Nov132008Vitaminc

Nov132008Vitaminc - DETERMINATION OF VITAMIN C (100 POINT)...

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Unformatted text preview: DETERMINATION OF VITAMIN C (100 POINT) EXERCISE K I O 3 I 2 I- K I Objective: To determine VITAMIN C content of an Unknown Concepts: Oxidation/Reduction Iodometry (Optical Activity) Techniques: Titration Weighing Apparatus: Buret Balance 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 C C O C C C C Reduced Form Oxidized Form C 6 H O 7 + H 2 O O O O H H HO OH H C C O C C C C HO O O H H HO OH 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 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 0.536 (-0.127 ) Redox Electrochemistry Redox Electrochemistry Reduction Half Reaction E 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 = +0.659 E = +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 + 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 4.) When ASCORBIC ACID has been consumed, The technique described above is general for the titration of reducing agents, and called Iodometry....
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Nov132008Vitaminc - DETERMINATION OF VITAMIN C (100 POINT)...

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