The Photodegradation of Dyes Chemistry IA

Transcription

1 Page 1 The Photodegradation of Dyes Chemistry IA Introduction This Internal Assessment provides experimentation and analysis into the topic of the photodegradation of dyes. Dyes, as a broad class of compounds, have many diverse and unique uses in industry today, whether it be to colour clothes, food, or even buildings. However, one problem that consistently faces the usage of dyes in industry is an effective method to degrade them, should the colour be required to be removed. For a long time, bleach (sodium hypochlorite), has long been used in the textile industry, to remove various dyes and stains from clothing. Bleach is also sold domestically, and my parents household, as with many others, bleach is used regularly to whiten and clean clothes. However, bleach does pose a number of different health hazards, such as lung irritation and skin irritation (The Clorox Company, 2012). Photodegradation has been suggested as an alternative for the degradation of dyes, which poses less safety risks to humans. To define the extent of my experiment on the photodegradation of dyes, the research question for this Internal Assessment is: Research Question How does the intensity of UV light, altered by the distance (5,7.5,10,12.5,15cm) of the UV lamp from the dye, affect the rate of photodegradation of Allura Red dye, with Zinc Oxide as the photo catalyst, as measured by the absorption of an aqueous solution of dye? Background Information I conducted some in-depth research into the degradation of dyes under different conditions, eventually settling on the photodegradation of dyes as my research topic, which helped me to narrow down my research and eventually settle on the research question above. One matter that needed to be developed was which food dye to use in the experiments. I had decided on a red dye towards the beginning of the IA experimentation, as I believed it would degrade the fastest under UV light as it was almost complementary to it. Red dye 40, or Allura Red Dye, is a wellknown food dye used in industry. As a compound, it has the molecular formula of: The structure of Red 40 dye is as shown below: CC 18 HH 14 NN 2 NNNN 2 OO 8 SS 2 Figure 1 Structure of Allura Red Dye (Pubchem.ncbi.nlm.nih.gov, 2017)

2 Page 2 Considering its properties, it does not degrade under any natural conditions, but does undergo photodegradation when irradiated with UV light using a photo catalyst. During my research, I also had to choose a photo catalyst to use, and settled on zinc oxide as the photo catalyst, as other photo catalysts were highlighted as carcinogens, and would not be permitted for experimentation. Zinc oxide undergoes a specific mechanism as described below (Lam, 2016): When exposed to UV light, the valence band electrons of the Zinc Oxide photo catalyst become excited, and as a result, the electrons that were initially in the valence band are moved to the conduction band. As a result, due to the lower number of electrons in the valence band, a positive hole is created, and with this there is also the generation of electron-hole pairs. This also helps to explain the semiconductor qualities of Zinc Oxide. ZZZZZZ + UUUU llllllhtt ZZZZZZ(ee cccc + h vvvv + ) After that, one of the constituents of the electron-hole pair is quite a powerful oxidising agent (h vvvv + ), which can undergo two different processes when it oxidises an external compound. One of the processes is direct oxidation, where the constituent itself oxidises the external compound. The other process is indirect oxidation, where the constituent can react with water and hydroxyl ions to form a strong hydroxide radical, which can then oxidise the subsequent external compounds. It should also be noted that when the two constituents of the electron-hole pair meet, it will cause a recombination: ee cccc + h vvvv + ZZZZZZ + HHHHHHHH Therefore, to prevent this happening, the other constituent can be used by electron acceptors to form more radicals, and could also possibly produce more hydroxide radicals. As a last step, the hydroxide radicals can react with the organic dyes, forming intermediates, and eventually forming the final products of the reaction, which could involve carbon dioxide and water. Additionally, the recombination of electron-hole pairs is also a possibility, as not all of the constituents are accepted by electron acceptors. Aim & Hypothesis The aim of this Internal Assessment is to determine the rate of degradation of Allura Red Dye under photocatalytic conditions, and with Zinc Oxide as the photo catalyst, and compare it to the rate of degradation when the UV light source is moved further away from the solution, decreasing the intensity of UV light available to the solution. It is hypothesized that as the UV light intensity is increased (moved closer to the beaker), the rate of degradation will increase, due to the higher energy that can be absorbed by the photo catalyst, thus speeding up the rate of photodegradation, and the number of electron-hole pairs that are produced by the catalyst, therefore increasing the number of hydroxide radicals available to react with the dye. Variables Variable Independent Variable The intensity of UV light that was present in the chemical system. This was altered by how far away the UV light source was from the beaker containing the Allura Red dye solution, at intervals of 2.5cm, the distances being 5cm, 7.5cm, 10cm, 12.5cm and 15cm.

3 Page 3 Variable Dependent Variable The rate of degradation of Allura Red Dye in a solution containing suspended Zinc Oxide nanoparticles. The rate was determined by taking samples of the solution at regular time intervals, and measuring their absorbance, which can be used to calculate the concentration of dye in the solution using Beer- Lambert s Law. The concentrations were then plotted against time, and the gradient was assumed to be the rate of degradation. Controlled Variables Temperature External Light Intensity Dispersion of Zinc Oxide nanoparticles Residue in centrifuge tubes Experiments were conducted at the same temperature (approximately 26 C), and controlled by the air conditioning present. Any disturbances that could be caused by air flow were negated by placing the solution in a dark shoebox. Solutions were placed in a dark shoebox to negate any outside light sources from interfering with the system, although the effect they would have is unknown. Solutions were consistently stirred with a magnetic stirrer at 40rpm, ensuring that the Zinc Oxide nanoparticles were equally dispersed throughout the chemical solution. The same centrifuge tubes were used for the experiments, and it was noticed that there was residue (supposedly Zinc Oxide) left after centrifuging the tubes, therefore they were washed and cleaned before being used again to prevent any of the Zinc Oxide residue affecting the results of the experimentation. Apparatus Equipment UV Light (20W Power) Beaker (250mL) Measuring cylinder 50mL Shoebox/ Dark area Magnetic Stirring Rod Magnetic Stirrer Centrifuge + Centrifuge Tube Colorimeter + Cuvette Drop Pipette Weighing Boat + Mass Scale (2 d.p.) Stopwatch Chemicals Red 40 dye (concentrated) ZnO (powder) Distilled Water Methodology The methodology for this experiment was adapted from an experiment conducted on another industrial dye (Lam, 2016), and has been used as it enables a high degree of control over the variables, as well as simple methods to obtain results without the use of expensive or advanced apparatus. Solution Preparation 1. Dilute 1mL (10 drops) of Allura Red dye in a beaker filled with 200mL of distilled water.

4 Page 4 2. After letting the dye diffuse for 2 minutes to ensure an equal distribution of the dye, weigh 0.3g of the ZnO photo catalyst, and add into the beaker. 3. Add in a magnetic stirrer rod, and place the beaker inside the shoebox. 4. Place the shoebox on the magnetic stirrer, and set the stirrer to 40rpm, to ensure the Zinc oxide photo catalyst stays suspended. 5. After 15 minutes, the solution should have reached the absorption-desorption equilibrium, where the ZnO has been adequately suspended equally throughout the dye. Absorbance Measurements 6. Take four samples of the solution and place into the centrifuge tubes. 7. Put the centrifuge tubes into the centrifuge and turn it on to separate the Zinc oxide nanoparticles to prevent the nanoparticles from causing a higher absorbance reading when measured. Record the time at which the centrifuge was started. 8. Place the UV in the shoebox, 5cm away from the centre of the beaker filled with solution, and turn the UV light on. 9. After 10 minutes of the centrifuge being on, turn it off, and remove the centrifuge tubes from the centrifuge. 10. Wait until the UV light has been on for 10 minutes, then remove another 4 samples from the solution, to put into the centrifuge tubes. 11. With the samples that have already been centrifuged, extract the liquid from the centrifuge tubes, and place into a cuvette. 12. Measure the absorbance reading of the solution in the cuvette at 470nm, as this is the wavelength at which the most notable absorbance readings can be recorded. 13. Repeat steps 6 12 until the solution has been irradiated for 40 minutes, and the samples for the 40 minutes have been centrifuged, and leaving the solution for any longer would cause absorbance too low to be recorded. 14. Repeat steps 1-13, changing the distance that the UV light is away from the beaker of dye solution. Control Experimentation Two control experiments were carried out to ensure that the combination of ZnO and UV light was causing the photodegradation to occur. The same methodology was used as with the other experiments, although for one control, the ZnO was omitted, and the other was carried out in the absence of UV light. Safety Precautions Concern Reason Solution Exposure to UV light Harmful to eyes, could cause blindness and irritation of eyes. UV protective goggles were worn for the entirety of the experiment Zinc oxide (Science Lab, 2017) Slightly hazardous when in contact with skin or eyes (irritant). Ensure hands are washed after experimentation, and that goggles are worn to prevent eye contact. Disposal to comply with local regulations

5 Page 5 Raw Data Raw Data Tables Shown below are the raw data tables displaying the values of the absorbance for each intensity of UV light, recorded at intervals of 10 minutes. It should be noted as absorption is technically unitless, the arbitrary units AU, or absorbance units were used: Absorbance (AU) (±0.001AU) Distance of UV Light (cm)(±0.05cm) Repeat Control Experiments Time (min) Control (no UV) Control (no ZnO) Absorbance (AU) (±0.001AU) Quantitative Observations During the experimentation, there was an obvious colour change in the dye solution, turning from milky pink to milky white, the milky colour as a result of the ZnO suspended in the solution. It was also noticed that when the solution was prepared, it remained clear, until the magnetic stirred was turned on, causing the solution to go cloudy. Thirdly, an interesting observation was that the discolouration of the solution was uniform, occurring across all sides of the beaker. Finally, there did seem to be a visible change in the colour of the solutions in the centrifuge as time passed, going from a deep pink colour to a light pink, almost colourless solution. Data Processing Intensity of UV Light To calculate the intensities of the UV at the specific distance, we will need to first calculate the distance covered by the UV light. In order to do this, we can use the formula: SSSSSSSSSSSSSS AAAAAAAA oooo SSSSheeeeee = 4ππrr 2

6 Page 6 Where r is the distance of the beaker from the UV light in metres. The surface area of a sphere has been chosen as it is assumed that the UV light will be emitting light that will take the shape of a sphere, and it will be constant in all directions, and thus the surface area of a sphere will give the relative intensity of the light at a certain distance. Using the value of the UV light being 5.00cm (0.05m) away from the beaker: SSSSSSSSSSSSSS AAAAAAAA = 4ππ(0.05) 2 SSSSSSSSSSSSSS AAAAAAAA = 0.01ππ SSSSSSSSSSSSSS AAAAAAAA (tttt 3 ss. ff. ) Finally, as intensity is typically measure in watts per metre squared, we must divide the power rating of the UV light (20W), by the surface area of the sphere emitted by the UV light: IIIIIIIIIIIIIIIIII (WWWWWWWWWW pppppp mmmmmmmmmm ssssssssssssss) = IIIIIIIIIIIIIIIIII = WW mm2 (tttt 3 ss. ff. ) Concentration of dye at different time intervals To calculate the rate of degradation of the dye, we will need to use a law first formulated by Beer-Lambert in 1962, aptly called Beer-Lambert s Law (Illustrated glossary of organic chemistry - beer s law (Beer-Lambert law), no date). The law states that: AA = εεbbbb Where ε is known as the extinction coefficient, and is linked to the substance being analysed. b is the path length, which is the length which the light has to pass through when measuring the absorbance (in cm). For this experiment, b will be equal to 3mm, or 0.3cm, which is the length of the cuvette used in the experiments. A is the absorbance, which was measured, and is shown in the raw data tables. Finally, c is the concentration of the solution, in mols/dm 3, which is the variable that is to be calculated in this experiment. Shown below are a number of example calculations, pertaining to the distance of 5cm (637 W/m 2 intensity). As three repeats were completed, the first step is to find the average absorption of the repeats: AAAAAAAAAAAAAAAAAAAA(AAAAAAAAAAAAAA) = AAAAAAAAAAAAAAiioooo (AAAAAAAAAAAAAA) = (tttt 3 ss. ff. ) After that, we can input this value into the formula for Beer-Lambert s Law, taking as the molar extinction coefficient for the Red 40 dye (Pubchem.ncbi.nlm.nih.gov, 2017): = cc cc = cc = mmmmmmmm/dddd 3 Therefore, we can assume that the concentration of Red 40 dye at 0 minutes, with the intensity of 637Watts per metre squared is equal to: cc = mmmmmmmm/dddd 3

7 Page 7 This procedure was repeated for all other time intervals at the particular intensity, and was also repeated for the other 4 intensities of UV light. Rate of Degradation Shown below is a table displaying the concentration changes for the intensity of 637 watts per metre squared: Time (min) Concentration (mols/dm 3 ) It is clear that there is a decrease in the concentration of the red 40 dye present in the solution. These concentration values have been displayed below graphically: Concentration (mols/dm3) As seen by the graph, there is apparently a linear decrease in the concentration of Red 40 dye under the UV light, and we can assume that it is a second order reaction. In order to determine the rate of degradation, we will need to consider the gradient of the line of best fit. Using the linear regression tool in Excel, the resultant gradient is: GGGGGGGGGGGGGGGG = Therefore, we can assume that the rate of degradation is: Discussion of Error Degradation of Red 40 Dye under UV light of intensity 367 W/m squared R² = Time (minutes) RRRRRRRR oooo dddddddddddddddddddddd = mmmm/dddd 3 mmmmmm As with almost every experiment, there are a number of different systematic and random errors that need to be accounted for. In this section, the random errors will be determined, based upon the apparatus used to take measurements. Systematic errors will be discussed later in the internal assessment. Error in Intensity For the intensity, the only error that needs to be accounted for is the error in the ruler measurements made to ascertain the distance that the UV lamp was from the beaker of solution. As the distance that was measured was different for each value, the absolute and percentage

8 Page 8 uncertainties will be different, although the method for working them out will be the same. Given that the uncertainty in the ruler was ±0.05cm, we can assume that for the 5cm measurement: %UUUUUUUUUUUUUUUUUUUUUU RRRRRRRRRR = %UUUUUUUUrrtttttttttttt RRRRRRRRRR = ±1% As no other measurements were required to calculate the intensity of the UV lamp, we can assume that the percentage uncertainty in the UV lamp intensity will be equal to that of the ruler measurement, therefore: Converting it to an absolute uncertainty; %UUUUUUUUUUUUaaaaaaaaaa IIIIIIIIIIIIIIIIII = ±1% AAAAAAAAAAAAAAAA UUUUUUUUUUUUUUUUUUUUUU IIIIIIIIIIIIIIIIII = AAAAAAAAAAAAAAAA UUUUUUUUUUUUUUUUUUUUUU IIIIIIIIIIIIIIIIII = ±6.37 WW mm 2 This was repeated for all other intensities, and results placed in the data tables. Error in Rate of Degradation Considering the rate of degradation, a much higher number of experimental apparatus were used, therefore more uncertainties will need to be considered. As the rate was determined by the gradient of the concentration against time, the uncertainties of these two values need to be determined: Uncertainty in Concentration To calculate the concentration, a number of different values were calculated, all of which will be needed to take into account. The first uncertainty will be the path length of the cuvette, measured with a ruler with ±0.05cm uncertainty associated with it. As the path length measured was 3mm, or 0.3cm, we can assume that the percentage uncertainty of the path length will be: %UUUUUUUUUUUUUUUUUUUUUU PPPPPPh LLLLLLLLLLh = %UUUUUUUUUUUUUUUUUUUUUU PPPPPPh LLLLLLLLLLh = ±16.7% (tttt 1 dd. pp. ) This will remain constant for all calculations, as the identical cuvettes were used. Secondly, the uncertainty in the absorbance will need to be accounted for. As a digital piece of apparatus was used to measure the absorbance, we can assume the uncertainty to be equal to ±0.001 AU. As the average of three readings was used, we can calculate the percentage uncertainty in each reading, then calculate the average of it to determine a percentage uncertainty in the average uncertainty, using the values for the distance of 5cm, at 0 minutes: %UUUUUUUUUUUUUUUUUUUUUU RRRRRRRRRRRR 1 = = ±0.599% (tttt 3 ss. ff. ) %UUUUUUUUUUUUUUUUUUUUUU RRRRRRRRRRRR 2 = = ±0.571% (tttt 3 ss. ff. ) %UUUUUUUUUUUUUUUUUUUUUU RRRRRRRRRRRR 3 = = ±0.592% (tttt 3 ss. ff. ) 0.169

9 Page 9 We can then average these values to find an overall percentage uncertainty in the absorbance readings: %UUUUUUUUUUUUUUUUUUUUUU AAAAAAAAAAAAAAAAAAAA = ±0.587% (tttt 3 ss. ff. ) Therefore, as the concentration was calculated by rearranging Beer-Lambert s law to become: cc = AA εε bb We can add the percentage uncertainties of the path length and the absorption, as they were the only measurements made, to obtain an overall uncertainty in the concentration of the dye solution with the UV light at a distance of 5cm, at 0 minutes: %UUUUUUUUUUUUUUUUUUUUUU CCCCCCCCCCCCCCCCCCCCCCCCCC = ±17.3% (tttt 3 ss. ff. ) Uncertainty in Time Values Considering the nature by which the degradation was measured, although the samples were taken every ten minutes, the lowest division on the stopwatch was ±1 second, therefore we can assume that the percentage uncertainty in the time will be, if we use the value of 10 minutes: %UUUUUUUUUUUUUUUUUUUUUU TTTTTTTT = ±0.167% (tttt 3 ss. ff. ) This was calculated for all other time values, and the uncertainties associated with each time have been placed in the table below. These uncertainties will remain constant for all data points: Time Percentage Uncertainty (to 3 s.f.) 0 ±0% 10 ±0.167% 20 ±0.333% 30 ±0.500% 40 ±0.667% Overall Uncertainty in the Rate of Degradation Considering that the rate of degradation was calculated by the average change in concentration over the average change in time (the gradient of the line between the two), we should be able to assume that the uncertainty in the rate of degradation will be equal to the average percentage uncertainty in the two, added together: %UUUUUUUUUUUUUUUUUUUUUU AAAAAAAAAAAAAA CCCCCCCCCCCCCCCCCCCCCCCCCC = ±17.5% (tttt 3 ss. ff. ) %UUUUUUUUUUUUUUUUUUUUUU AAAAAAAAAAAAAA TTTTTTTT = ±0.334% (tttt 3 ss. ff. ) 5 Therefore; %UUUUUUUUUUUUUUUUUUUUUU RRRRRRRR oooo DDDDDDDDDDDDDDDDDDDDDD = = ±17.8% (tttt 3 ss. ff. ) Converting this to an absolute uncertainty;

10 Page 10 AAAAAAAAAAAAAAAA UUUUUUUUUUUUUUUUUUUUUU RRRRRRRR oooo DDDDDDDDDDDDDDDDDDDDDD = = ± mmmmmmmm/dddd 3 min (tttt 3 ss. ff. ) This was completed for all other intensities of UV light, and results placed in the processed data table. Processed Data Processed Data Table Shown below is a data table displaying the numerous values associated with particular intensities of UV light, as well as the uncertainties associated with each value: Intensity of UV Light (W/m 2 ) Rate of Degradation (mm/dm 3 min) 637 ± *10-4 ± 2.06* ± *10-4 ± 1.97* ± *10-4 ± 1.95* ± *10-4 ± 1.63* ± *10-4 ± 1.23*10-5 Processed Data Graph The above data with error bars was plotted into the graph below (to aid in the visual interpretation of the results, the sign of the rate of degradation was inverted to give the rate of reaction). A smooth trendline was added. Rate * 10-3 (mols/dm³ min) Rate of Red 40 Dye degradation against UV light Intensity Intensity (W/m²) Conclusion and Analysis From the processed tables and graph, it can be seen that there does seem to be an exponential decrease in the rate of reaction (a decrease as the processed data graph was put through an absolute function). However, an interesting point to note is that the rate of degradation does seem to plateau out after a certain intensity, which seems to suggest that the energy the photo catalyst can hold is limited, and that after it has absorbed a certain quantity of energy, it cannot absorb any more. It was also seen in the control experiments that there were only very slight changes in the absorbance, which implies that it was the combination of both zinc oxide and UV light that caused photodegradation to occur, and that it doesn t occur without them present.

11 Page 11 Overall, the uncertainties were significant enough to be noticed (above 10%). It is also notable that the major source of percentage uncertainty in the quantities was the measurement of the path length when determining the concentration of the solution, thus this is one aspect of the methodology that could be improved later. However, the uncertainties in the intensity were small, and could not be noticed on the final graph. No literature values pertaining to the rate was found, however the trendline shown in the processed data graph does hold true to other graphs determined with how the intensity of UV light affects the rate of photodegradation, although other experimentation utilised other dyes, but similar trends were still seen: Figure 2 Example graph of the photodegradation of Methylene Blue and the effect of intensity (ABHILASHA, ASHMA and MARAZBAN, 2016) Considering the strengths of the experiment, it did negate a number of other factors that could affect the photo degradation of the dye, such as the beaker being kept in a dark shoebox, to negate any effects of the light in the classroom. The magnetic stirrer also negated the chance that the ZnO would just settle at the bottom and not cause any photodegradation at all. Finally, the control experiments did help to narrow down the cause of the photodegradation to the combination of UV light and ZnO, showing that they did not act independently of each other in the chemical system of photodegradation. In order to further this investigation, it would be interesting to test the effect of catalyst loading on the rate of degradation, and compare it to how the rate of degradation changes with UV light intensity, seeing if there is also a maximum capacity that the chemical system can have for the catalyst before the rate is not increased anymore. Finally, as seen in the graph, there were large gaps between the rates in earlier readings, as well as an apparent plateau towards later readings, which would be interesting to investigate, and confirm the pattern shown in the graph. Limitations and Improvements Limitation Significance Improvement Slight changes in the initial concentration of dye, noted by variable initial absorbance. This would be quite significant as very small concentrations were used, thus any changes (even minor ones) would cause a visible effect to the rate of degradation. added. Exposure to light when samples were being removed. Quite insignificant, but the causes of other light sources are not known to that great an extent. Ensure initial absorbance of the solutions are constant, or within a very minute range from each other, instead of using the volume of dye Ensure experiment is carried out in a dark room.

12 Page 12 Limitation Significance Improvement Battery Power of the UV light, the batteries had to be changed once between repeats Quite significant, as new batteries are likely to cause a much higher intensity of UV light present to the photo catalyst, thus increasing the rate of reaction by a lot. Change batteries before every repeat, or ensure the batteries do not need replacement during repeats. The UV light would not have been dispersed as a sphere Initial rate was assumed to be equal to the overall rate. Bibliography and References In calculations, it was assumed that the light intensity would be dispersed in a sphere-like pattern, which would not be the case in reality, as the lamp was rectangular in shape. The rate may change by some degree overall, however the significance is quite minor, as the absorption had already reached an almost negligible level towards the end of the experiments. It would be much more accurate to use a point source of UV light, or a spherical lamp, which would make the assumption of a spherical distribution more accurate. Intensity could also be measured using a light meter. Wait until the absorption reaches zero, and remains at zero for a decent length of time, which would enable the overall rate to be measured. (Increase range of timings) ABHILASHA, J., ASHMA, A. and MARAZBAN, K. (2016). A GREENER APPROACH FOR THE DEGRADATION OF DYE METHYLENE BLUE BY ORGANIC ADDITIVE CATALYSED PHOTO - FENTON PROCESS. [online] Available at: Illustrated glossary of organic chemistry - beer s law (Beer-Lambert law) (no date) Available at: Lam, L. (2016). PHOTOCATALYTIC DEGRADATION OF SUNSET YELLOW DYE OVER ZINC OXIDE NANOPARTICLES UNDER FLUORESCENT LIGHT IRRADIATION. [online] Available at: Pubchem.ncbi.nlm.nih.gov. (2017). Allura red AC dye C18H14N2Na2O8S2 - PubChem. [online] Available at: Science Lab. (2017). Material Safety Data Sheet - Zinc Oxide. [online] Available at: The Clorox Company. (2012). Safety Data Sheet - Clorox Splash-less Regular Bleach. [online] Available at: