Lab 4: Volumetric Determination of Vitamin C*
* Adapted from "Chemistry in the Laboratory", J. L. Roberts, Jr., J. L. Hollenberg and J. M. Postma, 4^th ed., Freeman, 1997.

July 17, 2001

Purpose. The goal of this experiment will be to introduce you to volumetric methods of analysis, or titrations, one of the "classical" methods of chemical analysis. In  these procedures, the amount of an analyte species in a mixture is determined by measuring the volume of a solution (the titrant) that is required to completely consume it. Typically a change in the appearance of the solution signals the equivalence point of the reaction has been reached, that is, just enough titrant has been added to completely consume the analyte.
 

Introduction.  In this experiment, you will determine the mass of vitamin C (C₆H₈O₆) in a vitamin pill as well as in a sample of orange juice. You will use a solution of potassium iodate as the titrant; it does not directly react with vitamin C, but instead with iodide ion, which you will dissolve along with your vitamin tablet. These two species react to form iodine, I₂, which reacts with vitamin C nearly as quickly as it is generated. The reactions are:
 

When all of the vitamin C has been consumed, addition of IO₃^- to the solution results in the further generation I₂, which is no longer consumed by reaction with vitamin C. Instead, the iodine reacts with iodide to form the triiodide ion, which rapidly reacts with starch, which you will also have added to your solution, to form a deep blue color in the solution. The development of this color is the endpoint of the reaction and signals the point at which you have completely reacted the vitamin C.
 

By measuring the volume of iodate solution that is necessary to completely consume the vitamin C, you will be able to calculate the number of moles of iodate used, and therefore, by the above stoichiometric factors, the number of moles of vitamin C in your sample. Finally, you will analyze a sample of orange juice by the same procedure.
 
 

Procedure - Part A: Analysis of Vitamin C Tablet

1) Add about 50 mL of deionized water to a 100 mL beaker and place on a hot plate. Heat the water gently and add one vitamin C tablet and continue gently warming until the tablet completely "disintegrates" (the vitamin C will completely dissolve, but some other ingredients of the tablet may be insoluble). Transfer the mixture to a 100 mL volumetric flask and rinse the beaker with several small washings of deionized water, adding the washings to the volumetric flask as well. Fill the volumetric flask to the graduation mark. Mix the solution thoroughly.
 

2) Using a volumetric pipet, transfer 25.0 mL of the vitamin C solution to a 125 mL Erlenmeyer flask. Add 0.2 g solid KI, 1 mL of 1 M HCl (CAUTION!), and about 1 mL of 0.5% starch solution indicator. Record the initial volume of the buret and begin titrating with 0.0250 M KIO₃ and continue adding titrant stepwise until you see a permanent blue color (it may be more of a brown in some cases). Note that you may see a transient color that disappears quickly; if this is the case, you are getting close to the endpoint, but have not reached it yet. At the endpoint, record the final volume of the buret.
 

3) Repeat the titration on two additional 25.0 mL samples of your vitamin tablet solution. Calculate the amount of vitamin C in your tablet based on each of your samples and compute the average value. How does your measurement compare with the expected value?
 

Part B: Analysis of Orange Juice
 

4) Transfer 25 mL of orange juice to a 125 mL Erlenmeyer flask (do not use a pipet as the pulp could clog the tip; use a graduated cylinder instead). Add 0.2 g KI solid KI, 1 mL of 1 M HCl and about 1 mL of the starch indicator solution. Titrate with the 0.0250 M KIO3 solution to the blue endpoint. Repeat two additional times. Based on your result, calculate the amount of vitamin C in one 8-ounce serving of this orange juice.
 
 

average value of vitamin C in tablet: _______________
 
 
 
 

Orange Juice Analysis
 
 
 

	Sample 1	Sample 2	Sample 3
initial KIO3 volume
final KIO3 volume
volume KIO3 used
moles KIO3 used
moles vitamin C
mass vitamin C
mass vitamin C in 8 oz.

average value of vitamin C in 8 ounce serving: _______________