Chemistry 162 The following write-up is inaccurate for the particular chemicals we are using. Please have all sections up through and including the data tables ready before class on Wednesday, February 27. As usual, please write an abstract and paper-clip it to the front of your individual writeup. The abstract and the carbon-copy pages of the write-up is due in class on Wednesday, March 6. Lab 5: Calculating an equilibrium constant Introduction: Indicators are substances whose solutions change color due to changes in ph. They are usually weak acids or bases, but their conjugate base and acid forms have different colors due to differences in their absorption spectra. Indicators are typically weak acids or bases with complicated structures. For simplicity, we represent a general indicator by the formula HIn, and its ionization in a solution by the equilibrium equation: The equilibrium constant for this reaction (Keq) can be written omitting the water, since it is a pure substance. In this experiment we will determine the equilibrium constant (Keq) for the indicator bromothymol blue using a spectrophotometer and a ph meter. Keep in mind that bromothymol blue is blue when in the basic form (In 2- ) and yellow when in the acidic form (HIn - ). The balanced reaction for bromothymol blue s dissociation is: Note that the undissociated bromothymol blue (left) starts already as an anion, and proceeds to become a dissociated divalent anion (right). To measure how much bromothymol blue is in a given solution, you will use the measurements of a spectrophotometer on the bromothymol blue and Beer s Law: The absorbance (A) is the product of three factors, the concentration of the measured species (c), the molar extinction coefficient (ε) and the length of the path the light beam must travel through the solution (l). Since ε and l are fixed for a particular spectrophotometer and chemical species, Beer s Law reduces to the simple notion that the absorbance measured is proportional to the concentration of the measured species. Further complicating this experiment is the observation that bromothymol blue changes color depending on the acidity of the solution. At some wavelengths bromothymol blue
will absorb light intensely while at others it will be nearly completely transparent. Our goal is to tune the instrument to the wavelength that will give us the best signal. This will be accomplished by testing our calibration solutions and selecting the wavelength of maximum absorbance (λ max) for both the basic (blue) form and acidic (yellow) form. By using strongly acidic or basic solutions, we can shift the equilibrium nearly completely toward the basic or acidic forms of the indicator. The spectrophotometer should be set to record absorbance (A), but sometimes they are setup to record percent transmittance (%T), which is an intuitively easier concept. The conversion from %T to A is given by the following formula: Lab 5: Calculating an equilibrium constant Part 1. Purpose One goal is to calculate the equilibrium constant of the dissociation of bromothymol blue; you can certainly write that. But another goal is the method that you are using to figure out that constant. In your own words, add a sentence explaining succinctly what you will be looking for. At the bottom of this section, write the balanced equation for the reaction, then add the mathematical expression for calculating the reaction s equilibrium constant. Part 2. Materials and methods Spectrovis spectrophotometer and laptop Lots of 4.5 ml cuvettes 0.00025 M bromothymol blue solution 0.010 M hydrochloric acid 0.010 M sodium hydroxide solution Distilled water Digital ph meter Many test tubes 0.10 M K2HPO4 solution 0.10 M KH2PO4 solution Make a sketch of the experimental setup to be used. Each sketch should have the various components labeled. Part 3. Procedure and Observational Data Caution: Dispose of all solutions (including unused ones) in the waste beaker provided.
Phase 1 Determining the wavelengths of maximum absorption 1. Prepare six calibration solutions in medium-size test tubes using the 1 and 5 ml graduated pipettes. Stock solutions are those that are provided with the lab. Calibration solution Stock bromothymol blue (ml) Stock hydrochloric acid (ml) C1 1.00 4.00 C2 0.70 4.30 C3 0.50 4.50 Calibration solution Stock bromothymol blue (ml) C4 1.00 4.00 C5 0.70 4.30 C6 0.50 4.50 Stock sodium hydroxide sol n (ml) 2. Obtain a spectrophotometer (SpectroVis) and computer. Turn on the computer and attach the spectrometer via USB cable. Click on the Logger Pro icon on the desktop to initiate the spectroscopy software. 3. Once the software is up and running it is necessary to calibrate the spectrophotometer. To calibrate the SpectroVis, choose Calibrate Spectometer:1 from the Experiment menu. 5. Fill a clean cuvette about ¾ full with distilled water and place it in the sample chamber of the spectrophotometer. Follow the instructions in the dialog box to complete
the calibration, and then click OK. After calibrating the spectrophotometer you will be ready to collect data (there may be some last minute set-up instruction from the instructor, so this is a good point to check with the instructor). Remember to handle the cuvette on its ridged sides, not its clear sides. 6. Fill a clean cuvette about ¾ full with solution C1 and place it in the sample chamber of the spectrophotometer. Click Collect to begin data collection. 7. Click Stop to end data collection. Save the spectrum onto your flash drive (in non-.cmbl format); record the filename in your notebook. Also record λ max for this species (use either the mouse or the table to determine this). 8. Fill a clean cuvette about ¾ full with solution C4 and place it in the sample chamber of the spectrophotometer. Click Collect to begin data collection. 9. Click Stop to end data collection. Save the spectrum onto your flash drive (in non-.cmbl format); record the filename in your notebook. Also record λ max for this species (use either the mouse or the table to determine this). Phase 2 Determining the proportionality constant in Beer s Law 10. Set the instrument to the wavelengths of maximum absorbance determined in Part 1, by clicking on the Configure Spectrometer Data Collection button. Then select Abs vs. Concentration as the collection mode. Then change the Column Name to Sample #, the Short Name to sample, and delete the Units. 11. Select the maximum absorbance(s) desired, then click OK.
12. Fill your cuvette(s) about ¾ full with your solution(s); you should run all of your C solutions. Insert your sample into the sample holder and click Collect. When reading stabilizes, click Keep. Enter Sample # and click OK. 13. Repeat these steps until all samples have been read on the spectrometer. 14. Click Stop to end data collection; save the data and record the name of the file. Phase 3 Taking measurements from which the equilibrium constant is calculated 15. Prepare five standard solutions in medium-sized test tubes using 1 and 5 ml graduated pipettes, according to the table below: Standard solution Stock bromothymol blue (ml) Stock K2HPO4 solution (ml) Stock KH2PO4 solution (ml) S7 1.00 4.00 0.00 S8 1.00 3.00 1.00 S9 1.00 2.00 2.00 S10 1.00 1.00 3.00 S11 1.00 0.00 4.00 16. Since the spectrophotometer is still calibrated, fill your cuvette(s) about ¾ full with your solution(s); you should run all of your S solutions. Insert your sample into the sample holder and click Collect. When reading stabilizes, click Keep. Enter Sample # and click OK. After reading the sample absorbance, return the sample to its original test tube (i.e., don t throw it into the waste beaker yet). 17. Repeat these steps until all samples have been read on the spectrometer. 18. Click Stop to end data collection; save the data and record the name of the file. 19. Your instructor will show you how to calibrate the digital ph meters, if needed. Follow the instructions given to measure and record the ph of each of the S solutions. Remember to rinse the ph electrode in between measurements. Part 4. Original data and preliminary analysis You ll need to be careful in this section to lay out enough space ahead of time. For Phase 1, you ll want enough space to tape the two (C1 and C4) spectra into the notebook, along with a title or label for each one. You ll also want to mark λ max on each spectrum. For Phase 2, you ll want a table that has headings calibration solution, bromothymol blue concentration, and absorbance, with units, and all the numbers.
For Phase 3, you ll want a table that has headings standard solution, ph, and absorbance, with units, and all the numbers. Part 5. Calculated results Show the calculation of the Beer s Law proportionality constant for both the acidic and basic forms of bromothymol blue; make sure to include units. Make sure to recast the Beer s Law equation to indicate what you are doing. Make a plot of bromothymol blue concentration (x-axis) versus absorbance (y-axis) for the Phase 2 data. Use an overlay because you will be plotting both the acidic and basic form absorbances on this graph. Include a title, label axes and all the other features of a good graph. Draw a best-fit line for your points, and include the correlation coefficient (though with so few points, it s a bit meaningless). Make a table of your Phase 3 data that shows [H + ], [HBTB ], [BTB 2 ] and the equilibrium constant Keq. Below, for one of the rows of the table, explain how you calculated or derived each of the concentrations, referring to the calibration plot as necessary in the explanation. Determine your team s avarage Keq for bromothymol blue at room temperature. Explain any outliers you choose to omit from your calculation of this average. Find a reference that gives a published Keq (big hint: it may be called Ka ), cite the reference in the usual style, and calculate a percent error. Part 6. Group results Enter your group s average Keq on the spreadsheet. Calculate the mean and standard deviation of Keq for all groups. Comment on any outliers (which may even be your own values!) and state whether they were used in the calculation. Calculate the percent error for the class from the published value in Part 5. Part 7. Questions 1. Ideally, what shape does Beer s Law suggest that your calibration curve should have been? In your graph, were there any deviations from this ideal behavior? If so, suggest a reason for the deviation.
2. Background subtraction is a commonly-applied step on many spectroscopy experiments. What was the background, if any, in this experiment? How did you correct for it? Part 8. Conclusion First paragraph: Report your team s λ max values for both the acidic and basic forms of bromothymol blue. Give a brief description of how you obtained these values, and how confident you are of them. Note any problems that might cause you to doubt these numbers. Second paragraph: Report your team s Beer s Law proportionality constants for both the acidic and basic forms of bromothymol blue. Give a brief description of how you obtained these values, and how confident you are of them. Note any problems that might cause you to doubt these numbers. Third paragraph: Report your team s average Keq of bromothymol and the percent error from the published value. Note any problems that might cause you to doubt this number. Final paragraph: How confident overall are you of your results? In other words, was this a good experimental setup? Were there clear random or systematic errors, and, if so, how could they be corrected? Abstract In the standard abstract format and in a hundred words or less, state the class s equilibrium constant for bromothymol blue and the standard deviation, and report the percent error from the published value. Briefly summarize the manner in which it was measured. Report any major random or systematic errors, and note how this (these) error(s) would lead to the results you have reported.