Vitamin C tablets vs. Citrus fruit juice

PURPOSE: To determine, by titration, the vitamin C (ascorbic acid) content

in several fruit juices.

Titration – the process in which a solution of one reactant, the titrant, is carefully added to a solution of another substance until a color change is observed. The volume of titrant required for completion is measured.

End point – the point at which the indicator changes color and the titration is stopped.

Equivalence point – point at which chemically equivalent amounts of the reactants have been added.

Basic terminology:

•We will determine the amount of vitamin C using a classic starch-iodine titration:

•Iodine reacts with starch to make a blue-black color, but vitamin C reduces iodine to iodide, thus preventing it from creating the blue color. 

•When a known volume of solution containing vitamin C is combined with a small quantity of starch/dilute HCl, dropwise addition of iodine will not cause a color change as long as any vitamin C is present.

• As soon as all the vitamin C has been consumed, the iodine will be free to react with the starch, and a blue color will appear (this is the endpoint of the titration).

•The amount of vitamin C present will be proportional to the amount of iodine required to bring about the appearance of the blue color. 

Burette containing iodine solution

Vitamin C sample solution

Burette containing iodine solution

Vitamin C sample solution

FILL THE BURET:

1. Place 250 mL beaker under the spout to catch excess iodine solution. VERY IMPORTANT: Close the stopcock.  Otherwise, iodine will flow out all over your bench as you try to fill the buret.

2. Place a funnel in the buret, and fill with iodine titrant.  Add iodine slowly enough so that the funnel is nearly empty at all times.  Do not fill up the funnel or the buret may over fill and spill iodine every where... Stop adding iodine solution when the buret is filled to close to the top of the graduations. 

3. Shoot several streams of titrant (iodine solution) to drive bubbles out of the buret

spout.  Practice controlling the stopcock so that a single drop is added to the flask.

The thumb wheel on the side is rotated for precision filling or dispensing and the plunger may be pressed for quick emptying.

Prepare the vitamin C sample:

•Pipette 25.0 mL of the vitamin C sample in a 250 mL Erlenmeyer flask

TITRATE THE SAMPLE

Record the starting volume in the buret to nearest hundredth of a mL (remember that the graduations go from top to bottom of buret, the numbers increases as you go down, and that you read at the bottom of the meniscus). 

Place prepared flask of sample under spout.

Add titrant while simultaneously swirling the flask. 

When the color change begins to show while swirling, reduce the rate of titrant addition.  Continue to reduce the rate of iodine addition as you approach the endpoint. Begin adding titrant drop-wise, swirling to remove color after each addition.

Stop when a trace of blue is stable (hopefully after a single drop has been added).  The leftmost flask displays a good endpoint.  The flask in the middle has not been titrated, and  for the flask at the right, the endpoint was over shot (too dark a blue).

Record the finish buret reading to nearest hundredth mL

Group discussion-

•Note the volume of triiodide needed to titrate the vitamin C solution sample. How do you calculate the ml of triiodide needed to titrate one milligram of ascorbic acid? (titer factor)

Calculate the titer factor and record it.

From volume of triiodide needed to titrate the juice samples, how do you calculate the amount of ascorbic acid in each sample

Calculate the amount of ascorbic acid in each juice sample and record it.

How many ml are there in one fluid ounce?. Calculate the volume of each juice that is equivalent to a 500 mg vitamin C table

Calculate the amount of ascorbic acid in an 8 ounce glass of each type of fruit juice

How many 8 ounce glasses of orange juice provide the USA recommended daily value of Vitamin C ?