To determine the concentration of copper present in a sample of water by complexing it with neocuproine and comparing it to a Beer’s Law calibration curve prepared with standards of a similar matrix.
ThermoSpectronic Genysis 10 UV spectrometer
provided water sample
0.0510 µg/mL copper
The lab was carried out according to the Copper-Neocuproine Method found in the lab manual . Three standards were prepared from 20.00 µg/mL copper stock solution by taking increasing volumes of stock. From an unknown water sample, duplicate replicates were used of equal volume to the standards. Each standard, a blank (deionized water) and both unknown replicates were quantitatively transferred to separatory funnels. Each funnel was treated as follows: sulphuric acid was added to adjust the pH, hydroxylamine hydrochloride was added to reduce cupric (Cu2+) ions to cuprous (Cu+) ions and sodium citrate was added to complex any other metal ions that might interfere with the analysis (Greenberg, Clesceri, & Eaton, 1992). The pH was then adjusted to between 4 and 6 before adding neocuproine to complex the copper. Liquid/liquid extractions were performed with two equal portions of methylene chloride for each standard/sample with both portions being recombined before diluting to a constant volume. The blank was used to zero a spectrophotometer set to the appropriate wavelength. The absorbance of each standard and sample was read from the spectrometer.
(Budavari, O'Neil, Smith, & Heckelman, 1989)
Copper stock solution: Standard/unknown dilution volume: Volumes of stock taken: Reagents added to each separatory funnel: 20.00 µg/mL 50.00 mL 3.00, 6.00 and 9.00 mL 1.0 mL concentrated sulphuric acid 5.0 mL hydroxylamine hydrochloride 10.0 mL sodium citrate Enough ammonium hydroxide to adjust pH 10.0 mL neocuproine reagent
Dilution volume of standards/unknown: Cell: Instrument: Wavelength:
2 x 10.00 mL methylene chloride 25.00 mL Quartz; clean, good condition, minor scratches ThermoSpectronic Genysis 10 UV spectrometer 457 nm
Volume of 20.00 µg/mL Copper Stock (mL) 0.00 3.00 6.00 9.00 Unknown sample 1 Unknown sample 2 Mass of Copper (µg) 0.00 60.0 120 180 Absorbance 0.000 0.243 0.294 0.368 0.336 0.330
Mass of copper = (Concentration of copper stock) (Volume of copper stock) = (20.00 µg/mL) (3.00 mL) = 60.0 µg Intercept Slope r = 0.177 = 0.00104 = 0.994404648
Equation of best fit line Mass of copper in unknown 1
y = 0.00104166667x + 0.176666667 = = = = (absorbance – 0.176666667) / 0.00104166667 (0.336 - 0.176666667) / 0.00104166667 152.96 153 µg copper Mass of Copper (µg) 153 147 150 Absorbance 0.336 0.330 -
Unknown Sample 1 Sample 2 Average
Original concentration of unknown copper = = = =
Average mass of copper / Volume of unknown 150.08 µg / 50.00 mL 3.0016 µg/mL 3.00 µg/mL
The concentration of copper found in the provided unknown water sample was 3.00 µg/mL.
Evaluation of Results:
Overall, the results of this lab seemed to turn out decently. Without knowing the actual unknown sample value though, it is difficult to know certainly. The 3.00 µg/mL result seems to make sense based on the range of standards that were used. Also, the consistency of results between the two unknown samples suggests that the results are fairly accurate. There were however some issues that could have resulted in errors in the experiment. During the addition of ammonium hydroxide to adjust the pH, it was difficult to know exactly when the desired pH range was reached. It was very possible that slightly too much was added as the Congo Red paper could only be tested at intervals between additions. Also, due to the simple blue to red colour change of the paper there is no...