chem
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Red
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Ggvcghhhhjjcjxjxjd
Dodd
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Dd
Dc
f不ssgshdgshbdbdhdjsjsbdbdufhfud
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Red
F
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Ggvcghhhhjjcjxjxjd
Dodd
DDR
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Dc
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Red
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DI
id. (Commercial vitamin C is often a mixture of ascorbic acid and other ascorbates.) Ascorbic acid, C6H8O6, is a reducing agent that reacts rapidly with iodine (I2) in acidic medium to produce iodide ion (I-) and dehydroascorbic acid, as shown in the following equation: [IMAGE]
+ I2(aq) -----------> + 2H+(aq) + 2I-(aq)
Ascorbic acid (Vit. C) Dehydroascorbic acid
However, since iodine is only slightly soluble in water, ascorgic acid should not be titrated directly by a standard iodine solution, since the end point of titration is not o obvious. Instead, back titration will be employed.
The titration of a reducing agent with iodine to produce iodide ion is referred to as an iodometric titration. Iodine is generate by adding a weighed amount of standard potassium iodate (KIO3) to an excess of potassium iodide (KI) and then add adding strong dilute sulphuric acid
(H2SO4) to produce iodine as shown below:
(1) KIO3(aq) + 5KI(aq) + 3H2SO4(aq) → 3I2(aq) + 3H2O(l) + 3K2SO4(aq)
The known excess of I2 generated by the reaction is immediately reacted with the ascorbic acid sample; finally, the excess unreacted iodine is “back-titrated” with standardized sodium thiosulphate(Na2S2O3) as shown in the following equation:
(2) 2 Na2S2O3(aq) + I2(aq) → Na2S4O6(aq) + 2NaI(aq)
The amount of ascorbic acid is determined by the stoichiometry of the equations and the difference between the total amount of iodine present and the
Dodd
DDR
D
F f F f非鞏 這
Dd
Dc f不ssgshdgshbdbdhdjsjsbdbdufhfud Jdh dhd. hdhdhdhdhdhdhd:nd
Sjnbd
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Red
F
F
D
D
D@!!gd
Ggvcghhhhjjcjxjxjd
Dodd
DDR
D
F
f
F
f非鞏
這
Dd
Dc
f不ssgshdgshbdbdhdjsjsbdbdufhfud
Jdh dhd. hdhdhdhdhdhdhd:nd
Sjnbd
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Red
F
F
D
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D@!!gd
Ggvcghhhhjjcjxjxjd
Dodd
DDR
D
F
f
F
f非鞏
這
Dd
Dc
f不ssgshdgshbdbdhdjsjsbdbdufhfud
Jdh dhd. hdhdhdhdhdhdhd:nd
Sjnbd
Hhb dd
Red
F
F
D
D
D@!!gd
DI
id. (Commercial vitamin C is often a mixture of ascorbic acid and other ascorbates.) Ascorbic acid, C6H8O6, is a reducing agent that reacts rapidly with iodine (I2) in acidic medium to produce iodide ion (I-) and dehydroascorbic acid, as shown in the following equation: [IMAGE]
+ I2(aq) -----------> + 2H+(aq) + 2I-(aq)
Ascorbic acid (Vit. C) Dehydroascorbic acid
However, since iodine is only slightly soluble in water, ascorgic acid should not be titrated directly by a standard iodine solution, since the end point of titration is not o obvious. Instead, back titration will be employed.
The titration of a reducing agent with iodine to produce iodide ion is referred to as an iodometric titration. Iodine is generate by adding a weighed amount of standard potassium iodate (KIO3) to an excess of potassium iodide (KI) and then add adding strong dilute sulphuric acid
(H2SO4) to produce iodine as shown below:
(1) KIO3(aq) + 5KI(aq) + 3H2SO4(aq) → 3I2(aq) + 3H2O(l) + 3K2SO4(aq)
The known excess of I2 generated by the reaction is immediately reacted with the ascorbic acid sample; finally, the excess unreacted iodine is “back-titrated” with standardized sodium thiosulphate(Na2S2O3) as shown in the following equation:
(2) 2 Na2S2O3(aq) + I2(aq) → Na2S4O6(aq) + 2NaI(aq)
The amount of ascorbic acid is determined by the stoichiometry of the equations and the difference between the total amount of iodine present and the