the determination of ascorbic acid

8/17/2019 The Determination of Ascorbic Acid

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THE DETERMINATION OF ASCORBIC ACID (VITAMIN C) IN FOOD PRODUCTS

OBJECTIVE Determine the contents of vitamin C (ascorbic acid) in various food products using small scale

redox titration.

INTRODUCTION It was as long ago as in mid eighteenth century that the beneficial effects of vitamin C werediscovered. This is when citrus fruits rich in vitamin C were used to prevent scurvy in the seasailors. Since then vitamin C has been claimed a miracle agent in curing and preventing commoncolds and !nown as one of the anti"cancer antioxidants. #ure ascorbic acid is a white crystalline solid with a molecular formula C $%&'$. It is well soluble in water. Its main chemical property is that it oxidies extremely easily in solution thus reducinganything that can accept electrons. The oxidation process is impeded in acidic environment and atlow temperatures. The process is accelerated by addition of light heat in basic solutions and in

the presence of metal ions such as Cu

*

or +e

,*

. Chemically ascorbic acid is similar to sugar molecules. In the body it is oxidied todehydroascorbic acid the fully functional molecule affecting biochemical reactions. To read more about vitamin C and its physiological role as well biochemical processes involvingthis nutrient clic! %--. /itamin C is not produced or stored in the human body0 therefore there is a need for a constantand regular supply of this vitamin in our everyday diet. /itamin C is present in many fruits andvegetables the best sources being broccoli sweet peppers parsley 1russels sproutsstrawberries papayas !iwi oranges lemons and grapefruits. 2maingly potatoes are areasonable source of vitamin C particularly when coo!ed with s!in on. 1ecause vitamin C is soeasily destroyed by the food processing techni3ues (due to its water solubility and reducing

properties) you may find or ascorbic acid listed as an ingredient on a nutritional label of groceryitems as foods are fre3uently artificially enriched in this nutrient.

Analysis of vitain C edox titration is used here to determine the vitamin C content in a sample. The oxidiing agentiodine reacts in an acidic environment with the reducing agent ascorbic acid to produce iodide ionand dehydroascorbic acid.

ascorbic acid dehydroascorbic acid ather than using tedious large "scale burette titrations you will perform the redox titration using asmall scale burette. This techni3ue is fast and easy and allows for comparison of the vitamin C content in many

http://www.vitamincfoundation.org/mega_1_1.htmlhttp://www.vitamincfoundation.org/mega_1_1.htmlhttp://www.chem.ox.ac.uk/mom/vitamins/vitaminc.htmlhttp://www.chem.ox.ac.uk/mom/vitamins/vitaminc.htmlhttp://www.planetkc.com/exrx/Nutrition/Antioxidants/VitaminC.htmlhttp://www.planetkc.com/exrx/Nutrition/Antioxidants/VitaminC.htmlhttp://www.vitamincfoundation.org/mega_1_1.htmlhttp://www.vitamincfoundation.org/mega_1_1.htmlhttp://www.chem.ox.ac.uk/mom/vitamins/vitaminc.htmlhttp://www.planetkc.com/exrx/Nutrition/Antioxidants/VitaminC.html


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different food items. The procedure consists of two steps4 5) the standardiation of I solution ) the analysis of a sample 2fter the analysis is completed some simple calculations are re3uired.

In this experiment you will determine the vitamin C content in a serving (typically 67m8) ofvarious 9uices and :or concentrates. ;ou may also choose to bring your own items including somesolids (brea!fast cereals vegetables fruits etc.). If you need to develop a techni3ue for analying solids (for extra credit) remember that they needto be processed (crushed chopped mashed etc.) and sometimes vitamin C must be water"extracted from them. ;ou will also need the to !now the exact mass of the food sample and:or thevolume of extract prepared as well as the volume of an ali3uot tested in order to calculate thevitamin C content in a serving (e.g. ,7 g is a serving for Total cereal but will you test ,7 g orless


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Table 5 for sample sie and prep. . #erform titrations on two separate portions of the same sample (9uice etc.) and determine theaverage A of drops of iodine solution used. 3. ecord the manufacturer claim on vitamin C content per serving (chec! product label). IMPORTANT 0 Since various foods contain different amounts of vitamin C the amount of 9uice or

solid ta!en for titration or the re3uired concentration of the I solution depends on how rich the fooditem is in vitamin C. If only or ! drops of the standard iodine sol"tion are used for titrating aportion of food you need to4 a. as! the instructor to show you how to properly dilute the I solution in a ,45 ratio. %owever I solution will have to be standardied again to find the proper number of drops per 5 mg of vitaminC ' b. use at least triple the sample sie and repeat the titration with the original I solution. Tal% /.

Sample Sample sie: preparation

fresh or re#onstit"ted 9uice (lemon orange andgrapefruit)

.7 g (or . m8 9uice)B *

5 drop of dilute sulfuric acid * drops ofstarch

#on#entrate of 9uice froen undiluted5.7 g (or 5. m8) BB * 5 drop of dilute sulfuric acid* drops ofstarch

BTransfer 9uice with a plastic pipette directly to a >"well tray tared on the balance to weigh it oruse a calibrated plastic pipette to deliver proper volume of 9uice to the well. ecord the actualvolume or mass of 9uice used. BB @se a spatula to transfer concentrate to a tared >"well tray on a balance or if thawed use apipette.

Vitain C ,ont%nt of some solid food items and 9uices Vitain C in f&-it of t% $o&l'

Cl%an -* Dispose of the solutions from the tray in an appropriate waste container. ash all e3uipment usedusing soapy water rinse well and replace on the instructors cart.

Cal,-lations an' &%*o&t Pa&t A0 Standardiation 5) Calculate the average number of drops of iodine solution per 5 mg vit.. C4

Av1 2 '&o*s *%& / 1 vit C + (2 '&o*s $%ll/ 3 2 '&o*s $%ll! 3 2 '&o*s $%ll4) 5 4 *a&t B ) Calculate the average A of drops of iodine solution used for titration of your samples4 Av1 2 '&o*s B + (2 '&o*s fo& sa*l% / 3 2 '&o*s fo& sa*l% !) 5 !

,) Calculate the number of mg of vitamin C in a sample of food used4 1 vit. C in a sa*l% t%st%' + Av1 2 '&o*s B 5 Av1 2 '&o*s *%& / 1 vit. C

http://www.aracnet.com/~seagull/Guineas/VitaminC.shtmlhttp://www.naturalhub.com/natural_food_guide_fruit_vitamin_c.htmhttp://www.aracnet.com/~seagull/Guineas/VitaminC.shtmlhttp://www.naturalhub.com/natural_food_guide_fruit_vitamin_c.htm


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>) Calculate the content of vitamin C in a serving (for 9uice ? 67 ml)4 vit. C in !67 l 8-i,% ? 1 vit. C in a sa*l% t%st%' 9 (!67 l 5 vol-% of sa*l% in :) -xample4 7.6 mg vit C x (67 m8 :. ml) ? 7.6 mg x 56 ? $.6 mg vit C:serving 4a) +or the concentrate4 fin' t% s%&vin1 si;% on t% *&o'-,t la%l an' ,onv%&t it to 1&as + 1&as of ,on,%nt&at%

5s%&vin1. Hint0 if a serving is in fl. o convert the fl. o to m8 then use density of 5.7 g:m8 to calculate themass m? dx/ Then calculate the content of vitamin C in a serving4 vit. C in on% s%&vin1 of ,on,%nt&at% + 1 vit. C in a sa*l% t%st%' 9 (1&as of

,on,%nt&at% 5s%&vin1) -xample4 (7.$ mg vit. C :5.7 g sample) x (5&7 g concentrate:serving) ? 57& mg vit C:serving

In your R$PORT show all calculations and results and include answers to the 3uestions on yourdata sheet. $%tra #redit &

If you chose additional solid food items brought from home for which the procedure had to bedeveloped please give an outline of the steps followed (include the sample preparation method(chopped raw coo!ed...) amount ta!en for testing etc.

DATA SHEET

P&%la 5. @sing the lin!s provided in this document find the ,-&&%nt D2 (ecommended Daily2llowance) for vitamin C for adults of either sex.

Adult Males 90 mg/day

Adult Females 75 mg/day

http://www.iom.edu/Obe!t.File/Maste"/7/#9$/0.pd%

. @sing the lin!s4 http4::www.msdsonline.com or http4::msds.pdc.cornell.edu:msdssrch.asp find thehealth haards associated with as,o&i, a,i' and io'in%.

WE ARE NOT AWARE OF ANY HAZARDS FOR THE ABOVE PRODUCT.

THE ABOVE INFORMATION IS BELIEVED TO BE CORRECT BUT DOES NOT PURPORT TO

BE ALL INCLUSIVE AND SHALL BE USED ONLY AS A GUIDE. SIGMA, ALDRICH,

FLUKA SHALL NOT BE HELD LIABLE FOR ANY DAMAGE RESULTING FROM HANDLING

OR FROM CONTACT WITH THE ABOVE PRODUCT. SEE REVERSE SIDE OF INVOICE OR

PACKING SLIP FOR ADDITIONAL TERMS AND CONDITIONS OF SALE.

COPYRIGHT 2001 SIGMA-ALDRICH CO.

LICENSE GRANTED TO MAKE UNLIMITED PAPER COPIES FOR INTERNAL USE ONLY

HIGHLY TOXIC (USA)

TOXIC (EU)

MAY CAUSE HARM TO THE UNBORN CHILD.

http://www.slcc.edu/schools/hum_sci/chemistry/labs/chem1140/VitaminC/vitCdatasheet.htmhttp://www.msdsonline.com/http://msds.pdc.cornell.edu/msdssrch.asphttp://www.slcc.edu/schools/hum_sci/chemistry/labs/chem1140/VitaminC/vitCdatasheet.htmhttp://www.msdsonline.com/http://msds.pdc.cornell.edu/msdssrch.asp


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TOXIC IN CONTACT WITH SKIN AND IF SWALLOWED.

CAUSES BURNS.

MAY CAUSE SENSITIZATION BY INHALATION AND SKIN CONTACT.

&'('' *A+,-MAO.

,. hat precautions must be ta!en when wor!ing with a3ueous iodine solution

the determination of ascorbic acid

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