Preadjustment of analyte oxidation state ssary to adjust the oxidation state of the analyte to one that can ith an auxiliary oxidizing or reducing agent. Preadjustment by auxiliary reagent Fe(II), Fe(III) Fe(II) 4 – Titrati on Ce 4+ Preoxidation : Peroxydisulfate ( (NH 4 ) 2 S 2 O 8 ) 2– ) Sodium bismuthate ( NaBiO 3 ) Hydrogen peroxide (H2O2) 920310 1 http:\\asadipour.kmu.ac.ir 33 slides tion : Stannous chloride ( SnCl 2 ) Chromous chloride Jones reductor (zinc coated with zinc amalgam) Walden reductor ( solid Ag and 1M HCl)
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Preadjustment of analyte oxidation state It is necessary to adjust the oxidation state of the analyte to one that can be titrated with an auxiliary oxidizing.
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Preadjustment of analyte oxidation state
It is necessary to adjust the oxidation state of the analyte to one that can be titratedwith an auxiliary oxidizing or reducing agent.
Ex. Preadjustment by auxiliary reagent
Fe(II), Fe(III) Fe(II)4–
Titration
Ce4+
Preoxidation : Peroxydisulfate ( (NH4)2S2O8 )2– )
Sodium bismuthate ( NaBiO3)
Hydrogen peroxide (H2O2)
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Prereduction : Stannous chloride ( SnCl2) Chromous chloride Jones reductor (zinc coated with zinc amalgam) Walden reductor ( solid Ag and 1M HCl)
5) Sodium oxalate and oxalic acid dihydarte Na2(COO)2 , (COOH)2·2H2O
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Sodium thiosulfate, Na2S2O3
Thiosulfate ion is a moderately strong reducing agent that has been widely used to determine oxidizing agents by an indirect procedure that involves iodine as an intermediate. With iodine, thiosulfate ion is oxidized quantitatively to tetrathionate ion according to the half-reaction:
2S2O3 2– S4O6
2– + 2e Eo = 0.08
Ex. Determination of hypochlorite in bleaches [CaCl(OCl)H2O]:
we expect the color change to occur in the approximate range
1.088 V to 1.206 V with respect SHE
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Starch is the indicator of choice for those procedures involving iodine because it forms an intense blue colourwith iodine. Starch is not a redox indicator; it responds specifically to the presence of I2,
not to a change in redox potential.
Starch-Iodine Complex
Structure of the repeating unit of the sugar amylose.
Ex. Redox titration ( hydroquinone vs dichromate standard solution )
HO OH O O + 2H+ + 2e Eo= 0.700
Cr2O72– + 14H+ + 6e 2 Cr3+ + 7 H2O Eo= 1.33
3
3 HO OH + Cr2O72– + 8H+ 3 O O + 2 Cr3+ + 7 H2O
Eo= Eocathode – Eo
anode = 1.33 – 0.700 = 0.63 V
K = 10 nEo/0.05916 = 10 6(0.63) / 0.05916 = 10 64
redox indicator : diphenylamine
colorless to violet
Very large : quantitative : complete reaction
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Iodimetry and iodometry
• Iodimetry: • a reducing analyte is titrated directly with iodine
(to produce I−).• • iodometry : • an oxidizing analyte is added to excess I− to
produce iodine, which is then titrated with standard thiosulfate solution.
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I2 + V.C→ 2I- + ……
I- + Cu2+→ I2 + Cu+
I2 + S2O32- → 2I- + S4O6
2-
1) Iodine only dissolves slightly in water. Its solubility is enhanced by interacting with I-
2) An excellent way to prepare standard I3- is to add a
weighed quantity of potassium iodate to a small excess of KI. Then add excess strong acid (giving pH ≈ 1) to produce I3- by quantitative reverse disproportionation:
3) Cu2++4I- 2CUI + I2
standard I3-
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Stability of I2 Solutions
• In acidic solutions of I3- are unstable because the excess I− is slowly oxidized by air:
• In neutral solutions, oxidation is insignificant in the absence of heat, light, and metal ions.
• At pH ≳ 11, triiodide disproportionates to hypoiodous acid (HOI), iodate, and iodide.
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I2 + OH- ⇌ IO- + I- + H+ 3IO- ⇌ IO3- + 2I-
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Iodimetry
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iodometry
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Bromatimetry
BrO3– + 5Br– + 6H+ 3Br2 + H2O
2I– + Br2 I2 + 2Br–
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I2 + 2 S2O32– 2I– + S4O6
2–
Addition reactions
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Determining water with the Karl Fisher Reagent
The Karl Fisher reaction :
I2 + SO2 + 2H2O 2HI + H2SO4
For the determination of small amount of water, Karl Fischer(1935) proposed a reagent prepared as an anhydrous methanolic solution containing iodine, sulfur dioxide and anhydrous pyridine in the mole ratio 1:3:10. The reaction with water involves the following reactions :
It is always advisable to use fresh reagent because of the presence of various side reactions involving iodine. The reagent is stored in a desiccant-protected container.
The end point can be detected either by visual( at the end point, the color changes from dark brown to yellow) or electrometric, or photometric (absorbance at 700nm) titration methods. The detection of water by the coulometric technique with Karl Fischer reagent is popular.