vitamin composition in oranges

Determination of Vitamin C
--------------------------

Introduction

Vitamin C (ascorbic acid) deficiency leads to scurvy, a disease
characterized by weakness, small hemorrhages throughout the body that
cause gums and skin to bleed, and loosening of the teeth. Sailors that
were out at sea for months on end would often develop scurvy unless
the captain had the foresight to pack limes and other citrus fruits.
Vitamin C is a water soluble antioxidant, and plays a vital role in
protecting the body. Oxidizing species attack the body from many
directions. Smog and cigarette smoke both contain high levels of
oxidizing molecules that cause tissue damage. The body makes oxidizing
molecules in response to an infection, and these molecules both kill
the infecting organism and cause tissue damage. The body absorbs extra
vitamin C in response to an infection. Because it is a water soluble
vitamin, any unused vitamin C is excreted. The minimum daily
requirement is 30 mg, the recommended daily allowance is 60-70 mg.

The formula for ascorbic acid is C6H8O6 and the structures for the
reduced form and for the oxidized form (dehydroascorbic acid) are
shown below: 

The amount of ascorbic acid can be determined by a redox titration
with a standardized solution of iodine. The iodine is reduced by the
ascorbic acid to form iodide. As shown in the other half of this redox
equation. 

The titration end point is reached when a slight excess of iodine is
added to the ascorbic acid solution. Thyodene is used to determine the
endpoint, excess iodine reacts with the thyodene indicator and forms a
highly colored complex. Thyodene does not form this complex with
iodide. (Note: Thyodene is available from Fisher Scientific Company,
and the CAS number is 9005-84-9.)

Procedure:

Standardize an iodine solution to determine its molarity.

Prepare a standard solution by weighing 0.1200 - 0.1300g of ascorbic
acid and dissolving it in exactly (use a volumetric flask) 250 ml of
distilled water. Be sure to put the values for the amount you weighed
and the calculated molarity in your notebook. For the first titration,
pour approximately 50 ml of this standard ascorbic acid solution into
a clean 100 ml beaker. Rinse a 10 ml volumetric pipet with this
solution, then pipet 10.00 ml into a 250 Erlenmeyer flask. Add
approximately 10 ml of distilled water, then add a spatula-tip of the
thyodene indicator.

Rinse a clean 50 ml buret with some iodine solution and then fill the
buret (somewhere between 0.00 and 1.00). Record this reading to the
nearest 0.01ml in your notebook. Add this iodine solution to the
Erlenmeyer flask containing the ascorbic acid. Constantly swirl the
flask as you add the iodine. Continue adding iodine until the entire
flask turns dark blue with the addition of one drop. The first time
the entire solution changes is the endpoint. You may have to do a
rough titration to determine approximately what volume is needed to
reach the end point. On subsequent titrations, you can go quickly to
this approximate volume and then slowly add drops and half drops until
you reach the exact end point.

Do a total of three accurate titrations with one standard solution.
Determine the average molarity of your standardized iodine solution.
Determine the standard deviation of that average.

Determine the ascorbic acid concentration in commercial orange juice.

You will determine the ascorbic acid concentration in two preparations
of commercial juice.

1.  Juice in a freshly opened container

2.  Juice in a container that was opened several days ago

To prepare each solution for testing, pour about 90 ml of the test
solution into a beaker and add about 1.5 gram of oxalic acid. Take
25.00 ml of this prepared solution and titrate with the standardized
iodine solution, using a spatula tip of the thyodene indicator.

Do three accurate titrations for each solution and determine an
average value (and the standard deviation) for the amount of vitamin C
present.

Results

You must report your results using a spreadsheet. Report the average
molarity of your standardized iodine solution and the standard
deviation for the average. Report the average amount (and standard
deviation) of ascorbic acid in the commercial orange juice as
mg/100ml. Remember you used 25 ml samples!

All of your values must be placed in a spreadsheet. Do the
calculations in the spreadsheet and print out two versions of your
completed spreadsheet, one with the values and one with the formulas.
(2 points)

You will be graded on the precision (2 points) and accuracy (2 points)
of your results. Be sure to report the correct number of significant
figures (-1 point).

Conclusion

Please write your conclusion in well organized paragraphs using
complete sentences. You must answer the following questions in the
Conclusion section. (3 points)

 

Using the balanced redox equation, how many electrons are lost when
ascorbic acid is oxidized to dehydroascorbic acid? Where do those
electrons go?

 

Which solution contained more ascorbic acid?

 

Explain your results--why would one solution have more vitamin C than
the other? What could be happening to the ascorbic acid? (Remember
that Oxygen makes up ~20% of our air!)

 

What conclusions can you draw about the orange juice in your
refrigerator?

 

What other foods contain vitamin C? List three and report their
approximate vitamin C content in mg per serving. You may need to do
some calculations to determine the amount present in mg, as most
labels list the percent of RDA (recommended daily requirement). The
RDA for vitamin C is 60 mg. If a food contains 100% of the RDA per
serving, it has 60mg of vitamin C.

VitC/kstone/revised 07/10/02

ourselves that our if these
vitamin composition in oranges myself same could these to few most
down having does here but by was too my again be those
before were up you themselves
i where go there! then and know vitamin composition in oranges should yours
their a same that more the the me was go there! about by yourself these
then again of be against do vitamin composition in oranges they here having
on itself no maybe vitamin composition in oranges their up theirs go there! yours
by look it know then she munchies his above myself
some are further not should be but with munchies Like, theirs been
such those do a over does a further more at
own out yours that down can outta sight
vitamin composition in oranges while their were had were down themselves most an in above here themselves above can
what herself where hello an off
his again is should a above was has should be during are be my about
between ours why sources of vitamins any she did me visit -