Kinetics of Dye Fading Lab
Kinetics of Dye Fading
The indicators used in this experiment are all large organic molecules with similar structures, and they undergo the same general type of reaction in the presence of hydroxide ions. As an example, phenolphthalein has the colorless structures shown in Figure 1 when the solution PH < 8. As the solution become basic and the PH increase, the phenolphthalein molecule loses two hydrogen ions to form the red violet dianion shown in Figure 2.
The colorless to red transition of H2P to P2- is very rapid and the red color develops instantly when the PH reaches its transition range (PH 8-10). If the concentration of hydroxide ions remains high, the red P2- dianion will, slowly combine with hydroxide ions to form a third species, POH 3-, which is again colorless. The rate of this second reaction is much slower than the first and depends on the concentration of phenolphthalein and hydroxide ions. Thus, the color of the red P2- species will gradually fade in a basic solution (PH > 3)
The kinetics of the "fading" reaction can be followed by measuring the concentration of P2- versus time and graphing results. Figure 3 illustrates how the concentration of a reactant decreases with time over the course of a reaction. Notice the is graph of concentration versus lime is a curved line, not a straight line. The curve levels off as it approaches the x-axis. This means that the reaction slows down as the reactant concentration decreases.
Exactly how much the reate decreases as the reactant concentration decreases depends on the rate law for the reaction. In the case of the reaction of P2- with OH- ions, the rate law has the general form.
The exponents n and m are defined as the order of reaction for each reactant and k is the rate constant for the reaction at a particular temperature. The values of the exponents n and m must be determined by experiment. If the reaction is carried out under particular temperature. The values of the exponents on and m must be determined excess of hydroxide ions- then the rate law will reduce to the form.
Methylene blue (MB) is another indicator dye that undergoes a color fading reaction. In this reaction, the color fading results from the reduction of methylene blue by ascorbic acid (Asc). See Equation 5. Methylene blue was added to a solution of 0.1 M ascorbic acid and the solution immediately turned blue. After 80 seconds, the color faded and the solution was almost colorless. The following absorbance measurements were recorded in Table 1.
MB + + Asc -> MBH
Table 1
Materials
Indicator solution, 1 drop
Phenolphthalein solution, 0.2%
Crystal violet solution, 1% alcoholic
Bromphenol blue solution, 0.10%
Sodium hydroxide, NaOH, 0.2 M, 5 mL
Sodium hydroxide solution, 1.0 M, 5 mL
Sodium hydroxide solution, 0.1 M, 5 mL
Thermometer
Tissues or lens paper, lint-free
Wash bottle and distilled water
Cuvette with lid
Colorimeter sensor or spectrophotometer
Computer interface system (LabPro)
Computer or calculator for data collection
Data collection software (LoggerPro)
Procedure
1. Handle the cuvette by its ribbed sides or its top to avoid getting fingerprints on the surface.
2. Connect the interface system to the computer or calculator and plug the colorimeter sensor into the interface.
3. Select Setup and Sensors from the main screen and choose "Colorimeter."
Note: Many newer sensors have an automatic calibration feature that automatically calibrates the colorimeter before use.
If the sensor has the autocalibration feature, set the wavelength on the colorimeter to the corresponding proper wavelength for your assigned indicator. Press the autocalibration key, and proceed to step 9. If the sensor does not have the autocali-bration feature, follow steps 4-8 to calibrate the colorimeter for 100% transmission (0 absorbance) with a "blank" cuvette containing only the sodium hydroxide solution.
4. Select Calibrate and Perform Now from the Experiment menu on the main screen.
5. Obtain a clean and dry cuvette and fill it about 2/3 full with the proper sodium hydroxide solution. Wipe the cuvette with a lint-free tissue, then place the cuvette in the colorimeter compartment.
6. Set the wavelength knob on the colorimeter to 0%T- the onscreen box should read zero. Press Keep when the voltage is steady.
7. Turn the wavelength knob on the colorimeter to the indicator wavelength-the onscreen box should read 100. Press Keep when the voltage is steady.
8. Return to the main screen and set up a live readout and data table that will record absorbance as a function of time.
9. Select Setup followed by Data Collection. Click on Selected Events to set the computer for manual sampling.
10. Remove the "blank" cuvette from the colorimeter compartment. Measure and record the initial temperature of the sodium hydroxide solution.
11. Add one drop of the indicator solution to the cuvette and immediately press Collect on the main screen to begin measuring time. This ensures that the absorbance versus time measurements will accurately reflect the time of reaction from the time of mixing.
12. Place the lid on the cuvette and carefully invert the cuvette several times to mix the solution.
13. Place the cuvette in the colorimeter compartment. When one or two minutes have elapsed from the time of mixing, press
Keep on the main screen to automatically record the absorbance.
14. Continue making absorbance measurements at regular time intervals (at least every two minutes). Press Keep on the main screen to automatically record the absorbance at each desired time.
15. When 16 minutes have elapsed from the time of mixing, press Stop on the main screen to end the data collection process.
16. If possible, save the data on the computer or calculator and obtain a printout of the absorbance versus time data table and graph. Otherwise, record the absorbance and time measurements in the data table.
17. Remove the cuvette from the colorimeter compartment. Measure and record the final temperature of the dye solution.
18. Rinse the cuvette several times with distilled water and allow it to air dry.
19. (Optional) Calculations and graphical analysis may be done on the computer or calculator using either the data collection software that accompanies the technology interface system or a conventional spreadsheet program such as Microsoft
Indicator: Bromphenol Blue Solution
The temperature saw a slight increase, but if it were a snapper increase.
The concentration of sodium hydroxide is assumed to be constant throughout the reaction and is thus included in the reduced rate law expression. It is valid because when I graphed In (Abs) vs time, I did indeed get a straight line with a negative slope. There is such a high amount of sodium hydroxide that reducing the amount of the NaOH would not drastically effect the concentration.
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