Determination of Vitamin C

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Determination of Vitamin C

• ???C?????

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• Vitamin C (ascorbic acid) is a water soluble
  antioxidant, and plays a vital role in protecting
  the body.
• It can be obtained from citrus fruits, tomatoes,
  potatoes, and fresh vegetables, particularly from
  red and green peppers.
• 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

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• 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.
• 1 IO3- 5 I - 6 H ? 3 I2 3
  H2O
• C6H8O6 I2 ? C6H6O6 2 I- 2
  H
• The titration end point is reached when a slight
  excess of iodine is added to the ascorbic acid
  solution.
• The reaction with potassium iodate titration
  utilizes starch as the indicator.
• When all of the ascorbic acid has reacted with
  the KIO3, the excess KIO3 oxidizes the KI
  produced in the reaction (which is colorless) to
  I2 which forms a deep blue color with the starch
  indicating that the reaction is complete.

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Procedure
Part 1 Preparation of iodine solution

• 0.1g of potassium iodate and 2g of potassium
  iodide were weighted accurately by an electric
  balance.
• Then was dissolved in 50 mL distilled water and 1
  mL conc. HCl.
• It was then dissolved in a volumetric flask to
  make up to 250 mL iodine solution.

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Part 2 Preparation of vitamin C

• Accurately weigh out a sample of ascorbic acid
  (about 0.3 g) .
• The sample was dissolved in distilled water.
• It was made up to 100 mL by a volumetric flask.

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Part 3Titration of iodine solution against
vitamin C.

• The burette was filled up with the standard
  iodine solution.
• 25 mL of the vitamin C solution, were pipetted
  out and added into a conical flask. Starch was
  added as an indicator in this moment.
• Titrate with the standard iodine solution until
  the first appearance of a blue endpoint.
• Do a total of three accurate titrations with one
  standard solution. Determine the average molarity
  of your sample. Determine the standard deviation
  of that average.

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????

• ??????????
• ??????????????????????,????,???????C???????,?????
  ???C?

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????

• 1. ??????
• (1) ???0.1 g???KIO3,??150 mL???,????2 g KI?
• (2) ?50 mL?????1 mL?????????,????250 mL?
• 2. ???C???
• (1) ?????C?0.3 g,???????,? ???100 mL?
• (2) ?????25 mL,??1 mL 3?????,???1mL
  0.5?????
• (3) ?????????,???????,????C?????

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????

• ??????????(IO3-)????????(I-,??)???(???)?????????(I
  2)(??????????????)?
• ??????????C?????????,?????????????,???????????????
  C????
• ?????????????C????,?????C???????????????,?????????
  ????????
• ???????,????C???,???????????????30?????

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• ????????????????????C????????????
• IO3- 5 I- 6H ? 3 I2 3 H2O
• C6H8O6 I2 ? C6H6O6 2 I- 2H
• (???C) (?????C)

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??

• ?????????????????????????????,??????????????????C?
  ??????????

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???? (?) ??????
???0.1 g???KIO3
?
??150 mL???
?
????2 g KI
?
?50 mL?????1mL???,???????
?
????????250 mL???
?
????????,?????????(??????)
?
???????????,???????
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???? (?) ???C???
???0.3g ????C
?
??150 mL???,?????????
?
????????100 mL???
?
????????,?????????(??????)
?
???????????,???????
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? ?? ?????25 mL?????250 mL????
?
??1 mL 3?????,???1 mL 0.5????
?????????,???????(???????30????,??????)
?
???????
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????
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??????(standard deviation) SD
?????
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A.??????
HCl

(1)???0.1g???KIO3??150mL??????2g?KI?
(2)??50mL????????????1mL????????????250mL?
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B.???C???
(3)?????C?0.3g???????????????100mL?
(4)??????????25mL,???????1mL3?????,???1mL0.5????
?
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(5)?????????,???????,????C?????
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