Metal-ammonia solutions color

Solvated electrons were first produced in liquid ammonia when Weyl (1864) dissolved sodium and potassium in it the solution has an intense blue color. Cady (1897) found the solution conducts electricity, attributed by Kraus (1908) to an electron in a solvent atmosphere. Other workers discovered solvated electrons in such polar liquids as methylamine, alcohols, and ethers (Moissan, 1889 Scott et al, 1936). Finally, Freed and Sugarman (1943) showed that in a dilute metal—ammonia solution, the magnetic susceptibility corresponds to one unpaired spin per dissolved metal atom. 

The model of metal-ammonia solutions that has emerged is based on ionization of the metal atoms to produce metal ions and electrons that are both solvated. The solvated electron is believed to reside in a cavity in ammonia, and thus it may behave as a particle in a three-dimensional box with quantized energy levels. Transitions between the energy levels may give rise to absorption of light and thereby cause the solutions to be colored. The dissolution process can be represented as... 

FIGURE 14.17 Sodium dissolves in liquid ammonia to form the deep blue solution in the lower half of the tube. At higher concentrations, the metal ammonia solution becomes bronze in color, as in the top half of the tube. 

Contents Theory of Electrons in Polar Fluids. Metal-Ammonia Solutions The Dilute Region. Metal Solutions in Amines and Ethers. Ultrafast Optical Processes. Metal-Ammonia Solutions Transition Range. The Electronic Structures of Disordered Materials. Concentrated M-NH3 Solutions A Review. Strange Magnetic Behavior and Phase Relations of Metal-Ammonia Compounds. Metallic Vapors. Mobility Studies of Excess Electrons in Nonpolar Hydrocarbons. Optical Absorption Spectrum of the Solvated Electron in Ethers and Binary Liquid Systems. Subject Index. Color Plates. 

At the beginning of the nineteenth century, the British chemist Davy first prepared alkali metal-ammonia solutions. Davy was also the first person to make alkali metals using electrochemistry. Ammonia condenses at -33.35°C and becomes solid at -77.7°C. Alkali and some other metals dissolve readily in anhydrous ammonia solutions. Already in 1908, on the basis of conductivity measurements, Kraus proposed that alkali ions A+ exist in ammonia together with cavities containing a single electron (solvated electrons), in equilibrium with dissolved alkali metal atoms. At not too high concentration, alkali solutions are all deep blue, suggesting that the color arises from the electron cavities rather than directly from the metal ion. 

Color. The most remarkable property of metal-ammonia solutions is their color. In dilute solutions the color is blue, as is also found for solutions in methylamine and other amines. In concentrated solutions, the solution has a metallic copper-like appearance and reflects light at normal incidence much more than the non-metallic solutions and liquids. 

Stability. On evaporating freshly prepared metal-ammonia solutions, a residue of the metal is left indicating that there are no chemical changes in the solution. But on allowing the solution to stand for some time, the blue color gradually disappears with evolution of hydrogen according to the reaction... 

The mauve colored cobalt(II) carbonate [7542-09-8] of commerce is a basic material of indeterminate stoichiometry, (CoCO ) ( (0 )2) H20, that contains 45—47% cobalt. It is prepared by adding a hot solution of cobalt salts to a hot sodium carbonate or sodium bicarbonate solution. Precipitation from cold solutions gives a light blue unstable product. Dissolution of cobalt metal in ammonium carbonate solution followed by thermal decomposition of the solution gives a relatively dense carbonate. Basic cobalt carbonate is virtually insoluble in water, but dissolves in acids and ammonia solutions. It is used in the preparation of pigments and as a starting material in the preparation of cobalt compounds. 

A series of metal cations (Ni, Fe, Co, Cu, Pt) form colored complexes with dimethylglyoxime in ammonia solution or weakly acidic medium. 

Heavy Metals. — The solution of 2 gm. of sodium phosphate in 20 cc. of water, acidulated with 1 cc. of hydrochloric acid, should appear unchanged on the addition of hydrogen sulphide water. On now adding to the liquid 5 cc. of ammonia water and a few drops of ammonium sulphide solution, no precipitate should form, nor should a green color develop. 

Salts of the bases MOH are crystalline, ionic solids, colorless except where the anion is colored. For the alkali metal ions the energies required to excite electrons to the lowest available empty orbitals could be supplied only by quanta far out in the vacuum ultraviolet (the transition 5p6 —5p56s in Cs+ occurs at 1000 A). However, colored crystals of compounds such as NaCl are sometimes encountered. Color arises from the presence in the lattice of holes and free electrons, called color centers, and such chromophoric disturbances can be produced by irradiation of the crystals with X rays and nuclear radiation. The color results from transitions of the electrons between energy levels in the holes in which they are trapped. These electrons behave in principle similarly to those in solvent cages in the liquid ammonia solutions, but the energy levels are differently spaced and consequently the colors are different and variable. Small excesses of metal atoms produce similar effects, since these atoms form M+ ions and electrons that occupy holes where anions would be in a perfect crystal. 

The carboxylic acids are considerably stronger acids than the phenols. They turn litmus red, and yield alkali metal salts which are neutral to litmus. They do not turn Congo red paper blue, however at best only a violet coloration is formed. In contrast to the phenols, the carboxylic acids dissolve even in bicarbonate solutions, and very easily in carbonate and ammonia solutions. The free acids are regenerated from their salts by strong mineral acids. 

Germanium, tin, and lead give alloy-type binaries of the alkali metals of variable stoichiometry, but when dissolved in ammonia with the alkali metals they produce colored Zintl-type anions see Zintl Compounds) in solution which can be... 

Liquid ammonia (600 mL) is condensed into a 2-L three-necked round-bottomed flask equipped with a dry ice/acetone condenser fitted with a calcium chloride guard tube, and a mechanical stirrer. The flask is kept at 198 K by a cooling bath (dry ice/acetone mixture). Benzoic acid (38.65 g, 0.32 mol) is placed into a 250-mL round-bottomed flask and dry and freshly distilled ethanol (150 mL) is added. This solution is transferred into the ammonia solution by means of a cannula. Small pieces of sodium metal (21.7 g, 0.94 mol), which are kept under hexane, are added to the reaction mixture with rapid stirring over a period of 30 min. The reaction mixture turned dark blue and is stirred further for about 25 min. Then NH4CI (35 g) is added. The color is discharged within about 2 min. The reaction mixture is stirred for a further hour, the cold bath is removed, and the ammonia left to evaporate under N2 flow and allowed to reach ambient temperature overnight. The white solid in the vessel is dissolved in chilled distilled water (500 mL), which is then acidified by a slow and careful addition of cone, hydrochloric acid (ca. 11 M) to pH 1-2. 

Thallous nitrate reacts with (XXXII) in the same way as lead iodide and nitrate. Red-brown 2T1(NS)3 NH3 may be precipitated from ammonia solution and may be converted to red-brown T1(NS)3 (54). Ochre colored T13(NS)8 is precipitated from alcoholic solution. With copper(I) chloride, brown Cu(NS)2 is precipitated, while with silver nitrate red-brown Ag(NS)2 is obtained (54). The structures of these metallic derivatives are not yet established. 

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