Chemistry 121 Laboratory: Spectrophotometric Determination of Cu2+ Background On June 7, 1991, the Environmental Protection Agency published a...
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     Chemistry 121 Laboratory: Spectrophotometric Determination of Cu2+Background

On June 7, 1991, the Environmental Protection Agency published a regulation to control the concentration of copper (and lead) in drinking water, known as the Lead and Copper Rule. This rule establishes an action level of 1.3 mg/L (which is also ppm) of Cu2+. Long-term exposure to copper can cause irritation of the nose, mouth, and eyes and it causes headaches, stomachaches, dizziness, vomiting and diarrhea. Intentionally high uptakes of copper may cause liver and kidney damage and even death. Whether copper is carcinogenic has not yet been determined. There are scientific articles that indicate a link between long-term exposure to high concentrations of copper and a decline in intelligence with young adolescents. Whether this should be of concern is a topic for further investigation.

http://www8.nationalacademies.org/onpinews/newsitem.aspx?RecordID=9782


In this investigation you will determine the concentration of copper(II) ions in a solution of unknown concentration. The unknown solution is a hypothetical drinking water sample.

Questions to be answered in the investigation:

Does the concentration of the copper(II) ions in the unknown solution meet or exceed the action level established by the EPA.

Is the method used in the experiment to measure copper ion concentrations in water a valid method to use for samples that have concentrations near the EPA action level? To answer this question, determine the concentration limit of the UV-Vis spectrophotometer given absorbance values below 0.1 are not reliable.


Solutions containing copper(II) ions, i.e. Cu2+ ions, have a characteristic pale blue color, assuming the anion present is colorless and no other colored substances are present. The intensity of the color is dependent upon the concentration of the copper(II) ions present. When aqueous ammonia, NH3(aq), is added to the solution, a different dark blue color develops as the [Cu(NH3)4]2+ complex forms. The ammonia is known as a derivatizing agent.


Cu2+(aq) + 4NH3(aq) → [Cu(NH3)4]2+(aq)


You and your group will make a series of solutions of know concentration in order to make a calibration curve to determine the copper(II) concentration in an unknown sample.


The [Cu(NH3)4]2+ system may deviate somewhat from Beer's Law because of the tendency of the [Cu(NH3)4]2+ ion to react with water to form [Cu(NH3)3(H2O)]2+ and other ions that do not absorb at the same wavelength maximum as the [Cu(NH3)4]2+ ion. This is an indication that the formation of the [Cu(NH3)4]2+ ion is an equilibrium reaction. This must be "driven" by the addition of excess ammonia. Therefore, you must use your calibration curve instead of Beer's Law in your analysis of the unknown.



Part 1: Determination of the Appropriate Solution to Use to Analyze for Cu2+


Fill one cuvette with 3 mL of stock Cu2+ solution. The stock solution is the solution of known concentration from which dilute solutions will be prepared.

1


Make another cuvette containing appropriate volumes of Cu2+ solution and the ammonia solution, the derivatizing agent, so that the total volume in the cuvette does not exceed 3 mL. Decide upon "appropriate volumes" by choosing a volume of the 0.10 M Cu2+ solution and calculating the volume of the 0.50 M NH3 solution needed so that you have 8-10 time as many moles of ammonia as Cu2+. If the sum of the volumes exceeds 3.00 mL, try a different volume. Try not to just guess, however, but reason your way to a different volume based upon the result of your first calculation.

Transfer, using pipets, the agreed upon volumes of Cu2+ and NH3 solutions to a cuvette. Cap the cuvette and mix well. Mixing measured amounts of two solutions to make a new solution with a total volume of 3.00 mL assumes that the volumes are additive. Although that is not always the case, it has been verified that it is a reasonable assumption in this experiment.

You need to prepare two blanks to calibrate the spectrophotometer: one for use with the Cu2+ solution and one for use with the [Cu(NH3)4]2+ solution. A blank is a solution that contains all of the substances present in the samples being analyzed except the analyte (the substance of unknown concentration). Decide upon appropriate blanks and then check with your instructor before you prepare the blanks in the cuvettes.

Measure the visible spectrum of each of the two solutions (Cu2+ solution and [Cu(NH3)4]2+ solution). Scan the entire visible range so that you can determine the wavelength of maximum absorbance for each of your solutions.

Analyze the results of the two spectra to determine whether to use the Cu2+(aq) or [Cu(NH3)4]2+(aq) in preparing the calibration curve to ultimately determine an unknown copper(II) concentration. You can also determine whether or not you should use a solution as concentrated as the stock solution when preparing your calibration curve. The absorbance range for analytical purposes is between 0.1-1.5. Discuss with your group members whether the stock solution is an acceptable concentration. If it is, it can be used as a data point on the calibration curve.

Save the appropriate blank.


Part 2: Preparation of the Solutions and the Calibration Curve


Make at least four different Cu2+ solutions beginning with the stock solution. Use a separate volumetric flask for each solution. Use deionized water as the solvent. These dilute solutions are called standard solutions because their concentrations are known.

Good analytical procedure requires that all samples be treated in the same manner. If your group decided to use the Cu2+ solution alone, then pour each of the standard solutions into separate cuvettes. If your group decided to use the [Cu(NH3)4]2+ solution, pipet the same amount of each of the standard solutions (use the same volume as the volume of stock solution you used in Part 1 of this experiment) into separate cuvettes and then add the appropriate volume of aqueous NH3 (as determined in Part 1 of this experiment).

Measure the absorbance of each of the calibration standards with the spectrophotometer set at the wavelength of maximum absorbance. Remember, the absorbance needs to be between 0.1-1.5.


Part 3: Determination of the Copper(II) Concentration of an Unknown


Obtain a solution of an unknown Cu2+ concentration from your instructor. Treat the unknown exactly as you did the standards. Your instructor may ask each person in the group to analyze a different unknown. 


Calculations

To ensure valid results, Parts 1-2 should be done during the lab period so that data that do not fit the calibration curve can be re-measured.

2



Plot a calibration curve showing the absorbance as a function of the concentration of either Cu2+ or [Cu(NH3)4]2+ in the standard solutions (choose whichever species was present in your cuvettes). If you used [Cu(NH3)4]2+ in each of the standard solutions, assume complete reaction between Cu2+ and the NH3 with Cu2+ as the limiting reactant.

Examine the calibration curve. If any points are grossly out of line from the curve or if any of the measured absorbance values are out of the appropriate range, adjust the dilutions as needed and repeat the measurements.

Use the calibration curve to determine the concentration of Cu2+ in your unknown.

Determine if the concentration of Cu2+ in the unknown meets or exceeds the action level for copper ions set by the EPA.

Obtain the expected concentration of Cu2+ in our unknown from your instructor and calculate a percent error.


Report

Be sure to submit all data, calculations, and the calibration curve (complete with a title, labeled axes, a best fit equation, and the R2 value in the data section of the report.


Discuss the quality of the calibration curve based on the R2 value and the percent error of the concentration of the unknown solution in the error analysis section of the report.





Spectrophotometric Determination of Cu2+ Lab Report Name:___________________________________ 

Course and Section:________________________ 

Date:____________________________________ 

Unknown A Concentration (3 sig figs):_______________ Unknown B Concentration (3 sig figs):_______________ Unknown C Concentration (3 sig figs):_______________ 

Question #1 What do you expect the y-intercept to be for your plot of concentration and  absorbance? What is the actual value? Explain any differences. 


Question #2 What is the equation for your "best fit" line ? Use this equation to show how  you calculated your unknown concentrations.  


Question #3 Summarize the experimental method used to analyze the unknown. Was the  method you used to determine the concentration of Cu2+ in the unknown effective?  Explain.

Question #4 Industries, such as environmental laboratories, use spectrophotometric  methods to determine the concentration of different substances in solutions. They  typically ignore the dilution factor when the analyte solution is added to the cuvette along  with other derivatizing agents if they treat the unknown in exactly the same manner as the  standards. For example, in this lab, you can ignore the dilution factor introduced when  you prepared the cuvettes if you used the same volume of Cu2+ solution and the same  volume of NH3 solution in each cuvette. Explain why the dilution factor can be ignored  under these conditions. 


Question #5 Discuss whether all points on the graph were valid. If not, explain why you  needed to repeat any measurements. Explain any discrepancies in the values of the  absorbance and concentration. 

Question #6 Did the volume of the pipet used impact the calculated concentration?  Explain. Do you expect the volume of the pipet used to impact the calculated  concentration? Why or why not? 

Question #7 Turn in the correctly completed MS Excel plot of your data and the UV-vis  spectrum.

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