EQUIPMENT NEEDED o buret and stand 100-ml volumetric flask 5- and 10-ml pipets @ 10- and 100-ml measuring cylinders © 25
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EQUIPMENT NEEDED o buret and stand 100-ml volumetric flask 5- and 10-ml pipets @ 10- and 100-ml measuring cylinders © 250-ml conical flasks SOLUTIONS NEEDED You prepare the first two: 1. Use stock solution (A, B, or C): You will pipet 5 mL into titrate with the provided iodine solution 2. Method sample preparation for vitamin C tablets: Crush up a single tablet (500 mg) in -50 mL of distilled water until completely dissolved, then transfer the entire solution to a 100-ml volumetric flask and fill to the mark. This is your stock solution, from which you pipet 5 mL to titrate with the provided iodine solution STOCKROOM PROVIDED SOLUTIONS o lodine solution: (0.005 mol L ") o Starch indicator solution: (0.5%) TITRATION 1. Pipet a 5-mL aliquot of the stock unknown solution into a 250-ml conical flask and add about 50 mL of distilled water and 1 mL of starch indicator solution 2. Titrate the sample with 0.005 mol L' iodine solution. The endpoint of the titration is identified as the first permanent trace of a dark blue-black color due to the starch-iodine complex 3. Repeat the titration with further aliquots of sample solution until you obtain concordant results (titres agreeing within 0.1 mL). Perform Steps 1, 2 and 3 for unknown vitamin C sample (A, B, or provided from the stockroom) Perform Steps 1, 2 and 3 for unknown commercial vitamin C pill.
CALCULATIONS 1. Calculate the average volume of iodine solution used from your concordant titres. 2. Calculate the moles of iodine reacting 3. Using the equation of the titration (below) determine the number of moles of ascorbic acid reacting ascorbic acid + l, -21 + dehydroascorbic acid + 2H 4. Calculate the concentration in mol L of ascorbic acid in the solution obtained from unknown stock solution and the pill. Also, calculate the concentration, mg/1g of ascorbic acid, and percent (by mass) in the sample of pill. ADDITIONAL NOTES 1. lodine stains both skin and clothing so proper care is advised. If staining does occur, alcohol may remove skin stains and cleaners are available for fabric stains. 2. Vitamin C, or ascorbic acid, is a water soluble antioxidant that plays a vital role in protecting the body from infection and disease. It is not synthesized by the human body and therefore must be acquired from dietary sources- primarily fruits and vegetables. The chemical structure and antioxidant (re- ducing) action of ascorbic acid are illustrated in the redox half equation below. 3. The concentration of the prepared iodine solution can be more accurately determined by titration with a standard solution of ascorbic acid or a stan- dard solution of potassium thiosulfate using a starch indicator. This should be done if possible as iodine solutions can be unstable. HO HO oxidation НО. НО. +2 H+ + 2e reduction HO OH O o FIGURE 10.2 Redox Reaction of Vitamin C
4. The average titre volume should ideally be in the range of 10-30 ml. If the titre required for a 20-ml. aliquot of sample solution is well outside this range, then a larger or smaller aliquot volume should be chosen. If the volume of the titre is too low, dilute the standard. If the titre volume is too high, dilute the sample. 5. Ascorbic acid is susceptible to oxidation by atmospheric oxygen over time. For this reason, the samples should be prepared immediately before the titrations. However, if the samples have to be prepared several hours earlier, oxidation can be minimized by the addition of a small amount of oxalic acid (e.g., 1 g oxalic acid per 100 mL of sample solution). 6. Identification of the endpoint in this titration is significantly affected by the coloration of the sample solution used. If the solutions are colorless or are pale in color, there is no problem identifying the endpoint. For strongly col- ored juices there can be a problem with the endpoint and it is advised to carry out a rough" titration in order to become familiar with any distinct color change which occurs at the endpoint (it may just be a darkening of the color). This will also help by establishing an approximate volume of io- dine solution required
DETERMINATION OF VITAMIN C CONCENTRATION REPORT SHEET A. STANDARDIZATION Unknown (A, B, or C): TABLE 10.1 Trial 1 Trial 2 Trial 3 volume pipetted 5mL 5ml 5ml Titration Initial reading Final Kereading 2.5ml 15.7mL B.2m 15.7 ml 28.7mL Dome 28.7ml 41.7 ml 13 mL volume of lodine solution Calculations mols of 12 mols of vitamin C molarity of unknown Average molarity:
B. ANALYSIS OF VITAMIN C TABLET Mass of pill: 0.379 TABLE 10.2 Trial 1 Trial 2 Trial 3 Titration final reading initial reading 16.8ML 17.3 mL 0 0 16.8 17.3 30.5h 11.3 13.1 volume of jodine solution Calculations mols of 12 mols of vitamin C volume of vitamin C molarity of vitamin C grams of vitamin C Determine the mass of vitamin C in your pill: (Experimental mass / initial pill mass) x 100 = %
3. How many grams of (pure) vitamin C is contained in your unknown stock solution (Part A)? Hint: The solution is made in a 1-L bottle.