Analysis of Vitamin C

Transcription

1 Analysis of Vitamin C In today's experiment, you will be titrating samples that contain vitamin C with potassium iodate, KIO 3. The KIO 3 is added to the reaction mixture using a buret while stirring constantly with a magnetic stir plate. As KIO 3 is added to the reaction container, it reacts with loose I - (aq) ions to form I 2(aq). The I 2(aq) molecules are then reduced by vitamin C ( a strong reducing agent) to reform I - (aq). Thus, I 2(aq) never builds up to appreciable levels because of the vitamin C that is present. However, at some point, all of the vitamin C is consumed and the concentration of I 2(aq) increases. The end of the titration is signaled by the build up of I 2(aq) and its reaction with I - to form I 3 -. This in turn reacts with starch to turn the solution blue. This point, when the solution turns faintly blue, is known as the endpoint of the titration. Sequential reactions - a. IO 3 + 5I - + 6H + 3I H 2 O (buret) KI HCl b. C 6 O 6 H 8 +I 2 C 6 O 6 H H + + 2I - vitamin c c. I 2 +I - I 3 - (blue in presence of starch)

2 Sequential calculations: 1. Use iodate volume (experimentally determined) and molarity information to calculate moles of iodate. 2. Use reaction "a" and the appropriate mole ratio to determine the number of moles of I 2 produced. 3. Use the moles of I 2 from reaction "a" in reaction "b" to calculate the number of moles of vitamin C consumed. 4. Convert moles of vitamin C into grams and mg of vitamin C. Procedure Buret operation suggestions: Read the buret from top to bottom (reverse of a graduated cylinder) Rinse the buret twice with small amounts (2-3 ml) of the liquid to be dispensed before beginning the experiment. Buret measurements have 2 decimal places (e.g ml) A piece of paper positioned in back of the buret can make it easier to see the meniscus and make the measurement. Flush out bubbles in the tip of the buret before beginning the experiment (see figure at right). Do this by quickly turning on and off the valve until all bubbles have passed through. A bubble that comes out during the experiment will adversely affect your results! Before filling the burret (use a funnel) be sure the valve is in the off position. Never allow the level of the liquid to drop below the lowest 25 ml mark ("?" on figure). Since there is no scale in this region, there is no way to determine how much liquid was dispensed. Remove the filling funnel before beginning the titration. The funnel can affect the initial measurement in some instances. Standardization: In this part of the experiment you will experimentally determine the amount of vitamin C in a vitamin C tablet 1. Rinse (twice) a buret with 2-3 ml portions of potassium iodate solution. 2. Fill the buret and eliminate air bubbles that may have lodged in the valve assembly by draining bursts of liquid into a waste beaker. When the bubbles are no longer present, refill the buret as needed. 3. Obtain a 100 mg vitamin C tablet. Weigh the tablet and record its mass.

3 4. Place the vitamin C table in a 250 ml Erlenmeyer flask. Add approximately 50 ml of distilled water to the flask. 5. Use a glass stirring rod to break up the tablet and stir the mixture for several minutes. Does all the solid dissolve? Why or why not? 6. Add approximately 1g KI and continue stirring until dissolved. Stir. 7. Add approximately 5mL of 1M HCl. Stir. 8. Add approximately 2-3 ml of 0.5% starch solution. Stir. 9. Set up the apparatus as above. 10. Add the stir bar (available at front lab counter) to the Erlenmeyer flask and position the flask on the stir plate. Start the magnetic stirrer being careful to leave the heat turned off. When properly adjusted, the liquid should be thoroughly stirred with no splashing. 11. Record the initial buret reading. 12. Slowly add (titrate) the M KIO 3. As you get close to the endpoint of the titration, a blue color burst will be observed to fade away. Slowly add KIO 3 until a faint blue color persists. 13. Record the final buret reading. 14. Subtract the initial and final buret readings to determine the dispensed volume of KIO 3. Record this number in your notebook and on the blackboard in the lab. 15. Repeat the above procedure a second time using another 100mg vitamin C tablet. 16. From your results, calculate the experimentally determined mass of vitamin C contained in each tablet. Record your result both in your notebook and on the black board. Finish these calculations and compare with a friend BEFORE continuing with the fruit juice part of the experiment!!! Titration of an unknown fruit juice.

4 1. Measure out and pour 50.0 ml of one fruit juice into a clean 250 ml Erlenmeyer flask. i. Record the exact amount of juice you dispensed in your notebook. ii. Record the type and brand name of juice in your lab notebook. iii. Locate the nutrition information on the carton and record the % daily vitamin C intake information for the juice sample. 2. Add approximately 1g KI and continue stirring until dissolved. Stir. 3. Add approximately 5mL of 1M HCl. Stir. 4. Add approximately 2-3 ml of 0.5% starch solution. Stir. 5. Titrate the mixture as you did above with KIO 3. Note: The expected color change will be affected/disguised by the original coloration of the juice. 6. Repeat the titration a second time if possible.

5 IV. Report Standardization: For each vitamin C tablet titrated report: i. molarity of KIO 3 ii. mass of tablet iii. initial buret reading (titration) iv. final buret reading v. ml of KIO 3 dispensed vi. moles of IO 3 - (calculate knowing the volume and molarity) vii. moles of ascorbic acid (use mole ratio from equations on pg 1) viii. milligrams of ascorbic acid (use molar mass for vitamin C) Calculation: Show the calculations you used to determine the mass of ascorbic acid for one tablet beginning with the original buret measurements. (Handwritten is okay) Juice trial(s): For each juice sample titrated report: i. molarity of KIO 3 ii. ml of original juice titrated iii. initial buret reading iv. final buret reading v. amount of KIO 3 dispensed vi. moles of IO 3 - vii. moles of ascorbic acid viii. milligrams of ascorbic acid Calculations: i. Show the calculations you used to determine the mass of ascorbic acid for one juice trial beginning with the original buret measurements. (Handwritten is okay) ii. Divide the mg of vitamin C you determined from part i by the initial volume of juice (approx. 50mL.use your numbers). The result of this calculation is the number of mg vitamin C per ml of original juice sample (mg Vit.C /ml Juice ) for each juice sample titrated. iii. Convert the results in step ii into mg/cup of juice. (convert ml to cups don t use the conversion factor on the side of OJ carton it isn t correct). iv. Calculate the percent of the recommended daily vitamin C intake requirement is met by one cup of juice. (The USDA recommends the average adult consume 60 mg of vitamin C daily.) How does this compare to the percentage stated on the juice carton? (calculate the Δ% comparing the stated value to your exp. value)

6