Metal and G/ml Copper Stock free essay sample

Lab Partner: Date Performed: Purpose: 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. Analyte: Copper Instrument: ThermoSpectronic Genysis 10 UV spectrometer Unknown: provided water sample Result: 0. 0510 Â µg/mL copper Procedure: The lab was carried out according to the Copper-Neocuproine Method found in the lab manual . Three standards were prepared from 20. 00 Â µg/mL copper stock olution 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 citra te was added to complex any other metal ions that might interfere with the analysis (Greenberg, Clesceri, Eaton, 1992). We will write a custom essay sample on Metal and G/ml Copper Stock or any similar topic specifically for you Do Not WasteYour Time HIRE WRITER Only 13.90 / page 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. Structures: (Budavari, ONeil, Smith, Heckelman, 1989) Average mass of copper / Volume of unknown 150. 08 Â µg / 50. 00 mL 3. 0016 Â µg/mL 3. 00 Â µg/mL Conclusion: The concentration of copper found in the provided unknown water sample was 3. 00 Â 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 Â 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 indication of whether or not the desired pH has been passed (anything over pH 5. 2 will show red). Since the experiment is so sensitive to this pH adjustment, there is a possibility of error. This error can be seen in the correlation coefficient (r = 0. 994) and visually on the calibration plot. The linearity of the standards could definitely have been better. On top of this, having only 3 points in the calibration curve compounds this error further and removes the possibility of dropping a standard point. Another source of error resulted from the liquid/liquid extraction step. Since the neocuproinecopper complex is much more soluble in methylene chloride than water , it was a necessary step to ensure all the copper was quantified. During the process of the extraction however, it is virtually impossible to not lose some sample/standard while venting. Finally, since methylene chloride was used to zero the spectrometer, it theoretically should have been able to be used as a point in the calibration curve. The results however show that this point would be far from linear with the other standards so that it was not used. Since the unknown sample fell between the second and third standard, it would only increase error to use the blank as a data point. This procedure for testing copper content in various samples has great applications for labs lacking some of the more expensive equipment. The major advantage to using this method is that it can be performed with simply a few chemicals, glassware and a basic visible spectrometer. If an atomic absorption flame or furnace spectrometer was available though, they are greatly superior for performing such analyses. Not only do they eliminate a lot of time consuming, tedious wet chemistry techniques and chemicals, the results would tend to be more reliable and more quickly obtained. Post-Lab Questions: Hydroxylamine hydrochloride was added to reduce cupric (Cu2+) ions to cuprous (Cu+) ions (Greenberg et al. , 1992). Sodium citrate was added complex metallic ions that could precipitate causing analysis interference when the pH was raised (Greenberg et al. , 1992). Some compounds that could interfere if present include high concentrations of chromium and tin (Greenberg et al. , 1992). In order to prepare 2. 00 L of 20. 0 Â µg/mL copper stock, 40. 0 mL of 1000 mg/L copper solution would be needed.