloride fractions in water. The solutions in this series would of course range in color from green through blue to colorless, the green color demonstrating the presence of complex ions. Three further series, each with fractions of the same concentrations as the first, but using 3 N, 6 N, and 12 N hydrochloric acid as solvent would be prepared also, being placed above the standard series. These would illustrate by their different colors the effects of pH on Bli , complex ion formation. 1 2a .ro ca ca loo la ire im eo mo m do Below the initial series, three further series using 3 N, AWmic wehht. Figvrs 1. Graph of Atomic Vo1um.s: Atomic Weights. 6 N, and 12 N ammonia as solvent should be arranged; 4
,
APRIL, 1948
20b
these mill demonstrate the preeence of the ammouiacopper complex. The intense color of this complex should he manifested by the greater number of dilute fractions required to reach a colorless condition than in the caee of the initial series. Color and Valency. The colors of the compounds derived from the, three oxides CrO, CroOa,and CrOa lend themselves well to a striking demonstration of the effects of valence changes on color. I n the case of chromous sulfate, CrS04,should the preservation of this compound prove possible, the concentrations would again be based on the standard procedure; but in order to introduce the same weight of chromium, the agent responeible for the color, into each comparable fraction-that is, in each vertical column of fractions in the h a 1 collection-the following relationship should he followed in the series containing chromous, chromic, and chromate compounds: CrdSOA CrSO, = ---2
KICrOl
In the arrangement outlined below each formula represents a series, i. e., a series of solutions having concentrations throughout the agreed range; although the valency of chromium is t.he same in chromate and dichromate, dichromate has been included to illustrate the color difference between the Cr06-- chromate ion, and the C&O,- - dichromate ion. The inclusion of tn'chromate is suggested because, should it prove possible to preserve trichromate solutions, it would be of interest t o compare their colors with those of dichromates. CrSOl CrdSOdd2 KaCrO, KzCrz0,/2 K.CrsOto/3
(Blue) (Purple) (Yellow) (Orange) (Red ?)
It may be added that while the preparation of the three series in the center of the scheme would offer no diffitulty, preparation and preservation of the outside members would require considerable skill, and probably preliminary research work. Manganese is another element whose compounds manifest the effect of valency changes on color in a striking manner. The following scheme is proposed, in which, as above, each formula represents a series of solutions: MnSO. KnMnO, KMnO,
ent colors in its compounds than vanadium. Depending on valency, lavender, green, blue, colorless, orange, and red solutions may be obtained, and the preservation of even the least stable lavender solutions should not prove an insuperable difficulty. A representative collection of vanadium compounds would indeed provide a most strikmg demonstration of the relationship between valency and color. Color and Temperature. A number of compounds suffer molecular rearrangement in solution with change in temperature. Of these, chromium salts may be mentioned, and an interesting illustration would be provided by the preparation of two series containing chromic sulfate; the first, a t room temperature would consist of violet colored fractions, the second, held a t 80' C., would be green in color. To prevent evaporation in the fractions of the second series, the containers would be required to withstand a pressure of about atmosphere. Other compounds showing this change will come to mind, and could well be included in displays. Color and Solvent. Series containing iodine dissolved in \ \ ~ t c rpotnisium . iodide, nlcol~ol,ethw, nirl)m ilisultide. ,mi chloroform \voulil show dilhences in coli~rdue to Qe solvent employed, and would also, by their ranges, suggest the varying solubility of the element in the solvents used. The violet solutions, similar in color t o iodme vapor, should be very striking. A comparison of the colors of aqueous and alcoholic solutions of inorganic compounds would also be of interest. Color and Particle Size. While thus far only series of colored solutions have been considered, it is highly probable that instructive collections of colored solids, graded according to particle size, could be prepared. In general we should expect color intensity to decrease with particle size, B point which would he elucidated by inspection of series of such compounds as the following, carefully screened within narrow size limits, and displayed under standard conditions with suitable precautions t o prevent decomposition: chrome alum, copper sulfate, manganous sulfate, potassium rhromate, potassium dichromate, potassium permanganate, vanadous ammonium sulfate. Many other suitable compounds will come to mind, and the persistence of color in spite of reduced particle size would be an interesting field of study. In connec-
(Pink) (Green) (Purple)
The small number of colored solutions given by manganous sulfate would contrast strongly with the large number provided by potassium permanganate, because of the greater color intensity of the MnOl- ion. The preservation of the series containing mangauate would require some thought, but could doubtless be accomplished by the use of air-free potassium hydroxide solution as solvent. Perhaps no element shows a greater variety of differ-
F i e 2.
Rimsl. Showing Compositions
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mixing when necessary, to form fractions of uniform volume representing each of the intersecting points on the diagram. On coolmg to room temperature, varying amounts of salt will crystallize out according to the solubilities of the compounds involved; that is, relative solubilities would he indicated by mere inspection. The principle may now be extended to produce the arrangement shown in Figure 3, in which the same system of nomenclature has been used, V, for instance, indicating vanadous ammonium sulfate. Consideration of this diagram will show that all colors will fade out toward the center to "colorless" zinc ammonium sulfate, and that the other compounds have been placed so as to provide the most suitable color combinations. This collection should of course he arranged horizontally for the best effect. Once again the student would be contion with the preservation and display of these collec- fronted with a graduated scale of difficulty; some of the tions it may be mentioned that easily oxidizable suh- compounds could be weighed out directly, some would stances, such as vanadous ammonium sulfate, may be require standardization by analysis, ferrous ammonium preserved unchanged under mineral oil, which effects sulfate would require special precautions to avoid oxidalittle change in appearance and color; probably it would tion and hydrolysis, and vanadous ammonium sulfate beadvisable to standardize the preeervation of the frac- would provide an interesting example of an electrolytic tions in these collections under this material, and to use preparation. A still more difficult step would be the attempt to replace one of the elements listed by chroflat glass containers for display. mium, as the blue, easily oxidized compound chromous Examples showing the effect of water of crystallizaammonium sulfate. Partial substitution of the sulfate tion on the color might well be included also, such as: group by selenates would afford a further example of NiSO,, yellow; NiSO,. 6H20, blue; NiYO, .%O, green. isomorphism. A similar but much snialler collection CuCb, brown; CuC12.2H90,green. could be prepared using the alums which are also isoCuSO,, white; CuS01.5H~0,blue. morphous. In this case also careful screening to within narrow size Elements k i n g on Other Minima. The elements limits, and standardization of the method of mounting ruthenium, rhodium, palladium, and osmium, iridium, would he essential. platinum, and gold, which lie on other minima of the curve of atomic weight and atomic volume, and form MORE ADVANCED STUDIES colored compounds, are so rare and expensive that fracWhile the number of interesting and instructive tions of the size already suggested cannot he considered colored compounds suitable for illustrating theoretical in their case. But by the use of small, or "micro" fracrelationships is of course almost unlimited, a few sug- tions, diluted in accordance with the basic proposal, it gestions regarding rather more advanced collections should be possible to illustrate adequately these littleknown compounds, and their color relationships, with&ay be of interest. Color, Isomorphism, and Solubility. There is a very out undue expense. This would be particularly true if laree class of salts. re~resentedhv nickel ammonium a fairly large magnifying glass were so mounted that it &ate, NiSO,. (NH&SO~.GH~O, "which are isomor- could be caused to traverse all the fractions a t will. Most of the compounds of these elements being unphous, and of which many are colored. The ability to form mixed crystals in all proportions due to isomor- stable and little known, the preparation and preservsr phism and solubility relationships in these compounds tion of representative colored series would once again would be attractively shown by a collection prepared provide some interesting and instructive problems. In this category, series showing the color effects due according to the following principle. In the triangle shown in Figure 2, Zn represents a to valence changes in uranium, as shown by uranyl and solution containing only zinc ammonium sulfate; Ni, uranous sulfates, should he included. It is hoped that enough has been said to indicate the 100 per cent nickel ammonium sulfate solution, and Cu, 100 per cent copper ammonium sulfate solution, while wide possibilities available in the field of colored inorintersecting points indicate mixtures of solutions with ganic chemicals and to provide convincing evidence that systematic collections, prepared in accordance with the compositions according to the usual convention. Now suppose the solubility of the least soluble of the schemes outlined above, would offer great possibilities three salts is x gm. per liter at. 80°C., and that solutions from the educational point of view. I t may be that the of each containing x gni. per liter a t 80°C. are prepared. temporary display of such colored collections would Appropriate volumes of these solutions are then used, prove a stimulating attraction.
