Exp. 8 (Iodimetric Analysis for Vitamin C)

Exercise No. 8 Iodimetric Analysis Analysis for Vitamin C Introduction

Vitamin C, known chemically as ascorbic acid, is an important component of a healthy healthy diet. diet. Humans Humans are one of the few members members of the animal animal kingdom kingdom unable unable to synthesie !itamin C, resulting in the need for regular ingestion in order to pre!ent the malady called scur!y scur!y and to remain healthy. healthy. "he National National #cademy #cademy of $ciences has estab establis lishe hed d the thresh threshold old of %& mg'd mg'day ay for for adult adults s as the (eco (ecomme mmend nded ed )ietar )ietary y #llowance #llowance *()#+. inus -auling, a chemist whose many contributions contributions to chemical bonding theory theory should be wellknown wellknown to you, recommended recommended a le!el of /&& mg'day to help ward off off the common cold. cold. He also suggested suggested that that large doses of !itamin !itamin C are helpful in pre!enting cancer. "he !itamin C content of foods can easily be determined by oxidiing ascorbic acid, C%H%0%1 C%H80% C%H%0% 2 3H2 2 3e "his reaction is !ery slow for ascorbic acid in the dry state, but occurs readily when in contact with moisture. # reagent that is particularly good for the oxidation is an a4ueous solution of iodine, 5 3. $ince $ince iodine iodine is not !ery !ery solub soluble le in water water,, we disso dissol!e l!e it in a  solution of potassium iodide, 65 in which the 5 3 exists mainly as 5 7 , a complex ion. "he reaction with ascorbic acid in!ol!es 5 3, which is reduced to I- ion. 3e 2 53

35

5n the o!erall reaction, one mole of ascorbic acid re4uires one mole of 5 3 for complete oxidation. 5n fact, iodimetry is one of the most accurate of !olumetric processes hen the redcolo redcolored red 5 3 solution is added to the ascorbic acid solution, the characte characterist ristic ic iodine iodine color disappears disappears because because of the abo!e reaction. reaction. #ltho #lthough ugh we could use the first permanent appearance of the yellow color of dilute iodine to mark the endp endpoin ointt of the the titrat titration ion,, bett better er resul results ts are obtain obtained ed when when starc starch h is adde added d as an indicator. $tarch reacts with 5 3 preferentially with ascorbic acid, and so its concentration remains !ery low until the ascorbic acid is all oxidied. #t that point, the 5 3 concentration begins to go up and the reaction with the indicator occurs. 53 2 starch yellow

starch 9 5 3 complex blue

:ecause an 5 3 solution cannot be prepared accurately by direct weighing, it is necessary necessary to standardie standardie the 5 3 against a reference substance of known purity. e will will use pure ascorbic acid acid for this reference, or primary standard. #fter standardiation you

can use the iodine solution for the direct determination of !itamin C in any kind of sample. Objectives

;. "o prepare and standardie an iodine solution< 3. "o analye an unknown sample containing !itamin C through titration method. Materials

aluminum foil ascorbic acid analytical balance buret 3/&m Erlenmeyer flasks used carbon paper

distilled water ; !olumetric flask solution container =&&m beaker 5 3 crystals

starch indicator iron stand and buret clamp wash bottle funnel filter paper

Procedure

#. $tandardiation of the iodine solution 0btain from the stockroom a buret and an unknown !itamin C sample. eigh out accurately on the analytical balance three ascorbic acid samples of approximately &.;& g into clean 3/&m Erlenmeyer flasks )issol!e each sample in about ;&& m of water. Clean the buret thoroughly. eigh about ;&3& grams of 5 3 crystals in a =&& m beaker and add approximately ;/& m of distilled water. $tir thoroughly and co!er with a piece of aluminum foil. (inse the buret with a few milliliters of the 5 3 solution three times. )rain and then fill the buret with the 5 3 solution. "ake the initial reading of the buret, add ; m of starch indicator to the first ascorbic acid sample and titrate with the iodine solution. Note the change of the 5 3 color as you swirl the flask gently and continuously during the titration. Continue adding the iodine solution, using progressi!ely smaller !olume increments, until the sample solution >ust turns a distinct blue. #fter reading the buret, titrate the other two samples being sure to add the starch indicator and to read your buret before and after each titration. :. #nalysis of an unknown containing Vitamin C ?i!en your experience with the standardiation process, de!ise an analogous procedure to determine the !itamin C content of your unknown sample. @ou will need to select a sample sie, and you may need to carry out an initial treatment of the sample. Aor example, if your instructor assigns you a fruit >uice sample, it is but desirable to first filter the sample through cheese cloth, followed by rinsing of the filter with water. 5t may be helpful in choosing the sample sies to calculate an iodine solution parameter called the titerthe number of mg of ascorbic acid which reacts with ; m

of iodine solution. "his number is easily found from the 5 3 concentration and the mass relationship in the reaction. 5t is desirable to ha!e the !olume of 5 3 for each titration be at least ;/ m. Bsing a small initial sample will gi!e you an indicator of how much to scale up for your final titrations. (eport your results in percent !itamin C, if a solid sample is used. Aor li4uid samples, report mg of !itamin C per ;&& m. 5n each case, calculate the sample sie re4uired to gi!e the ()# of !itamin C. C. )isposal of (eaction -roducts #ll reaction products may be diluted with water and poured down the drain. Questions

;. rite a balanced e4uation for the reaction between 5 3 and ascorbic acid. 5dentify the oxidiing agent and the reducing agent. 3. # solution of 5 3 was standardied with ascorbic acid. Bsing a &.;&& g sample of pure ascorbic acid, 3/.73 m of 5 3 were re4uired to reach the starch end point. *a+ hat is the molarity of the iodine solution *b+ hat is the titer of the iodine solution 7. # sample of fresh grapefruit >uice was filtered and titrated with the abo!e 5 3 solution. # ;&&m sample of the >uice took D.8/ m of the iodine solution to reach the starch end point. *a+ hat is the concentration of !itamin C in the >uice in mg !itamin C';&& m of >uice *b+ hat 4uantity of >uice will pro!ide the ()# amount of !itamin C

)ata and Calculation #. $tandardiation of 5odine $olution $ample ass of #scorbic acid sample, g oles of #scorbic acid * G ;% g+ 5nitial buret reading, m Ainal buret reading, m Volume of 5 3 added, m oles of 5 3 consumed olarity of 5 3, mol' "iter of 5 3, mg #sc'm 5 3

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:. #nalysis of an unknown containing Vitamin C $ample ass or !olume of unknown 5nitial buret reading, m Ainal buret reading, m oles of iodine added oles of !itamin C in sample ass of !itamin C in sample, mg -ercent !itamin C in solid sample Conc of !itamin C in li4uid sample, mg';&& m ()#, m or mg

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Exp. 8 (Iodimetric Analysis for Vitamin C)

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