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Nov132008Vitaminc - 2 I DETERMINATION OF VITAMIN C(100...

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DETERMINATION OF VITAMIN C (100 POINT) EXERCISE KIO 3 I 2 I - KI
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Objective: To determine VITAMIN C content of an Unknown Concepts: Oxidation/Reduction Iodometry (Optical Activity) Techniques: Titration Weighing Apparatus: Buret Balance
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Normal VITAMIN C is REDUCED form C 6 H O 6 C 6 H O 6 - 2H HO HO O H H HO OH H H C C O C C C C Reduced Form Oxidized Form 7 + H 2 O O O O H H HO OH H H C C O C C C C HO O O H H HO OH H H C C O C C C C OH C 6 H O 6 Reduction is the addition of H 2 Oxidation is the removal of
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PROPERTIES OF ASCORBIC ACID PROPERTIES OF ASCORBIC ACID Molar Mass 176 mg / mmol Solubility in H 2 O 300 mg / mL pK a ’s: 4.17, 11.57 Analytical Wavelength 245 nm (UV) Reduction Potential -0.127 V Since VITAMIN C is a reducing agent, we will use an oxidizing agent, I 2 ( iodine ) to react with it. I 2 is produced in situ by reacting IO 3 - ( iodate ) with excess I - ( iodide ) that we add to the solution
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0.536 – ( -0.127 ) Redox Electrochemistry Redox Electrochemistry Reduction Half Reaction E 0 a I 2 + 2 e - 2 I - +0.536 b 2 IO + 12 H + + 10 e - I 2 + 6 H 2 O +1.195 (1/2) b – (5/2) a a c E 0 = +0.659 E 0 = +0.663 1 I 2 1 C 6 H 8 O 6 +1.195 0.536 I 2 + C 6 H 8 O 6 2 I - + C 6 H 6 O 6 + 2 H +
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CHEMISTRY CHEMISTRY 1.) IO 3 - (iod ate ) oxidizes I - (iod ide ) to I 2 ( iod ine ) IO 3 - + 5 I - + 6 H + 3 I 2 + 3H 2 O 6 6 6 Involves neither I - nor I 2 as reactants ! Requires acidic environment
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4.) When ASCORBIC ACID has been consumed, The technique described above is general for the titration of reducing agents, and called Iodometry. 1. Add excess KI to acidified reducing agent 2. Titrate with KIO 3 to a Starch-iodine end point How will we know when reaction is complete?
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