NOTE REGARDITU’G T H E COLLOIDAL NATURE OF CUPRA?rlMOKIUM SOLUTION* BY ALFRED J. STAMM’
The’nature of the complex salts of copper and ammonia, and the reason for their intense blue color is one of considerable interest. Earlier investigations on the lowering of the freezing point,2 vapor pressure depre~sion,~ distribution c~efficient,~ and change in viscosity5 all showed that four ammonia radicals are combined with one copper to form the complex ion Cu(NH3)1++. This relationship of copper to ammonia is not exact, however. According to Dawson,6 dilution of the solution or decreasing the ammonia concentration relative to that of copper causes the equilibrium between the components to be displaced so that the ratio of combined ammonia to copper diminishes. Copper finally precipitates from solution as copper hydroxide. Bhatnagar, Goyle, and Prasad7 have recently shown that this deviation from the true stoichiometric relationship and the intense blue color of the solutions may be explained on the assumption that for all concentrations and proportions of ammonia and copper, part of the copper is colloidally dispersed as copper hydroxide. The amount of colloidal copper hydroxide present and its degree of dispersion, which affects the color of the solution, will vary with the relative concentrations of the constituents and the method of preparation. These investigators have further substantiated their theory by comparing four properties of copper hydroxide sols with those of cuprammonium solutions, namely the absorption spectra, cataphoresis, dialysis, and flocculation with electrolytes. In all cases the data indicate that the cuprammonium solutions contain some colloidally dispersed copper hydroxide. The author had occasion, in connection with his study of the dispersion of cellulose in cuprammonium so1ventJ8to make a brief study of the colloidal nature of cuprammonium solvent itself, using the ultracentrifuge apparatus of Prof. The S ~ e d b e r g . In ~ this study the concentration gradients existing in the sedimenting systems were determined by the new index of refraction method developed by Ole Lamrn’O in this laboratory. * Contribution from the Laboratory of Physical Chemistry of the University of Upsala, Fellow of the Rockefeller Foundation. Chemist on leave from the U. S. Forest Pro-
ducts Laboratory, Madison, Wis. * Reychler: Bull. 3, 13, 387 (1895). Gaus: 2. anorg Chem., 25, 2 j 9 (1900). Dawson: J. Chern. SOC., 89, 1668 (1906). Blanchard! J. Am. Chem. SOC.,26, 131j (1904). Dawson: J. Chem. SOC.,95, 371 (1909). Bhatnager, Goyle and Prasad: Kolloid-Z., 44,79 (1928). * Stamm: J. Am. Chem. Soc. 52 3047, 3068 (1930). Svedberg, “Colloid Chemistry,” pp. 146-167 (1928). ‘OLamm: 2. physik. Chem., 138, 313 (1928); 143, 177 (1929).
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COLLOIDAL NdTCRE O F CCPRAWMONIUM SOLUTION
The cuprammonium solution was prepared by drawing CO1-free air slowly through a tall cylinder containing concentrated ammonia (sp. gr. 0.9 I ) and strips of electrolytic copper foil for 6 hours. The copper concentration was determined by evaporation of z cc. portions, igniting and weighing as the oxide. The ammonia concentration was determined by titrating a diluted portion against 0 . 1 X HC1 using methyl orange as an indicator. The stock solution contained 1.28 per cent copper and 2 2 . 0 per cent ammonia. W t h the high-speed oil-turbine centrifuge a definite sedimentation of a polydisperse material was observed using the above stock cuprammonia solution. The sedimentation velocity ranged from 5 X IO-^ cm. 'see. per em. 'sec.? to I O to I O O times this value. The upper limit of the sedimentation velocity was not determined, as the author was primarily interested in the finer dispersion. This sedimentation value could have been definitely obtained by making measurements with the centrifuge operating a t a lower speed. If it is assumed that the colloidal material present is simple Cu(OH)*,the density of which is 3.37, t,he particle size range can be calculated using Stokes' law of settling. This gives a particle size range from 6. j mp in diameter to more than 20.0 mp. Unfortunately, the percentage of the total copper concentration that is colloidally dispersed can not be definitely determined without the knowledge of the relationship existing between the sol concentration and the refractive index, I t is very likely less than I O per cent of the total copper concentration, however.
Conclusions I t has been shown with the aid of the ultracentrifuge that cuprammonium solutions do contain a material in colloidal dispersion. This colloidal material is undoubtedly a copper hydroxide sol, according to the deductions drawn from the data of Bhatnagar, Goyle and Prasad.7 This sol is polydisperse and contains particles ranging in diameter from 6. j mp to more than 20.0 mp for the concentration investigated. The author wishes to express his sincere thanks to Prof. Svedberg for the use of the ultracentrifuge equipment of this laboratory, and for the help which he always freely gave towards the carrying out of this research.
