The purpose of this experiment is to measure the formation constant of the tetraamminecopper(II) ion by colorimetry. Anhydrous copper sulfate (CuSO4) is white, which means that it does not absorb light in the visible region of the spectrum. The hydrated copper sulfate (CuSO4 - 5H2O) is blue. The structure of the compound can be represented more accurately as Cu(H2O)4 SO4 - H2O where four water molecules are bound to the copper ion and the fifth is a water of crystallization. The water molecules are arranged at the corners of a square, with the copper at the center. Such an arrangement is called square coplanar. The oxygen of each water molecule shares one pair of electrons with the central copper ion. The absorption spectrum of 0.01M copper sulfate is shown, in Figure 1, by the dotted line, A. Absorbance is plotted against wavelength in angstroms (Å). Notice that the compound absorbs light of wavelengths from 6000 to above 8000 Å, which is the yellow-to-red region of the visible spectrum. The light transmitted through the solution comes out richer in light of blue wavelengths (4000 to 5000 Å) than white light, and so the solution looks blue. When ammonia is added to a solution of copper(II) cation, a deep blue color is formed immediately. The blue color is due to the complex. ion Cu(NH3)42+. Cu(H2O)42+ + 4NH3 Cu(NH3)42+ + 4H2O
This complex ion, the tetraamminecopper(II) cation, has a square co-planar geometry also. The absorption spectrum of this complex ion in 0.05M ammonia is shown in Figure 1 as the solid line, B. In this complex, also, the light of yellow and red wavelengths is absorbed more than the blue, so the solution appears blue. The tetraamminecopper(II) cation is the principal copper species present in ammonia solution of concentration 0.01 to 5M. However, at lower concentrations of ammonia, other copper species having 3, 2 or 1 molecules of ammonia may be present. At higher concentrations of ammonia, a pentaamminecopper(II) cation, [Cu(NH3)5 H2O]2+ is formed also. In Figure 1, the dashed line, C, represents the absorption spectrum of 0.01M tetraamminecopper(II) cation in a solution of 1M ammonia. It is evident that the absorbance is larger in 1M ammonia than in 0.05M ammonia, and the wavelength for the maximum absorption is slightly different. These differences are due to the formation of perhaps 25% of the pentaamminecopper complex. The tetraamminecopper complex is practically the only copper species present under the conditions of this experiment: an ammonia-ammonium chloride buffer with a 0.05M concentration of ammonia.
Figure 1. Absorbance spectrum of 0.01 M CuSO4 in
(B) 0.09 M NH3 and 0.09 M NH4NO3; and
(C) 1M NH3.
Cupric ion reacts with ammonia to form a deep blue complex ion Cu(NH3)4 2+
Cu2+ + 4NH3 Cu(NH3)42+ (1)
The mass action expression for this equilibrium can be written as follows; K, is the formation constant of the tetraamminecopper(II) ion: Kf = [Cu(NH3)42+]
The equilibrium for Reaction (1) lies very far on the side of the complex ion; so far that the cupric ion concentration is too small to measure, except by an electrochemical cell. In this experiment, the formation constant will be measured by studying the equilibrium between solid copper hydroxide and cupric ammine complex in ammonia. When cupric hydroxide is treated with ammonia, one expects the reaction
Cu(OH)2 (s) + 4NH3 Cu(NH3)42+ + 2OH- (2)
The concentration of the cupric ammine can be estimated from the color of the solution. The hydroxide ion concentration should be twice as great. The situation is not, however, so simple. The pH of the solution is less than that expected on the basis of Reaction (2), probably because of the acidity of the cupric ammine ion, which contains two water ligands. [Cu(NH3)4(H2O)2]2+...
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