Copper forms an intensely colored complex with ammonia, NH3. The copper content of a copper-containing complex can thus be determined by freeing the copper ion from the complex; complexing it with NH3; and measuring the amount of light absorbed by the resulting copper-ammonia complex, Cu(NH3)42+. The copper ion can be freed from the complex by digesting the complex with nitric acid, HNO3. The carbon-containing portion of the complex will be completely oxidized by the acid, leaving Cu2+ in solution.
Analysis Procedure
Generation of Standard Analysis Curve. Prepare 25 mL of a 0.050 M stock solution of Cu(NO3)2.6H2O in a CLEAN 25-mL volumetric flask. Obtain five CLEAN 10-mL volumetric flasks, label them 1-5, and use a graduated pipet to transfer copper stock solution to them according to the following table. NOTE: You may use 5-mL volumetric flasks, but cut all volumes in half.
| Solution Number | Volume 0.05 M Stock Solution, mL | [Cu2+], M |
|---|---|---|
| 1 | 1 | 0.005 M |
| 2 | 2 | 0.01 M |
| 3 | 3 | 0.015 M |
| 4 | 4 | 0.02 M |
| 5 | 5 | 0.025 M |
Fill each flask to the mark with 7.5 M aqueous NH3 solution. Stopper and vigorously shake each flask to get thorough mixing and homogeneity. Then measure the absorbance of each of these 5 solutions at the wavelength of maximum absorbance. This should be around 600 nm. Construct a plot of absorbance versus the molarity of copper.
Analysis of the Copper Complex. Weigh a small amount (0.02-0.05 g)of your copper amino acid complex on the analytical balance. Record the mass to 4 decimal places. Transfer the solid completely to a small beaker, and, IN THE HOOD, add 0.5-1 mL of concentrated nitric acid. Warm gently until the solid dissolves and the solution is the light blue color characteristic of Cu2+. Cool the solution, and add 6M NaOH dropwise until the pH of the solution is greater than 5 (monitor the pH using pH paper, not a pH meter). Carefully pour the solution into a 10-mL volumetric flask. Wash the beaker with two 1-mL portions of water, transferring these as completely as possible to the 10-mL volumetric flask. Add conc aqueous NH3 to raise the liquid level in the flask to the mark. Stopper and shake. Measure the absorbance of the solution at the same wavelength used for the standard solutions.