The Indirect Iodimetric Analysis Ascorbic Acid free essay sample

I: Purpose: To determine the percent weight of Ascorbic Acid in unknown sample. This was to be done by the means of an indirect iodimetric analysis. In an iodometric analysis, the oxidizing agent can be determined by a different means than titrating directly with standard iodide, because a high concentration of I- is needed to form the I3- complex. In this type of analysis, excess iodide is added to the oxidizing agent, and the triiodine is titrated with stand thiosulfate. This indirect analysis finds the number of moles of ascorbic acid based on the known number of moles of IO3- and subtracting half the amount of moles of the thiosulfate solution. II: Equations: Iodate with Iodide: IO3- + 8I- + 6H+ ? 3I3- + 3H2O Thiosulfate with Triiodide 2S2O32- + I3- ? S4O62- + 3I- Ascorbic Acid with Triiodide C6H8O6 + I3- +2H2O ? C6H6O6 +3I- + 2H3O+ III: Sample Calculations: A) Concentration Iodate: 1. We will write a custom essay sample on The Indirect Iodimetric Analysis Ascorbic Acid or any similar topic specifically for you Do Not WasteYour Time HIRE WRITER Only 13.90 / page 9502g KIO3 * * = 0. 01823 M KIO3 B) Concentration Thiosulfate (S2O32-) 0. 0250481 L KIO3 * = 0. 00045663 mole KIO3 0. 00045663 moles IO3- * * = 0. 0027366 M C) First and Second Derivative V(mL) E(mV) ?V(mL) ?E ?E/? V ml ? ?(?  E/V) ?(? E/? V)/? Vav 14. 536 265 0. 021 -181. 25 -8629. 05 14. 5305 0. 01 -4 -400 14. 541 14. 546 261 0. 011 -100 -9090. 9 14. 5465 0. 012 -6 -500 14. 552 14. 558 255 0. 047 500 10638 14. 5555 First Derivative: On the y-axis plot ? E/? V (column 5) and on the x-axis plot ml (column 6). The slope is steepest at the end point. Second Derivative: On the y-axis plot ? (? E/? V)/? (column 9). And on the x-axis plot Vav (column 10), where the graph crosses zero is the end point. D) % Weight Ascorbic Acid (fully automated) -Moles Ascorbic Acid = moles I3- ? (moles S2O32-) -Mols IO32- = 0. 025048 L IO3- * * = 0. 001370 mols I3- -Mols S2042- 0.  014872L S2032- *0. 07151 M S2O42- = 0. 00106 mols S2O42- -Mols Ascorbic Acid = 0. 001370 moles I3- ? (1. 00106 moles S 2O62-)= 0. 000838 moles Ascorbic Acid 0. 000838 moles Ascorbic Acid * = 0. 1476 g AA % weight = *100 = 50. 18% IV: Tables and Graphs: See attached for first and second derivative and chart used to obtain first and second derivative. Semi-Manual Titration mL mV mL mV mL mV 0 3040 14. 342 287 14. 632 164 2. 030 339 14. 390 284 14. 642 156 4. 026 336 14. 484 275 14. 660 152 5. 970 334 14. 494 274 14. 676 147 8. 010 330 14. 504 272 14. 692 144 10. 004 326 14. 536 265 14. 706 140 12. 026 318 14. 546 261 14. 736 137 12. 650  314 14. 560 255 14. 764 133 13. 186 310 14. 570 247 14. 796 131 13. 428 308 14. 578 241 14. 816 128 13. 912 301 14. 580 236 14. 864 126 14. 106 296 14. 600 193 14. 930 123 14. 154 295 14. 612 174 15. 054 120 14. 296 289 14. 622 168 15. 868 112 V. Conclusion: After completing three separate methods to determine the percent weight of ascorbic acid in a sample, in the manual titration the percent weight was 51. 45%. The semi-manual method gave a percent weight of 51. 40%, and the automatic titration gave a percent weight of 50. 18%. Averaged together, giving a percent weight of 51. 01% and a standard deviation of 0. 719 and an RSD of 14 ppt. A possible systematic error that could have occurred would have been while doing the manual titration to not wipe the tip of your 25 mL pipet. This would have caused the calibrated amount to not have been correct and you would have added a falsely high amount of potassium iodate. This would have given a value of moles I3- higher than what was actually given. This is turn would create a larger value for the moles of ascorbic acid and you would get a higher percent weight. Another possible systematic error would be if the automatic titrator was improperly calibrated and it caused the volume dispensed to be larger than what was shown. This would have given an underestimation of the end point. For example, if the machine showed the it had reached the end point at 15. 00 mL but in realty the machine had dispersed 15. 02 mL. , which would lead to a number of moles of S2O32- that are lower than what was actually required, which would in turn give a lower percent weight of ascorbic acid and be falsely low. VI: Questions: 1. Explain the difference between iodimetric and idometric methods of analysis. Why is this experiment classified as an indirect iodimetric method? Iodimetric uses reducing agents to be determined by direct titration with standard I3-. Examples are As(III), sulfide, cyanide etc. , which contain oxidizable functional groups. Whereas idometric is used for the determination of oxidizing agents. Excess iodide is added to the oxidizing agent and the triiodide that’s produced is titrated with thiosulfate. This experiment is classified as an indirect iodimetric method because since we are measuring content of ascorbic acid through the known moles of thiosulfate and I3-, the method becomes indirect but it is also idoimetric because we are determining an oxidizing agent. 2. What is the function of the KIO3 solution? Explain how the preparation of this standard solution differs from the preparation of the standard NaOH solution used in the Soda Ash experiment. Iodate was used because in the idometric analysis, a high concentration of I- is needed, so excess iodine is added to the iodate to form the triiodine which is then titrated with thiosulfate. This differs from the soda ash experiment because in the soda ash experiment a primary standard (KHP) was used. 3. Use a balanced net ionic equation to explain the purpose of the KI in this experiment. Why must KI be present in excess? IO3- + 8I- + 6H+ ?  3I3- + 3H2O KI is in excess because a high concentration of I- is needed to form the triiodine which then allows the method to proceed with idometric analysis, where the I3- can be titrated with thiosulfate. 4. What is the function of thiosulfate? Write the balanced net ionic equation for the reaction of thiosulfate and triiodide. What pH range is required for this equation to hold? 2S2O32- + I3- ? S4O62- + 3I-. S2O42- is used for titration after it has been standardized which can be determined by the equation moles of ascorbic acid = moles I3- ? (moles thiosulfate). This complex is oxidized to S4O62- as log as the pH is below 9. 5. Describe the preparation and standardization of the thiosulfate solution including all special procedures. The solution is made from Na2S2O3 5H2O, but since it is not a primary standard, it must be standardized. The solution has the potential to be oxidized by atmospheric oxygen, so deionized water which is bubbled with nitrogen prior to the solution preparation. Adding a few drops of chloroform prevents bacteria which can cause decomposition. 17 g of Na2S2O3 5H2Om was added and filled with water that has been bubbled with nitrogen and 0. 05g Na2CO3. The solution is then mixed and the chloroform was added. Three spate flasks were used for standardization. Each flask contained excess solid KI. For the titration, when the color turned pale yellow, the starch indicator was added and the titration was complete when the blue color disappeared. 6. What is the indicator for titrations involving triioidine ion? Why is the indicator added just prior to the end point? When the yellow color turns pale, the helical amylase component of soluble starch is added. This indicator must be added after most of the triiodine has reacted because in large concentrations, the stable complex remains even after the end point is passed. 7. Give the common name and molecular formula for ascorbic acid. Write the balanced net ionic equation for the reaction of this compound with triiodide ion. The common name is Vitamin C and it has a molecular formula of C6H8O6. C6H8O6 + I3- +2H2O ? C6H6O6 +3I- + 2H3O+ 8. Explain what is meant by potentiometric end point detection. Describe data analyses by first and second derivative plots. Potentiometric end point detection, the end point is determined graphically and titrant is added in increments and the potential is measured. The first derivative is the change in voltage divided by the change in the titrant volume on the y-axis and it is plotted against the titrant volume midway between the two points. Where the slope is the steepest is the end point. The second derivative is the change in the first derivative divided by the change in the titrant volume midway between two points, on the y-axis and the average value of the midway titrant volume on the x-axis. The end point is where the graph crosses zero. 9. Explain the difference between an indicator electrode and a reference electrode. What items serve these functions in this experiment. An indicator electrode responds to changes in the activity of one of the ions in the solution, since it is impossible to measure a single electrode, a reference electrode is needed. The reference electrode remains constant, any changes are due to the indicator changes. 10. A possible source of systematic error in this experiment is partial decomposition of the thiosulfate solution. Suppose that decomposition occurred after the thiosulfate was standardized, but before it was used for analysis of ascorbic acid. Would the wt% ascorbic acid be falsely high, falsely low, or unaffected? Give all the reasoning to justify your answer. If the thiosulfate had decomposed after it had been standardized, it would have required more milliters of solution to titrate and obtain the same amount of moles since the concentration had been decreased. With more titrant being added, you are going to get a number of moles that is greater. Since to obtain the moles of ascorbic acid, you subtract the moles of the thiosulfate (divided by 2) from the moles of the IO3- you are subtracting a larger number and you are going to obtain a smaller number for the moles of ascorbic acid, thus the %wt would be falsely low.