The analysis of vitamin c

When the red-colored I2 solution is added to the ascorbic solution, the characteristic iodine color disappears because of the above reaction... After standardization you can use the iodi

Analysis for Vitamin C

A Introduction

Vitamin C, known chemically as ascorbic acid, is an important component of a healthy diet In the mid-eighteenth century the British navy found that the addition of citrus fruit to the sailors' diet prevented the malady called scurvy Humans are one of the few members of the animal kingdom unable to synthesize vitamin C, resulting in the need for regular ingestion in order to remain healthy The National Academy of Sciences has established the threshold of 60 mg/day for adults as the Recommended Dietary Allowance (RDA) Linus Pauling, a chemist whose many contributions to chemical bonding theory should be well-known to you, recommended a level of 500 mg/day to help ward off the common cold He had also suggested that large doses of vitamin C are helpful in preventing cancer The vitamin C content of foods can easily be determined by oxidizing ascorbic acid,

C6H8O6 to dehydro-L-ascorbic acid, C6H6O6:

+2H++2e

-O O

O O

OH OH

C

H C

C

H C H

OH H

Vitamin C Dehydro-L-ascorbic acid

This reaction is very slow for ascorbic acid in the dry state, but occurs readily when in contact with moisture A reagent that is particularly good for the oxidation is an aqueous solution of iodine, I2 Since I2 is not very soluble in water, we dissolve it in a solution of potassium iodide, KI, in which the I2 exists mainly as I3-,

a complex ion The reaction with ascorbic acid involves I2, which is reduced to I- ion:

) ( 2 )

(

2e− +I2 aq ⎯⎯→ I− aq

In the overall reaction, one mole of ascorbic acid requires one mole of I2 for complete oxidation When the red-colored I2 solution is added to the ascorbic solution, the characteristic iodine color disappears because of the above reaction

Although we could use the first permanent appearance of the yellow color of dilute iodine to mark the end point of the titration, better results are obtained when starch is added as an indicator Starch reacts with I2 to form an intensely colored blue complex In the titration I2 reacts preferentially with ascorbic acid, and so its concentration remains very low until the ascorbic acid is all oxidized At that point, the I2 concentration begins to go up and the reaction with the indicator occurs:

) (

)

2 aq starch starch I complex

yellow blue

Because an I2 solution cannot be prepared accurately by direct weighing, it is necessary

to standardize the I2 against a reference substance of known purity We will use pure

ascorbic acid for this reference, or primary standard After standardization you can use the iodine solution for the direct determination of vitamin C in any kind of sample

B Procedure

B-1 Standardization of the Iodine Solution

Obtain from the storeroom a buret and-an unknown vitamin C sample Weigh out accurately on the analytical balance three ascorbic acid samples of approximately 0.10

g into clean 250-InL Erlenmeyer flasks Dissolve each sample in approximately 100 mL

of water Clean your buret thoroughly Draw about 100 mL of the stock I2 solution from the carboy in the laboratory into a 400-mL beaker and add approximately 150 mL of water Stir thoroughly and cover with a piece of aluminum foil Rinse the buret with

a few milliliters of the I2 solution three times Drain and then fill the buret with the I2

solution

After taking an initial reading of the buret (you may find looking toward a light source will make it easier to see the bottom of the meniscus), add 1 mL of starch indicator to the first ascorbic acid sample and titrate with the iodine solution Note the change of the

I2 color as you swirl the flask gently and continuously during the titration Continue the addition of the iodine, using progressively smaller volume increments, until the sample solution just turns a distinct blue After reading the buret, titrate the other tvio sample solutions-being sure to add the starch indicator and to read your buret before and after each titration

B-2 Analysis of an Unknown Containing Vitamin C

Given your experience with the standardization reaction, you should be able to devise an analogous procedure to determine the vitamin C content of your unknown sample You will need to select a sample size, and you may need to carry out an initial treatment of the sample In particular, if your instructor assigns you a fruit juice sample, it will be desirable to first filter the sample through cheese cloth, followed

parameter called the titer-the number of mg of ascorbic acid which reacts with 1 ml- of iodine solution This number is easily found from the I2 concentration and the mass relationship in the reaction It is desirable to have the volume of I2 for each titration be at least 15 mL Using a small initial sample will give you an indication of how much to scale up for your final

titrations

Report your results in per cent vitamin C, if a solid sample was used For liquid samples, report mg of vitamin C per 100 mL In each case, calculate the size sample required to give the RDA of vitamin C

C Data and Calculations

C-1 Standardization of Iodine

Solution

Mass of Ascorbic Acid Sample (g)

Moles of Ascorbic Acid

Initial buret reading (mL)

Final buret reading (mL)

Volume of I2 added (mL)

Moles of I2 consumed

Molarity of I2 (M)

Titer of I2 (MM Ascorbic

C-2 Unknown Sample

Mass or volume of unknown

Initial buret reading (mL)

Final buret reading (mL)

Moles of iodine added

Moles of vitamin C in sample

Mass of vitamin C in sample (mg)

Percent vitamin C in solid sample

Concentration of vitamin C in liquid sample

Amount of sample which will furnish RDA

D Analysis for Vitamin C Pre-Lab

1 Write a balanced equation for the reaction between I2 and ascorbic acid Identify the oxidizing agent and the reducing agent

2 A solution of I2 was standardized with ascorbic acid Using a 0.1000-g sample of pure ascorbic acid, 25.32 mL of I2 were required to reach the starch end point

a What is the molarity of the iodine solution?

b What is the titer of the iodine solution (mg asc/mL)?

3 A sample of fresh grapefruit juice was filtered and titrated with the above I2 solution

A 100 mL sample of the juice took 9.85 mL of the iodine solution to reach the starch endpoint

a What is the concentration of vitamin C in the juice in mg vitamin C/100 mL of juice (mg/ 100 mL)?

Acknowledgements

Masterton, W L.; Slowinski, E J.; Wayne, C W Chem Principles in the

Laboratory 1997, 257-261