How changing pH affect the rate of decomposition of vitamin C

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How changing pH level affect the rate of decomposition of vitamin C, ascorbic acid. Please give proper reference to my IB student, Eileen if you use her material.

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How changing pH affect the rate of decomposition of vitamin C

  1. 1. 2206-008 Chemistry HLCandidate Name : Cham, Eileen Yee LinCandidate Number : 2206-008Date of Practical : 14th May 2009 Practical Assessment 24 – Investigating the relationship between pH and oxidation of ascorbic acidResearch QuestionHow does the pH value of a solution containing ascorbic acid (manipulated by adding different volumesof concentrated hydrochloric acid to the solution) affect the change in amount of the ascorbic acid in thesolution (which is determined using iodometric titration) due to oxidation by atmospheric oxygen?IntroductionVitamin C is a powerful anti-oxidant – it hinders the oxidation of other substances by undergoingoxidization itself.In the human body, free radicals exist and may cause cell damage when present in excess. Vitamin C isusually consumed to hinder reducing power of these free radicals; the vitamin C donates an electron(undergoes oxidization) to a free radical but itself does not become a radical. Vitamin C acts as“scavengers” and prevents the free radicals from oxidizing the cells in our body.Experiments with micellar and bilayer membrane models demonstrate that vitamin C is a more powerfulantioxidant at low pH. (Gramlich, Zhang, & Nau, 2002); meaning that vitamin C degrades (or is oxidized)more quickly at a low pH medium.An increased acidity (lower pH) in our extracellular fluids, especially blood, is detrimental to our health,as the vitamin C, intended for oxidation by the free radicals, is oxidized rapidly by the abundant H+ ionsinstead. Cell death, or even cancer, may arise due to the free radicals.In this experiment, ascorbic acid solution is manipulated into solutions of different pH (using differentvolumes of concentrated hydrochloric acid) and then subjected to iodometric titration to determine itsconcentrations at different pH values. A lower concentration than the original concentration at the normalascorbic acid pH value indicates there is degradation or oxidation.HypothesisH+ ions oxidizes a vitamin C molecule by grabbing one electron from it. The vitamin C molecule isoxidized and becomes dehydroascorbic acid. 1
  2. 2. 2206-008 Chemistry HLAt low pH solutions, the H+ concentrations are high. The lower the pH, the more H+ ions present.As such, the lower the pH value of an ascorbic acid solution, the more H+ ions present to oxidize theascorbic acid, and hence more ascorbic acid degrades to become dehydroascorbic acid.The hypothesis for this experiment is that the concentration of vitamin C solution will decrease with thepH of the solution, if the other factors of oxidation (such as temperature, light intensity, time) are keptconstant.Therefore, the expected graph of this experiment is as follow: Concentration of Vitamin C pH value Original pH of vitamin C Figure 1: Expected graph of concentration of vitamin C solution against pH value of solutionVariablesDependent variable : Concentration of ascorbic acid left in the solution – This can be determined by subjecting the ascorbic acid solution to iodometric titration.Independent variable : pH value of the solution in which the ascorbic acid is dissolved – The pre-prepared ascorbic acid solution is dissolved ascorbic acid powder in distilled water. The pH of the solution is manipulated by pouring in different amounts of concentrated hydrochloric acid, and also distilled water, if necessary. The pH value is determined using a pH meter. – When the acidified ascorbic acid solution has been added with excess potassium iodide powder to be subjected to iodometric titration, the pH value is determined once again as it may change due to reaction with potassium iodide powder. – The pH of the solution can only be manipulated to be lower than its original pH, as ascorbic acid is acidic (with pH value lower than 7).Controlled variables : 1. Temperature 2
  3. 3. 2206-008 Chemistry HL – The different vitamin C solutions are placed in the same area in the room, at room temperature. The area has to be free from heat sources. 2. Exposure to light – The acidified ascorbic acid solutions are placed in the same spot with the same amount of exposure to light, and not a sunny spot. 3. Time for which the ascorbic acid solution is treated with a pH solution – The ascorbic acid solutions has to be left with its acidified solution for a same period of time before subjecting it to iodometric titration. 4. The starting concentration of ascorbic acid solution – A pre-prepared ascorbic acid solution of 0.1 % concentration is used to be treated with different pH mediums. 5. For iodometric titration: Concentration of standard potassium iodate solution, volume of starch solution, concentration of starch solution.Apparatus and materials  0.1 % vitamin C, C6H8O6 solution  0.002 mol dm-3 potassium iodate, KIO3 solution  Starch solution  Potassium iodide, KI2 powder  Concentrated hydrochloric acid, HCl solution  pH meter  (50.00 ± 0.05) cm3 burette  (10.00 ± 0.02) cm3 pipette  250 cm3 conical flask  Pipette filler  Dropper  White tile  Beaker  Retort stand with clamp  Wash bottle  Filter funnel 3
  4. 4. 2206-008 Chemistry HLProcedure 1. 50 cm3 of 0.1 % vitamin C solution is pipetted out 4 times into 4 different beakers. The beakers are labeled with “1”, “2”, “3” and “4”. 2. Vitamin C solutions of different pH values are prepared by pouring in different amounts of concentrated hydrochloric acid into beaker 1, 2 and 3. Distilled water can be added to manipulate the pH value of the solution. The pH values of these acidified vitamin C solutions are recorded. 3. The pH value of the original vitamin C solution in beaker 4 (control) is determined using a pH meter, and is recorded. 4. These beakers containing vitamin C solutions are wrapped with aluminium foil and are placed in the same area in the room that is free from heat sources. 5. A burette is rinsed and is filled with pre-prepared 2.00 × 10-3 mol dm-3 of potassium iodate solution. The burette is clamped with a retort stand, and a white tile is placed under it. 6. 10 cm3 of acidified vitamin C solution is pipette out from beaker 1 into a conical flask. 3 plastic- dropper squirts of starch solution are added into the conical flask, followed by 3 scoops of potassium iodide powder. The end pH of the mixture in the conical flask is again determined using a pH meter and is recorded. 7. The conical flask is placed under the burette. The initial reading on the burette is recorded and the titration process is started. The titration is stopped immediately when the solution in the conical flask turns dark blue. The final reading on the burette is recorded. 8. Steps 6 – 7 is repeated until a consistent amount of titre (difference no more than 0.1 cm3) is obtained. 9. Steps 6 – 8 is repeated with the vitamin C solutions in beaker 2, 3 and 4. The vitamin C from beaker 4 has to be acidifed with dilute hydrochloric acid.Data collection – quantitative data Manipulated pH value End pH value (after having been added with differentBeaker containing 0.1 % vitamin (after having been added with potassium amounts of concentrated hydrochloric C solution iodide and starch solution) acid) (± 0.1) (± 0.1) 1 0.5 0.8 2 1.0 1.5 3 1.8 2.2 4 (control) 3.1 (control) 3.1 (control) Table 1: pH values of vitamin C solutions 4
  5. 5. 2206-008 Chemistry HL End pH value of 0.8 1.5 2.2 3.1 (control) Vitamin C (± 0.1) Titration 1 2 1 2 3 1 2 1 2 run Initial burette reading, Vi 13.20 23.60 22.50 5.20 15.10 4.20 13.3 25.00 0.00 / cm3 (± 0.05 cm3) Final burette reading, Vf 23.60 34.10 32.20 15.10 25.00 13.30 22.50 35.90 10.90 / cm3 (± 0.05 cm3) Total titre, Vpotassium iodide’ 10.4 10.5 9.7 9.9 9.9 9.1 9.2 10.9 10.9 / cm3 (± 0.1 cm3) Table 2: Collected data from titrationsData collection – qualitative dataThe colourless solution in the conical flask turns dark blue at the end of each titration. 5
  6. 6. 2206-008 Chemistry HLData processingThe average titre for each pH value is calculated using the data collected: Average titre, Vpotassium iodate pH value of vitamin C / cm3 (± 0.1) (± 0.1 cm3) 10.4 +10.5 0.8 2 = 10.5 9.7+9.9+9.9 1.5 3 = 9.8 9.1+9.2 2.2 2 = 9.2 10.9+10.9 3.1 (control) 2 = 10.9 Table 3: Calculation of the average titresThe chemical reactions involved in the iodometric titrations are: 1. Between potassium iodate solution, potassium iodide powder and the H+ ions from the acid, to form iodine and water. 1KIO3 + 5KI + 6H+ → 3I2 + 6K+ 3H2O 2. Between iodine and vitamin C, to form iodide ions and dehydroascorbic acid. 3I2 + 3C6H8O6 → 6I- + 3C6H6O6From the above chemical equations, we can deduce that 1 mol of potassium iodate is needed to react with5 mol of potassium iodide to form 3 mol of iodine, which in turn reacts with 3 mol of vitamin C.Therefore, 1 mole of potassium iodate is needed to produce 3 mol of vitamin C.To find the concentration of vitamin c, Mvit c,1 × amount of moles of vitamin C = 3 × amount of moles of KIO3 1 × Mvit c × Vvit c = 3 × Mpotassium iodate × Vpotassium iodate 1 × Mvit c × 10.0 = 3 × 2.00 × 10-3 × Vpotassium iodate 3 × 2.00 × 10 −3 × MVitamin C = 10.00(where Vvitamin C = volume of vitamin C, Mpotassium iodate = concentration of potassium iodate, Vpotassium iodate =average titre for each pH value) 6
  7. 7. 2206-008 Chemistry HL 3 × 0.002 × Using the equation MVitamin C = , the concentration of vitamin C for each pH value 10is calculated: Concentration of vitamin C, Mvit c pH value of Vitamin C 3 × 2.00 × 10 −3 × (± 0.1) = 10.00 / mol dm-3 3 × 0.002 × 10.5 0.8 10 = 6.30 × 10-3 3 × 0.002 × 9.8 1.5 10 = 5.88 × 10-3 3 × 0.002 × 9.2 2.2 10 = 5.52 × 10-3 3 × 0.002 × 10.9 3.1 (control) = 6.54 × 10-3 10 Table 4: Calculation of concentration of Vitamin CThe concentration of vitamin C in its original (control) environment of pH 3.1 is 6.54 × 10-3 mol dm-3.The concentration of vitamin C in a manipulated environment of pH 2.2 is 5.88 × 10-3 mol dm-3.The concentration of vitamin C in a manipulated environment of pH 1.5 is 5.52 × 10-3 mol dm-3.The concentration of vitamin C in a manipulated environment of pH 0.8 is 6.30 × 10-3 mol dm-3. 7
  8. 8. 2206-008 Chemistry HLData presentation Concentration of vitamin C, Mvit c / mol dm-3 against pH values of vitamin C 6.80E-03 6.60E-03 Concentration of vitamin C, Mvit c / mol dm-3 (Control) 1, 0.00654 6.40E-03 0.8, 0.00630 6.20E-03 6.00E-03 1.5, 0.00588 5.80E-03 5.60E-03 2.2, 0.00552 5.40E-03 5.20E-03 0.7 1.2 1.7 2.2 2.7 3.2 pH value Figure 2: Concentration of vitamin C solution , Mvit c / mol dm-3 against pH value of vitamin C 8
  9. 9. 2206-008 Chemistry HLUncertaintiesUncertainty in pH value due to pH meter = 0.1Uncertainty in titre or volume of potassium iodate solution, Vpotassium iodate: Vpotassium iodate Percentage uncertainty in Vpotassium pH value / cm3 iodate / % (± 0.1 cm3) 0.1 0.8 10.5 × 100 = 0.95 10.5 0.1 1.5 9.2 9.2 × 100 = 1.09 0.1 2.2 9.8 9.8 × 100 = 1.02 0.1 3.1 10.9 × 100 = 0.92 10.9 Table 5: Calculation of percentage uncertainty of average titresUncertainty in Vvit c due to pipette = 0.02 cm3 0.02Percentage uncertainty in Vvit C due to pipette = × 100 % = 0.2 % 10.00Uncertainty in Mvit C Percentage Percentage pH uncertainty in uncertainty in Vvit Total percentage uncertainty / % value Vpotassium iodate / % C /% 0.8 0.95 0.2 0.95 + 0.2 = 1.15 1.5 1.09 0.2 1.09 + 0.2 = 1.11 2.2 1.69 0.2 1.69 + 0.2 = 1.89 3.1 0.92 0.2 0.92 + 0.2 = 1.12 Table 6: Calculation of total percentage uncertainty in concentration of vitamin C, Mvit c 9
  10. 10. 2206-008 Chemistry HL Total percentage pH Mvit c Absolute uncertainty in Mvit c -3 uncertainty value / mol dm / mol dm-3 /% 0.8 6.30 × 10-3 1.15 6.30 × 10-3 × 1.15 % = 0.07 × 10-3 1.5 5.88 × 10-3 1.11 5.88 × 10-3 × 1.11 % = 0.07 ×10-3 2.2 5.52 × 10-3 1.89 5.52 × 10-3 × 1.89 % = 0.1 × 10-3 3.1 6.54 × 10-3 1.12 6.54 × 10-3 × 1.12 % = 0.07 × 10-3 Table 7: Calculation of absolute uncertainty in concentration of vitamin CConclusionThe concentration of vitamin C at its original, control pH value, 3,1 is (6.30 ± 0.07) × 10-3 mol dm-3.The concentration of vitamin C at a manipulated pH value of 2.2 is (5.5 ± 0.1) × 10-3 mol dm-3.The concentration of vitamin C at a manipulated pH value of 1,5 is (5.88 ± 0.07) × 10-3 mol dm-3.The concentration of vitamin C at a manipulated pH value of 0.8 is (6.54 ± 0.07) × 10-3 mol dm-3.When plotted in a graph, the relationship between concentration of vitamin C solution and the threemanipulated pH value is shown to be a negative linear regression curve, as the concentration of thevitamin C solutions decreases with decreasing pH values.The hypothesis, which is that the relationship should form a positive linear regression curve, is thus notaccepted.The result is obtained through stoichiometric calculations of the reaction between potassium iodatesolution, excess potassium iodide solids, and vitamin C solution. For each pH value, potassium iodatesolution is titrated against a mixture of vitamin C solution and potassium iodide solids, and the volume ofpotassium iodate solution in the reaction is obtained; then, using the obtained volume, and the chemicalequation of the reaction as a reference, the concentration of the vitamin C solution can be then calculatedfor each pH value.EvaluationThere are some possible explanations to why the results obtained do not concur with the hypothesis. 10
  11. 11. 2206-008 Chemistry HLThough the period of treatment should be fixed for all solutions, the vitamin C solutions were not treatedwith their respective pH mediums for the same period of time, due to time constraints with theexperiment. The vitamin C solution in pH 1.5 solution was left for 2 hours before it is subjected toiodometric titration; the vitamin C solution in pH 2.2 is left on the workbench for more than 20 minutesbefore it is subjected to iodometric titration; while the vitamin C solution in pH 0.8 is immediately titratedonce the it has been treated.Also, the vitamin C solutions were not treated at the same temperature, even though the temperatureshould also be fixed in this experiment. The vitamin C solution at pH 1.5 was put into the refrigerator for2 hours before it is subjected to iodometric titration; while the vitamin C solutions at pH 0.8, 2.2 and 3.1were titrated at room temperature without any refrigeration.Moreover, the oxidation of vitamin C solution might have been accelerated in the process of handling thebeakers containing the solution. Shaking the beaker causes a vigorous flow of oxygen from atmosphereinto the solution and accelerates oxidation of vitamin CThe above three missteps can explain the glaring divergence of the results from the divergence: pH 1.5and pH 2.2 vitamin C solution were exposed to more sources of error (prolonged period of treatment,drastic changes in temperature, and shaking during transfer of beaker from refrigerator to workbench)than pH 0.8 vitamin C solution. Hence they degrade more than as hypothesized, and their data plot pointsin Figure 2 is lower than expected in the hypothesis, causing the results curve to be a negative linearregression curve.The pH of the vitamin C solution changes when added with concentrated hydrochloric acid; this is due tothe common ion effect. It is the shift in equilibrium caused by the addition of a compound having an ionin common with the dissolved substance. The equilibrium of pH value of the solution shifts towards thatof concentrated hydrochloric acid, which is lower than that of vitamin C solution.Only three data plot points are available; this is because the range of pH tested can only be in between pH1 and 3.1. It is difficult to achieve a range of different pH value of the medium within this limited range.Ways of improvementThe period of treatment for each vitamin C solution, and the temperature of treatment, should bestandardized.More pH values of the vitamin C solutions should be used for iodometric titration so that more data plotpoints can be obtained for Figure 2.To minimize percentage uncertainty, a bigger volume of vitamin C solution can be used for theiodometric titration – this can be achieved by using a pipette that can suck a bigger volume such as 25.00cm3, as opposed to the 10.00 cm3 one used. 11
  12. 12. 2206-008 Chemistry HLWorks CitedGramlich, G., Zhang, J., Nau, W. M. (2002, August 31). Increased Antioxidant Reactivity of Vitamin Cat Low pH in Model Membranes - Journal of the American Chemical Society. Retrieved June 15, 2009,from ACS Publications: http://pubs.acs.org/doi/abs/10.1021/ja026927b#ja026927bAF1 12

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