Experiment 4: Measuring the Ksp of Copper Tartrate over Various Equilibrium Concentrations

SOLUBILITY PRODUCT OF COPPER (II) TARTRATE OBJECTIVE: Measure the solubility product (Ksp) of CuC4H6O6 over a range of different equilibrium concentrations. Use the concept of activity in the calculation of Ksp. INTRODUCTION: When an excess of slightly soluble salt is left in contact with water, a dynamic equilibrium between the solid and the ions in solution results. In the case of copper (II) tartrate:

CuC2H206(s)

Cu

+2 (aq)

+ C2H2062- (aq)

The equilibrium constant is expressed as a solubility product Ksp and is defined by: Ksp = [A Cu+2] [ A C2H2062- ] Or Ksp = f Cu+2 [Cu ]* f C2H2062- [C2H2062-] The activity of the pure solid, CuC2H206(s) is assumed to be a constant and so it does not appear in the expression. The Ksp is a function of temperature but should be constant in this experiment over a range of copper and tartrate ion concentration values. Spectrophotometric determination of this constant can be done quickly and simply. If an ion of a compound is colored or can be made to form a colored complex in solution, the solubility product constant may be determined by visible spectroscopy. In weakly acidic solutions Cu2+ ions exist predominantly as Cu(H2O)n2+, which has a characteristic pale blue color. The absorbance for this solution is relatively weak at the concentrations we want to study in this experiment. For the same concentration of Cu2+ the absorbance is higher when the aquo complex is converted to the ammonia complex, Cu(NH3)42+. To determine the concentrations of Cu2+ in your equilibria mixtures, NH3 will be added under conditions that produce predominantly Cu(NH3)42+. You will select a wavelength at which to measure the [Cu(NH3)42+]. The [Cu2+] in your samples will be determined by comparison with a standard calibration curve, which is a plot of absorbance as a function of [Cu2+] for a series of known samples. One complication here is that there are also mixed complexes such as Cu(H2O)2(NH3)22+ and Cu(H2O)(NH3)32+. The relative...