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 XXXXXXXXXXammonia 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 ammonia (CARE), and make up to the mark with deionised water. Determine the absorbance of this solution using the chosen filter.
7. Using your calibration graph, determine the concentration of copper ions in the solution prepared from the unknown.
8. Calculate the concentration of copper ions in the original unknown solution. Remember to take into account the dilution performed in preparing the unknown solution for analysis in the colorimeter.
Result:
Concentration of Copper:
1. 250mol dm-3
2. 125mol dm-3
3. 83.3mol dm-3
XXXXXXXXXXmol dm-3
5. 50 mol dm-3 | Water (control) is 0.00
440 filter-0.03
470 filter-0.08
490 filter- 0.20
520 filter-0.36
550 filter- 0.53
580 filter-0.66
590 filter-0.73
680 filter- 0.60 | Chosen filter-590:
5 is 0.73
4 is 0.57
3 is 0.40
2 is 0.28
1 is 0.11 | Unkown is 0.60 Abs |
Discussion question:
A) Calculate the concentration (in mol dm-3) of the standard solutions.
(C1 V1 = C2 V2) (5 marks)
B) Tabulate the absorbance values of the highest concentration standard using different wavelength filters (5 marks)
C) Plot a graph of absorbance (Y-axis) versus wavelength of different filters (X-axis) using the highest concentration standard (from B) (5 marks) [REMEMBER TO LABEL THE AXIS AND TITLE THE GRAPH]
D) From (C), choose the best filter and give your reason for choosing that filter.
(2 marks)
E) Tabulate the absorbance values obtained for all the standard solutions and the unknown sample you have measured with the selected filter (wavelength)
(5 marks)
F) Plot a calibration graph of absorbance (y-axis) versus concentration of the standards (x-axis) and draw a trend line using Excel together with the equation for the trend line. (10 marks)
G) Find the concentration of unknown sample from the calibration graph using either linear regression or interpolation, and then calculate the concentration of copper ions in the original unknown solution XXXXXXXXXXmarks)