UV/VIS SPECTROMETRIC DETERMINATION OF AN EQUILIBRIUM CONSTANT

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1 EXPERIMENT 8 UV/VIS SPECTROMETRIC DETERMINATION OF AN EQUILIBRIUM CONSTANT A. Prelab Perform the experimental portion of the lab in groups of 68. Divide the work so everyone participates. You have one lab period to complete the experimental portion of the lab. Everyone is responsible for completing and handing in a Lab Report B. Experimental Overview In this experiment the equilibrium constant of the week acid bromthymol blue, C 27 H 28 Br 2 O 7 S (mw = g/mol), will be measured. This weak acid is commonly used as an indicator for acidbase titrations. The chemical equilibrium can be written: HIn (aq) + H 2 O (l) H 3 O + (aq) + In (aq) where, HIn = C 27 H 28 Br 2 O 7 S and In = C 27 H 27 Br 2 O 7 S. K = [H 3 O + ][In ]/[HIn] The equilibrium constant, K, will be determined by measuring the [In ] and [HIn] and the ph since [H 3 O + ] = 10 ph. The equilibrium ratio of [In ]/[HIn] can be controlled by adjusting the ph of the solution. The concentrations of In and HIn will be determined spectrophotometrically using the Hitachi U2001 uv/vis Spectrophotometer. HIn, the acidic form of bromthymol blue is yellow, and In, the basic form, is blue. Because these species absorb photons at different wavelengths it is possible to determine their concentrations independently. C. Procedure Prepare a working standard of bromthyol blue Pipet 10 ml of stock bromthyol blue into a 100 ml volumetric flask and dilute to the line with 50/50 H 2 O/C 2 H 5 OH. The stock solution contains 1.007g bromthymol blue in 250 ml solution. Prepare solutions A1, A2, A3, A4, and A5 Prepare the above solutions by adding to a 100 ml volumetric flask 25 ml of H 2 O, 1 ml of ph adjusting reagent (1.0 M HCl) and a variable volume of bromthymol blue stock solution in that order. The volumes of bromthyol blue to be added are: A1 1 ml, A2 2 ml, A3 3 ml, A4 4 ml, A5 5 ml. Dilute to the line with H 2 O. Prepare solutions B1, B2, B3, B4, and B5 Prepare the above solutions by adding to a 100 ml volumetric flask 25 ml of H 2 O, 1 ml of ph adjusting reagent (1.0 M NaOH) and a variable volume of bromthymol blue stock solution in that order. The volumes of bromthyol blue to be added are: B1 1 ml, B2 2 ml, B3 3 ml, B4 4 ml, B5 5 ml. Dilute to the line with H 2 O. 1

2 Prepare solution C Prepare the above solution by adding to a 100 ml volumetric flask 25 ml of H 2 O, 1 ml of ph adjusting reagent (ph 7.00 buffer) and 5 ml of of bromthyol blue. Dilute to the line with H 2 O. Prepare solution D Prepare the above solution by adding to a 100 ml volumetric flask 25 ml of H 2 O, 1 ml of ph adjusting reagent (ph 7.40 buffer) and 5 ml of of bromthyol blue. Dilute to the line with H 2 O. Part 1: Spectral Analysis of Solution Sets A and B. Obtain a visible spectrum from 700 to 350 nm of each of the 5 solutions in both sets A and B. The directions for using the Hitachi U2001 Spectrophotometer are given at the end of this experiment. In solutions A, the [H 3 O + ] is so large that you can assume that the spectrum is due to only the acidic form of bromthymol blue, HIn, i.e., [In ] ~ 0. Similarly in solutions B, the [OH ] is so large that its spectrum is due only to the basic form of bromthymol blue, In, i.e., [HIn] ~ 0. Measure the absorbances, Abs Ai, and the associated wavelength, λ max A, corresponding to the maximum absorbances for the 5 solutions of A. Similarly, obtain Abs Bi and λ max B for the 5 solutions of B. Superimpose the spectra from solutions A1 and B1, A2 and B2,, A5 and B5 along the wavelength axis using the data handling capability of the spectrophotometer. Determine the isobestic point, λ I, the wavelength where the absorptivity of solution A equals that of solution B. At this wavelength the absorbance of solutions A, B, C, and D theoretically will be equal if the total concentration of bromthymol blue, [HIn] + [In ], were equal. Part 2: Spectral Analysis of Solutions C and D, determination of the equilibrium constant As in Part 1 with solutions A and B, obtain a visible spectrum from nm for both solutions C and D using the Hitachi U2001 Spectrophotometer. From these spectra obtain the absorbances at λ 1 and λ 2 for both solutions, Abs 1 and Abs 2. For both solution C and D: Abs 1 = k A1 x c HIn + k B1 x c In Abs 2 = k A2 x c HIn + k B2 x c λ λ 2 Because Abs 1 and Abs 2 are known from the above measurement, and k A1, k B1, k A2, and k B2 were obtained in Part 1, for each solution c HIn ([HIn]) and c In ([In ]) can be obtained by solving the above equations for these variables. These concentrations, together with [H 3 O + ] = 10 ph enable one to calculate the equilibrium constant for this weak acid at room temperature, K = [H 3 O + ][In ]/[HIn]. You will be able to do this using data from both solution C and D. 2

3 Report Page for Part 1 Turn in this page, one superimposed spectra for solutions A and B, and linear regressions for solutions A and B. You may use Excel to perform the linear regression analysis. Table 1A Solution [HIn] λ 1 λ 2 A1 A2 A3 A4 A5 Linear Regression Results for Solution A: Table 1B Solution [In ] λ 1 λ 2 B1 B2 B3 B4 B5 Linear Regression Results for Solution B: Molarity of bromthymol blue stock solution: λ 1 = λ 2 = λ I = Beer s Law states that absorbance is proportional to concentration, Abs = kc, where the proportionality constant, k, is the product of the absorptivity, a, and the path length, b. For solutions A, plot Abs vs c Hin at both λ 1 and λ 2 and from linear regressions, determine the proportionality constants k A1 and k A2. k A at λ 1 (k a1 ) = k A at λ 2 (k A2 ) = Similarly, for solutions B, plot Abs vs c In proportionality constants k B1 and k B2. at both λ 1 and λ 2 and from linear regressions, determine the K B at λ 1 (k B1 ) = k B at λ 2 (k B2 ) = 3

4 Report Page for Part 2 Turn in this page, the spectra for solutions C and D, calculations, and the answers to questions Table 2 Variables Solution C Solution D Abs 1 Abs 2 c HIn C In C total = c Hin + C InpH [H 3 O + ] K K ave = Questions: B1. Using K ave, calculate [In ] in solutions A1 A5. B2. Using K average, calculate [HIn] in solution B1 B5. B3. For both solutions C and D measure Abs I (at λ I ). Solution C: A I = Solution D: A I = Average value A I = Since at λ I, k HIn = k In, show why the A I values of solutions C and D should be equal. 4

5 Short Instructions for Using Hitachi U2001 Spectrophotometer Clean hands before using the keyboard! 1. Turn on instrument (switch at the lower, left front) and wait for initialization to finish. 2. Place the sample and reference cells in the appropriate holders (sample toward the front). 3. Use the down arrow key, < >, to select the wavelength scan, WL Scan, and press <Enter>. 4. Use the up arrow key, < >, to select Test Setup, and press <Enter>. 5. On the left side of the menu use the up/down arrow keys to move through the menu. To return to the previous menu, press <Return>. a. At Start WL, enter the high wavelength for the scan. b. At Stop WL, enter the low wavelength for the scan. * c. At Scan Speed, for most purposes 1200 nm/s will be suitable. For better resolution, choose a slower speed, for lower resolution, a higher speed. d. For other options, the default values will usually suffice (Init Delay, 0; Num Cycles, 1; Cycle Time, 0; Display Format, Sequential). 6. Press the blue <Forward> key and a chart will appear. Press the green <Start> key. 7. When the scan is completed you may want to rescale the yaxis, the xaxis or both of the spectrum to improve the display. Choose <3> (Rescale) and then <Enter>. Select the appropriate option then <Enter> and choose the desired limits for the various options, then press <Return>. 8. To print the scan, make sure the printer is on (left purple button at top) and that it has paper. Choose <4> (Print). Release the paper by pressing the right purple button at the top of the printer. Then press <Return>. 9. To obtain quantitative information: a. Make sure printer is on as in step 8, choose <3> (Table) and press <Enter>. Values of the wavelengths for peaks and valleys and their respective absorbances will be printed. Then press <Return>. b. Or, choose <2> (Peaks) and press <Enter>. Move the cursor to each successive peak with the right or left arrow keys, < > or < >. The wavelength and absorbance values for each selected peak will appear, in black, on the display. After recording all pertinent data, press <Return>. c. Or, choose <1> (Trace) and again move the cursor to the left or right with the arrow keys. The speed of the cursor movement increases with time the arrow key is depressed. Again, the wavelength and absorbance values for cursor placement will appear, in black, on the display. After recording or printing (grey, <Print Data> key) all pertinent data, press <Return >. * If the instrument is to be left on for a significant period of time and the uv region is not being used, go to the Instrument Setup menu, and turn off the D 2 lamp. 5