COLORIMETRIC DETERMINATION OF COPPER
Introduction:
Colorimeter is used to study the concentration of a solution of a compound by measuring its absorbance at a specific wavelength of light. The complex formed between the Cu2+ ion and the ligand ammonia [Cu(NH3)4(H2O)2]2+ in this experiment exhibits an intense blue colour, and this can be used as the basis of a very sensitive method for the determination of copper ions in solution.
Materials and Apparatus:
Solution of copper ions of known concentration (0.25 M) Solution of copper ions of unknown concentration Saturated (0.880) ammonia solution (CARE: This solution must be dispensed in the fume cupboard) (note, 0.880 refers to the density, g/cm3, of the saturated solution, not its concentration) Colorimeter, cuvettes, 6 x 100 cm3 volumetric flasks. Appropriate graduated pipettes with fillers.
Method:
1. Pipette exactly 1, 2, 3, 4 and 5 cm3 portions of the 0.25 M Cu2+ solution into separate 250cm3 volumetric flasks. To each, pipette 2.5 cm3 of 0.880 ammonia (CARE), make up to the mark with deionised water, and shake well. Ammonia should be added in the fume hood.
2. Calculate the concentration of copper ions in each of the five prepared solutions.
3. Using the colorimeter as demonstrated, determine the absorbance of the most concentrated solution using each wavelength filter in turn and note the absorbance values down. Remember to collect a background each time you change the filter. From your results, decide which is the best filter to use in the analysis.
4. Using the selected filter, determine the absorbance of each of the five standard solutions.
5. Plot a calibration graph of the absorbance at the selected wavelength against the concentration of copper ions for each of the five standard solutions. Use Excel for this, and add a trendline and the equation for the trend line.
6. Pipette 5 cm3 of the solution of copper ions of unknown concentration into a 100 cm3 volumetric flask. Add 2.5 cm3 of 0.880…