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SPECTRAL ANALYSIS OF Cu2+
Reactions are often monitored by some form of spectroscopy. Many compounds are colored due to their absorption of visible light and absorbance spectrophotometry can be used to determine the concentration of these colored substances in solutions.
A linear relationship exists between the absorbance of the solutes in solution and the concentration of the solution; the more solute molecules in the path of the light source, the greater will be the absorbance of the light. Low concentrations will transmit more light (and the color will appear lighter or less intense), and high concentrations will transmit less light (and the color will appear darker or more intense). This relationship is known as Beer’s law and is given by the equation:
A= εbC
where A is the absorbance of the solution, ε is the molar absorptivity of the solute, b is the path length of light passing through the sample of solution, and C is the concentration of the solution in molarity (moles/L). The constant ε is specific to each solute at each wavelength and the units are M-1cm-1. If b and ε are constant, then the absorbance, A, is directly proportional to the concentration, C.
At a given wavelength plot of absorbance on the y-axis vs. concentration on the x-axis is predicted to be a linear plot, with a slope equal to εb, and an intercept of zero. In absorbance experiments, this is referred to as the standard (or calibration) curve. Once the calibration curve is complete, you will be able to interconvert between absorbance, A and concentration, C.
Notice that the intercept of the y-intercept of the line is zero – this makes sense, because if the concentration the light-absorbing compound is zero, then the absorbance should also be zero. In practice, the intercept will be a very small number, close to zero, but may not be exactly equal to zero.
In this experiment, students will prepare several dilute solutions from a standard stock solution of Cu2+. Using a spectrophotometer, students will measure the absorbance of each of the dilute solutions and construct a standard curve. Students will use the standard curve to determine the concentration of Cu2+ remaining in solution to investigate the reaction of copper (II) sulfate and potassium hydroxide.
References and further reading
Technique E: Volumetric Transfer Pipet Use of the laboratory manual
Technique I: Use of Spectrophotometer of the laboratory manual
Experiment 2501 Using Excel for Graphical Analysis of Data of the laboratory manual
Experiment 2508 Beer’s Law of the laboratory manual
2.0 SAFETY PRECAUTIONS AND WASTE DISPOSAL
| !!Wear your safety goggles!! Spectrophotometers and cuvettes are expensive. Use them carefully. Potassium hydroxide is a strong eye irritant. Be sure that the pathway to the eye wash station remains always unobstructed. Solid potassium hydroxide is caustic! Do not handle the pellets! Copper (II) solutions should be disposed of in the Inorganic Hazardous Waste container. Dissolved copper salts are very toxic to marine and aquatic life. |
3.0 CHEMICALS AND SolutionS
| Chemical | Formula | Concentration | Approximate amount | Notes |
| copper (II) sulfate pentahydrate | CuSO45H2O | N/A | About the size of a pea (0.25 g) | Very toxic to aquatic life with long lasting effects. |
| potassium hydroxide | KOH | N/A | 1 g | Corrosive and hygroscopic. |
| copper (II) sulfate | CuSO4 | 0.35 M | 100 mL | Very toxic to aquatic life with long lasting effects. |
| Item | Use | Notes |
| Beakers | Collecting and storage of samples | Start with clean, dry beakers. Label properly. |
| Two (2) 25 mL volumetric transfer pipets | Volume measurement | Label one ‘standard Cu2+’ and one ‘laboratory water’ |
| Three (3) 10-mL graduated pipets | Volume measurement | Label one ‘standard Cu2+’ and one ‘laboratory water’ |
| Pipet bulb | Volume measurement | |
| Two (2) watch glasses | Cover reactions to prevent contamination | |
| Two (2) gravity filtration set-ups | Separation of precipitate from supernatant | Includes: filter funnel, filter paper, and Erlenmeyer flask. Label one ‘Reaction A’ and one ‘Reaction B’ |
| Test tubes | Solution dilutions | Start with clean, dry tubes. Label 1 – 4 for standard dilutions |
| Spectronic 20 &/or 200 | % Transmittance &/or Absorbance measurement | Reference Technique I: Use of Spectrophotometer for proper use. |
| Square plastic cuvette or glass test tube cuvette | % Transmittance &/or Absorbance measurement |
5.0 PROCEDURE
Part A: Qualitative Investigation of Reaction
1. In a beaker add a small amount (1 cm3) of solid copper (II) sulfate pentahydrate (CuSO45H2O) to about 25 mL of laboratory water. Mix to dissolve. Record the appearance of the solution in Data Table 1 in Section 6.
2. In a second beaker add a small amount (one or two pellets) of solid potassium hydroxide (KOH) to about 25 mL of laboratory water. Mix to dissolve. Record the appearance of the solution in Data Table 1 in Section 6.
3. Pour the solutions together and mix. Record the appearance of the mixture in Data Table 1 in Section 6.
4. Based on the observations of the mixture, provide a balanced chemical equation including state designations in Data Table 1 in Section 6 that represents the reaction.
Part B: Quantitative Investigation of Reaction
1. Obtain about 100 mL of the standard Cu2+ solution in a clean, dry beaker. Please do not take more than you need! Any excess will be wasted. Record the concentration of the standard Cu2+ solution (approximately 0.35 M) in Data Table 3 in Section 6.
2. Label two clean dry 100 ml or larger beakers “Reaction A” and “Reaction B”. Into each mass 0.2 – 0.6 g of KOH using an analytical balance and record the exact mass in Data Table 2 in Section 6.
3. Using a volumetric transfer pipet, add 25.00 mL laboratory water to each beaker. Mix. Record the exact volume of laboratory water added in Data Table 2 in Section 6.
4. Once all the KOH has dissolved, add 25.00 mL standard Cu2+ solution to each beaker. Mix. Record the exact volume of standard Cu2+ solution added in Data Table 2 in Section 6. (You will use the rest of the standard Cu2+ solution in Part C.)
5. Allow the reactions to proceed for 10 minutes or more mixing periodically. (Feel free to complete Part C while you wait.) To prevent contamination, cover the beakers with watch glasses.
6. Prepare two filter papers for gravity filtration. The filter paper when folded should be a few mm below the rim of your funnel. Fold the filter paper into a cone by first folding it in half, and then in half again.
7. Place a funnel in the neck of an Erlenmeyer flask labeled “Reaction A”. Open the cone filter paper for Reaction A so that three layers are on one side and one layer is on the opposite side. Support the filter paper in the funnel and pour a small amount of the reaction mixture to wet the filter paper. Do not wet with laboratory water as this will dilute the concentration of Cu2+ in the supernatant (liquid portion of the mixture).
8. Pour the remaining mixture to be filtered through the filter paper, in portions if necessary. It may be necessary to filter several times to separate the supernatant from the precipitate (solid portion of the mixture). If additional passes are necessary, the same filter paper can be used. The filter paper and precipitate can be discarded according to your instructor’s directions. Record the appearance of the supernatant in Data Table 2 in Section 6, and retain the supernatant for analysis in Part D.
9. Repeat the filtration process for Reaction B using an Erlenmeyer flask labeled “Reaction B” and the second filter paper cone.
Part C: Preparation of Standard Dilutions
1. Prepare the following standard dilutions using test tubes and graduated pipets to withdraw the specified volumes of the standard Cu2+ solution from Part B and laboratory water. Make sure to record the actual volumes to two (2) decimal places. For example, if you must withdraw 2 mL of water, the water level can go beyond the 2 mL mark but record the exact volume, such as 2.01 mL in Data Table 3 in Section 6.
Dilution | Volume (mL) stock dye | Volume (mL) laboratory water |
1 | 2.00 | 8.00 |
2 | 4.00 | 6.00 |
3 | 6.00 | 4.00 |
4 | 8.00 | 2.00 |
To use the graduated pipet, first use the bulb to draw solution into the pipet above the zero mark. Cap the end of the pipet with your finger. Slowly allow air into the top of the pipet until the meniscus is just touching the ‘0.00’ calibration mark. Move the end of the pipet over to your receiving test tube. Drain the pipet until the meniscus is just touching the ‘2.00’ mark. Remove the pipet and return the additional liquid back to the stock you have in your beaker. 2.00 mL has now been transferred. Adjust these instructions for other volumes as appropriate.
2. Cover and securely close each test tube with a small piece of parafilm. Shake each solution well to mix.
Part D: Absorbance Measurements
The following steps can be performed on a Spectronic 20 or Spectronic 200 spectrophotometer (see Technique I: Use of Spectrophotometer of the laboratory manual for additional information). Ensure that the instrument has warmed up, stabilized, and blanked with laboratory water. Absorbance measurements should all be done on the same day, using the same spectrophotometer.
Spectronic 20
1. Measure the % transmittance of the standard Cu2+ solution and standard dilutions at 620 nm and record the data in Data Table 3 in Section 6.0. Note: Absorbance will be calculated in Section 7.0 Calculations and Data Analysis.
2. For each subsequent measurement, empty and rinse the cuvette with the next solution. Rinse the cuvette by adding a small amount of the next solution to be measured, swirling to coat the cuvette and empty before filling the cuvette until there is about 4 cm of the solution in the bottom.
3. With the same techniques measure the % transmittance of the filtered reaction supernatants at the same wavelength and record the data in Data Table 2 in Section 6.0.
Spectronic 200
1. Either in Scan mode, or in Live Display mode, measure the absorbance of the standard Cu2+ solution and standard dilutions at 620 nm and record the data in Data Table 3 in Section 6.0.
2. For each subsequent measurement, empty and rinse the cuvette with the next solution. Rinse the cuvette by adding a small amount of the next solution to be measured, swirling to coat the cuvette and empty before filling the cuvette until there is about 4 cm of the solution in the bottom.
3. With the same techniques measure the absorbance of the filtered reaction supernatants at the same wavelength and record the data in Data Table 2 in Section 6.0.
| Last Name | First Name | Partner Name(s) | Date | |
| Data Table 1. CuSO4 + KOH | ||||
| Appearance | ||||
| CuSO45H2O | ||||
| KOH | ||||
| Mixture | ||||
Balanced Chemical Equation:
| Data Table 2. Reaction Solutions | ||||||
| Reaction | A | B | ||||
| Mass KOH (g) | ||||||
| Laboratory water (mL) | ||||||
| Standard Cu2+ solution (mL) | ||||||
| Total volume (mL) | ||||||
| Supernatant Appearance | ||||||
| Absorbance | ||||||
| Data Table 3. Cu2+ Standard Dilutions Concentration of Standard Cu2+ Solution (mol/L): ________ Absorbance: ______ | ||||||
| Dilution # | Volume of Standard Cu2+ (mL) | Volume of Laboratory Water (mL) | % T (if measured) | Absorbance | Concentration* | |
| 1 | ||||||
| 2 | ||||||
| 3 | ||||||
| 4 | ||||||
| * See Section 7.0 Calculations and Data Analysis below | ||||||
1. Using the dilution equation, M1V1 = M2V2, calculate the concentration of Cu2+ (M2) in each of the diluted solutions, where M1 is the concentration of standard Cu2+ solution, V1 is the volume of standard Cu2+ solution recorded in Data Table 3, and V2 is the total volume of the dilution (sum of standard Cu2+ solution and water volumes). Record the values in Data Table 3. Remember to record the values in the Data Table with the appropriate number of significant figures and units. Show one sample calculation here.
2. If Spectronic 20 was used to measure the % transmittance, the absorbance of standard solution, dilutions, and sports drink samples will need to be calculated. Using the equation, A = log10 100%T, calculate the absorbances. Remember to record the values in the Data Table with the appropriate number of significant figures. Show one sample calculation here.
3. After completing the Data Table 3, make a graph of absorbance (y-axis) vs. the concentration of Cu2+ (x-axis). This is the standard curve. Notice that the data is linear. Obtain a best fit line or linear regression. Attach the graph when submitting and provide the equation here.
4. Based on the absorbances of the reaction supernatants in Data Table 2, determine the concentration in units of molarity of Cu2+ of each supernatant using the equation of the standard curve. Record the value in the Analysis Table below with the appropriate number of significant figures and units. Show one sample calculation here.
For example, if the equation of your standard curve is y = 1.2439x + 0.004637, and the measured absorbance for a reaction supernatant is 0.303. The concentration of Cu2+ in the supernatant is 0.240 M using the algebra shown.
(0.303-0.004637)1.2439=[Cu2+]
| Analysis Table | ||
| Reaction A | Reaction B | |
| Supernatant Cu2+ concentration | ||
| Moles of Cu2+ in supernatant | ||
| Moles of Cu2+ added | ||
| Actual moles of Cu2+ consumed | ||
| Moles of KOH added | ||
| Theoretical moles of Cu2+ consumed | ||
| % error | ||
5. Calculate the number of moles of Cu2+ remaining in the filtered supernatant of each reaction mixture using the supernatant concentration of Cu2+ in the Analysis Table and the total volume of the reaction mixture in Data Table 2. Record the values in the Analysis Table. Remember to record the value in the Analysis Table with the appropriate number of significant figures and units. Show one sample calculation here.
6. Calculate the number of moles of Cu2+ added to each reaction mixture using the concentration of standard Cu2+ solution in Data Table 3 and the volume of standard Cu2+ solution of each reaction mixture in Data Table 2. Record the values in the Analysis Table. Remember to record the value in the Analysis Table with the appropriate number of significant figures and units. Show one sample calculation here.
7. Calculate the number of moles of Cu2+ consumed in the reaction by subtracting the number of moles of Cu2+ remaining in the supernatant from the number of moles of Cu2+ added to each reaction mixture. Record the values in the Analysis Table. Remember to record the value in the Analysis Table with the appropriate number of significant figures and units. Show one sample calculation here.
8. Calculate the moles of KOH added to each reaction using the molar mass of KOH and mass of KOH added to each reaction in Data Table 2. Record the values in the Analysis Table. Remember to record the value in the Analysis Table with the appropriate number of significant figures and units. Show one sample calculation here.
9. Calculate the theoretical moles of Cu2+ consumed in each reaction using the moles of KOH added to each reaction and the balanced chemical equation in Data Table 1. Record the values in the Analysis Table. Remember to record the value in the Analysis Table with the appropriate number of significant figures and units. Show one sample calculation here.
10. Using the actual and theoretical moles of Cu2+ consumed in each reaction calculate the percent error of each reaction. Record the values in the Analysis Table. Remember to record the value in the Analysis Table with the appropriate number of significant figures and units. Show one sample calculation here.
% error= theoretical mol Cu2+ consumed-actual ol Cu2+ consumedtheoretical mol Cu2+ consumed×100%
8.0 POST-LAB QUESTIONS AND CONCLUSIONS
1. Locate and label the reaction mixture data points on your standard curve. Thinking about the appearance of the supernatants (Data Table 2), how do the qualitative observations correlate to the quantitate data? Provide a brief explanation to illustrate your reasoning.
2. In Data Table 1 you provided a balanced chemical equation for the reaction between copper (II) sulfate and potassium hydroxide, using the data in the Analysis Table, does your data support the chemical equation? Summarize data for and against your balanced chemical equation. Explain your reasoning using your data.
