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Tuesday, February 4, 2014

How much Vitamin C is in your fruit juice?



Lab: Determining the Amount of Vitamin C in Fruit Juices

I  have been doing this lab every year for a long, long time.  I always enjoy it, and so do my students. I often do this lab with my biology students when teaching about nutrition and digestion, but my favorite use of this lab is with my chemistry classes.  This lab is perfect to introduce the idea of titrations, equivalents, and as a review of dimensional analysis.

In this experiment the student will use a lab procedure known as a titration to determine the amount of Vitamin C found in a 6 ounce serving of various fruit juices.  I most often use orange juice, pineapple juice, and apple juice. 

A titration is the controlled addition and measurement of the amount of a solution of known concentration required to react completely with a measured amount of a solution of unknown concentration.  Titration provides a means of determining the chemically equivalent amounts of two substances.  

The materials list is short and consists of items found in almost all labs..... no fancy equipment required!  You will need:  Spot plate, Thin stemmed or microtip Beral pipets  (or medicine droppers),  White paper for background, Ascorbic acid standard solution,  Apple juice,  Orange juice,  Pineapple juice, Starch solution,  Iodine solution and Plastic Toothpick (stirrer).

In order to determine the amount of Vitamin C in the fruit juice, the student must first do a titration using a vitamin C (ascorbic acid) solution of known concentration.  An iodine/starch complex is used so that a color change can be detected.  The number of drops of iodine added will be used to determine the amount of Vitamin C present in the juice.  

When ascorbic acid (Vitamin C) comes into contact with iodine, it is oxidized to form dehydroascorbic acid.  When Vitamin C and iodine are in solution together, they will form iodide.  As iodine is added during the titration, iodide will continue to be formed until there is no more Vitamin C left in the solution.  At this point, iodine becomes present in the solution and the starch turns a blue-black color.  The starch is used as an indicator because it turns black in the presence of iodine, but not for iodide.  The amount of iodine that is added during the titration can be used to indicate the amount of Vitamin C present in the fruit juice.

Set up for the lab is quick and easy, and does not take a lot of advance preparation.  


This lab is available in my TpT store and can be viewed here:  Determining the Amount of Vitamin C in Fruit Juices.

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