Chemistry 12. Determining the Amount of Vitamin C (Ascorbic Acid) in a Sample

Transcription

1 Chemistry 12 Determining the Amount of Vitamin C (Ascorbic Acid) in a Sample Abstract Redox reactions are involved in a wide variety of techniques for quantitative analysis of chemical substances. The methods for calculating the amounts of substances involved in these reactions (expressed either as mass in grams or as the volume of a solution of known molarity) will have been introduced to in your studies of stoichiometry. The balanced equations for many of these redox reactions are complex and must be obtained either by adding balanced half-reaction equations or by using the oxidation number method. A substance that is often used in quantitative redox reactions is iodine, I 2. It is easily formed by oxidizing I -, but the reaction can be reversed just as easily, so a wide variety of reactions can involve I 2 as a reactant or product. In addition, it gives a characteristic deep blue color with starch solution even when it is in very low concentrations, making it easy to determine when the I 2 is all used up or when it begins to form. Starch is therefore used as the indicator in these reactions. In Part I of this experiment, you will prepare a solution of potassium iodate (KIO 3 ) of known concentration. This solution makes a good primary standard in redox titrations, since it is stable and can be obtained very pure. In Part II, known volumes of this solution will have excess H + and I - added and a reaction will occur in which I 2 is produced in a quantity determined by the moles of KIO 3 present initially. A solution of sodium thiosulfate, Na 2 S 2 O 3, of unknown concentration will then be titrated into the solution, which will react with the I 2. Starch will be added when most of the I 2 has reacted with S 2 O 3 2- and the titration will be continued until the blue color disappears. The concentration of the NaS 2 O 3 solution can now be calculated. This solution can be used to determine the amount of I 2 in other solutions. In Part III, the amount of vitamin C in a sample will be determined by analyzing it with a standard I 2 solution. Purpose 1. To prepare a solution of potassium iodate, KIO 3, of known molarity. 2. To standardize a solution of sodium thiosulphate, Na 2 S 2 O 3, by titrating it against standard KIO 3 (with excess I -, H + ). 3. To use a standard I 2 solution to determine the mass of ascorbic acid (vitamin C) in a vitamin tablet. Chemistry

2 Materials Apparatus 2 x 250 ml beaker 4 x 100 ml beaker Scale 25 ml pipet Pipet bulb Wash bottle 250 ml Erlenmeyer flask Mortar and pestle Funnel 10 ml graduated cylinder 250 ml volumetric flask 1 L Nalgene reagent bottle Stirring rod Buret Teflon coated magnet Magnetic stirrer Reagents Potassium iodate (KIO 3(s) ) 0.12 M Sodium thiosulphate (Na 2 S 2 O 3(aq) ) 1.0 M Potassium iodide (KI (aq) ) 1.0 M Sulfuric acid (H 2 SO 4(aq) ) Starch solution Procedure Part 1 Preparing a Standard Potassium Iodate Solution 1. Before coming to the laboratory, calculate the mass of KIO 3 required to make up ml of a M solution of KIO Put on your lab apron and safety goggles. 3. Accurately measure the mass of a clean, dry ml beaker and record it in your copy of Table 1 in your notebook. 4. Place solid KIO 3 in the beaker until you have approximately the mass calculated in step 1. Record the actual mass accurately in Table Dissolve the KIO 3 in water and pour the solution through a funnel into a ml volumetric flask. Rinse the beaker three times and add the rinsings to the flask. 6. Add water until the level of the solution is up to the mark, stopper the flask, and invert and swirl thoroughly to make the solution homogeneous. Part 2 Standardizing a Sodium Thiosulphate Solution 1. Transfer the standard KIO 3 solution to a clean, dry ml beaker. 2. Obtain in three ml beakers approximately 50.0 ml each of 1.0 M KI, 1.0 M H 2 SO 4, and starch solution. Label each beaker

3 3. Using a suction bulb on your pipet, withdraw ml of the KIO 3 solution and transfer it to a ml Erlenmeyer flask, touching the tip to the side of the flask to ensure that the correct volume is delivered. 4. Add to the flask containing the KIO 3 solution approximately 5 ml of 1.0 M KI and 5 ml of 1.0 M H 2 SO 4. These are excess amounts, so precise measurement is not required. Use a 10 ml graduated cylinder for adding the KI and H 2 SO 4. You will notice that a brown precipitate is formed at first and that it dissolves again to give a clear brown solution. This behavior is characteristic of iodine, I 2. The amount of precipitate formed is determined by the moles of KIO 3 originally present. The iodine will now be made to react with the sodium thiosulphate solution, Na 2 S 2 O 3, in order to determine the molarity of the Na 2 S 2 O Fill a 1 L nalgene reagent bottle about half full with the 0.12 M Na 2 S 2 O 3 solution provided and label it with your name. Use solution from this bottle for all subsequent parts of the experiment requiring Na 2 S 2 O Add about 10 ml of this solution the buret, rinse, and discard. Fill the buret with Na 2 S 2 O 3, then open the valve briefly to allow some to drain through the tip. 7. Determine the initial buret reading and record it in Table Run the Na 2 S 2 O 3 solution into the flask until the brown colour has faded to a light yellow. Add approximately 5 ml of starch solution and continue adding Na 2 S 2 O 3 drop by drop until the blue/black colour disappears. Note that the endpoint is a very precise one and that it is therefore important not to add the starch too soon. Take the reading on the buret and record it in Table Repeat steps 3 to 8 until you obtain consistent results (titrations agree within 0.10 ml). You now have enough data to calculate the molarity of the Na 2 S 2 O 3 solution. Part 3 Determining the Amount of Vitamin C (Ascorbic Acid) in a Sample 1. Obtain in a ml beaker about ml of the of the solution of iodine (in Potassium iodine, I 2 /KI) provided. You will need to standardize it by completing steps 2 to 5 before doing the rest of Part III. 2. Using a suction bulb, withdraw ml of the iodine solution into a pipet and transfer it to a 250 ml Erlenmeyer flask. 3. Refill the buret with the standardized Na 2 S 2 O 3 solution, take the initial reading, and record it in Table Run the Na 2 S 2 O 3 solution into the flask until the brown colour of the I 2 fades to pale yellow, add 5 ml of starch solution, and continue the titration dropwise until the dark blue colour just disappears. Again, take the final reading on the buret and record it in Table Repeat steps 2 to 4 until consistent results are obtained (titrations agree within 0.10 ml). You now have enough data to calculate the molarity of the I 2 solution when you later perform your analysis of results

4 6. Discard the Na 2 S 2 O 3 solution remaining in the buret, wash the buret with water, then rinse it with the I 2 solution. Discard this, then fill the buret again with I 2, and drain some to refill the tip. 7. Obtain a 250 mg (or 300 mg) tablet of vitamin C (ascorbic acid), grind it with a mortar and pestle, and place the powder in a 250 ml Erlenmeyer flask. Add 5 ml of starch solution. 8. Take the initial reading of the I 2 solution in the buret and record it in Table 4. Then allow it to run into the flask. Swirl constantly, watching for the first appearance of the characteristic dark blue colour, which stays even after swirling. Again, take the final reading on the buret and record it in Table Repeat steps 7 and 8 until consistent results are obtained (titrations agree within 0.10 ml). 10. Clean all glassware and return to their original locations. 11. Before leaving the laboratory, wash your hands thoroughly with soap and water. Observations Part 1 Preparing a Standard Potassium Iodate Solution Table 1: Preparing a Standard KIO 3 Solution Calculated mass of Potassium iodate, KIO 3, required for ml of M solution (g). Mass of beaker (g) Mass of beaker + KIO 3 (g) Mass of KIO 3 (g) Part 2 Standardizing a Sodium Thiosulphate Solution Table 2: Volume of Na 2 S 2 O 3 Needed to React with Iodine from 25 ml of KIO 3 Initial reading of Buret Final reading of Buret Volume of Na 2 S 2 O 3 required Average volume of Na 2 S 2 O 4 Trial 1 Trial 2 Trial 3 Trial 4 (if necessary) - 4 -

5 Part 3 Determining the Amount of Vitamin C (Ascorbic Acid) in a Sample Table 3: Volume of Na 2 S 2 O 3 Needed to React with 25 ml of I 2 Solution Initial reading of Buret Final reading of Buret Volume of Na 2 S 2 O 3 required Average volume of Na 2 S 2 O 4 Trial 1 Trial 2 Trial 3 Trial 4 (if necessary) Table 4: Volume of I 2 Needed to React with Ascorbic Acid (Vitamin C) Mass of Vitamin C in Tablet = mg Initial reading of Buret Final reading of Buret Volume of I 2 required Average volume of I 2 Trial 1 Trial 2 Trial 3 Trial 4 (if necessary) Post Lab Considerations In order to calculate the results for this experiment, you will need to work out the balanced overall redox equation for each reaction that is occurring, then use the mole relationships in the equation to relate reacting quantities The balanced half-reactions occurring in this experiment are as follows: Production of I 2 from KIO 3, KI, H 2 SO 4 : 2 IO H e - I H 2 O 2 I - I 2 + 2e - Standardization of Na 2 S 2 O 3: 2 S 2 O 3 2- S 4 O e - You need not show an equation for the reaction of vitamin C (ascorbic acid, C 6 H 8 O 6 ) with I 2. They react in a 1:1 mole ratio. The deep blue/black color formed by starch and iodine is very useful in a variety of situations. Its formation is not really a chemical reaction. The iodine molecules just happen to fit closely inside the long spiraling starch molecule and interact strongly with - 5 -

6 it. The combination is unstable above 50 C. It is interesting to note that here you are using starch to test for the presence of iodine, but in food chemistry the reverse occurs, iodine solution is used to test for the presence of starch. You may have performed such a test in an earlier science course. Note that iodine is only slightly soluble in water, but does dissolve well if I - ions are present. The reaction that occurs is I 2 + I - I 3 -. However, the complex I 3 - breaks down very readily when any chemical is present that can react with I 2 (such as S 2 O 3 - ) and the solution therefore acts as though it were a solution of I 2. Questions Part 1 Preparing a Standard Potassium Iodate Solution 1. Calculate the concentration of KIO 3 solution formed when you dissolved your calculated mass of KIO 3 in water and made the volume up to ml. 2. Calculate the number of moles of KIO 3, and therefore of IO 3 -, in ml of this solution. Part 2 Standardizing a Sodium Thiosulphate Solution 1. Work out the overall redox equation for the reaction of IO 3 -, I -, and H + to give I Work out the overall redox equation for the reaction of S 2 O 3 2- and I 2 to give S 4 O 6 2- (tetrathionate ion) and I State the relationship between moles of IO 3 - and moles of S 2 O 3 2- working through moles of I From this relationship and the number of moles of KIO 3 in ml of solution, calculate the number of moles of S 2 O 3 2- with which the KIO 3 reacts. 5. Knowing the average volume and the number of moles of S 2 O 3 2- used, calculate the [S 2 O 3 2- ]. Part 3 Determining the Amount of Vitamin C (Ascorbic Acid) in a Sample 1. Calculate the number of moles of S 2 O 3 2- used from the average volume used and the molarity from Part II. 2. Calculate the number of moles of I 2 present, using the relationship obtained in Part II. 3. Calculate [I 2 ] from the number of moles used and the volume in litres. 4. From the average volume of I 2 solution used in the vitamin C titration, calculate the number of moles of I 2 required to react with the vitamin C

7 5. Recalling that it was stated in the Post Lab Considerations that vitamin C and I 2 react in a 1:1 mole ratio and that vitamin C (ascorbic acid) has the formula C 6 H 8 O 6, calculate the mass in milligrams of vitamin C present. 6. Compare your result with the rated amount of vitamin C for that tablet and calculate the percentage deviation between your result and the rated amount. Conclusions State the mass of vitamin C present in the tablet and the percentage deviation from the given value on the bottle