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 (H 2 O 2 ) Prereduction : Stannous chloride ( SnCl 2 ) Chromous chloride Jones reductor (zinc coated with zinc amalgam) Walden reductor ( solid Ag and 1M HCl) 901013 1 http:\\ asadipour.kmu.ac.ir
). 2. –. Preoxidation : Peroxydisulfate ( (NH 4 ) 2 S 2 O 8 ). Sodium bismuthate ( NaBiO. ). 3. Hydrogen peroxide (H. O. ). 2. 2. Preadjustment of analyte oxidation state. - PowerPoint PPT Presentation
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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)
Prereduction : Stannous chloride ( SnCl2 )
Chromous chloride
Jones reductor (zinc coated with zinc amalgam)
Walden reductor ( solid Ag and 1M HCl)901013 1http:\\asadipour.kmu.ac.ir
Sodium oxalate and oxalic acid dihydarte Na2(COO)2 , (COOH)2·2H2O
Titanium trichloride TiCl3
Potassium ferrocyanide K4Fe(CN)6 · 3H2O
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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]:
Primary standard : potassium iodate (KIO3), K2Cr2O7, KBrO3
Titration reactions:
KIO3 + 5KI + 6HCl 3I2 + 6KCl + 3 H2O
I2 + 2Na2S2O3 2NaI + Na2S4O6
KIO3 3I2 6Na2S2O3·5H2O 6 Equivalent
mw 214.02 248.21
214.02 g 6 × 248.21g
214.02 g / 6 1 N × 1000 ml
35.67 g 1 N × 1000 ml
a g x N × V ml
x N = ( a g × 1 N × 1000 ml) / (35.67 g × V ml)
Stabilizer for sodium thiosulfate solution : Na2CO3
Na2S2O3 + H2O + CO2 Na2CO3 + H2S2O3
H2S2O3 H2SO3 + S
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Calculations
Equivalent weight = ( formula weight) / ( e– change)
Equivalents = g / eq. wt. meq = mg / eq. Wt.
Normality (N) = eq / L = meq / ml
Reaction eq. wt of reactant
Fe2+ Fe3+ + e FW Fe ÷ 1
KMnO4 + 5e Mn2+ FW KMnO4 ÷ 5
Na2S2O35H2O ½ S4O6– + e FW Na2S2O35H2O ÷ 1
Cr2O72 – + 6e 2 Cr3+ FW Cr2O7
2 – ÷ 6
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Molecular model of thiosulfate ion.
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16-2 Finding the end point
• A redox indicator is a compound that changes color when it goes from its
oxidized to its reduced state.
or
For ferroin, with E° = 1.147 V
we expect the color change to
occur in the approximate range
1.088 V to 1.206 V with respect SHE 901013 9http:\\asadipour.kmu.ac.ir
A redox titration is feasible if the difference between analyte and titrantis > 0.2 V.If the difference in the formal potential is > 0.4 V, then a redox indicator usually gives a satisfactory end point.
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• Starch is the indicator of choice for those procedures involving iodine because it forms an intense blue complex with iodine. Starch is not a redox indicator; it responds specifically to the presence of I2, not to a change in redox potential.
• The active fraction of starch is amylose, a polymer of the sugar α-d-glucose.
• In the presence of starch, iodine forms I6 chains inside the amylose helix and the color turns dark blue
Starch-Iodine Complex
Structure of the repeating unit of the sugar amylose.
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Arsenious oxide, As4O6
As4O6 + 6H2O = 4H3AsO3
H3AsO3 + I3– + H2O = H3AsO4 + 3I– + 2H+
The As4O6 molecule consists of an As4 tetrahedron with a bridging oxygen atom on each edge
2KMnO4 5 Na2(COO)2 10 Equivalent mw 158.03 mw 134.01 158.03 g / 5 134.01 g / 2 1 Eq.
31.606 g 67.005 g
1N × 1000 ml 67.005 g
x N × V ml a g
x N = ( a g × 1N × 1000 ml) / (67.005 g × V ml)901013 17http:\\asadipour.kmu.ac.ir
Preparation of 0.1 N potassium permanganate solution
KMnO4 is not pure. Distilled water contains traces of organic reducing substances which react slowly with permanganate to form hydrous managnese dioxide. Manganesse dioxide promotes the autodecomposition of permanganate.
1) Dissolve about 3.2 g of KMnO4 (mw=158.04) in 1000ml of water,
heat the solution to boiling, and keep slightly below the boiling point for 1 hr.
Alternatively , allow the solution to stand at room temperature for 2 or 3 days.
2) Filter the liquid through a sintered-glass filter crucible to remove solid MnO2.
3) Transfer the filtrate to a clean stoppered bottle freed from grease with cleaning mixture.
4) Protect the solution from evaporation, dust, and reducing vapors, and keep it in the dark or in diffuse light.
5) If in time managanese dioxide settles out, refilter the solution and restandardize it.
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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16-7 Methods Involving IodineIodimetry 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.
• Iodine only dissolves slightly in water. Its solubility is enhanced by interacting with I-
• A typical 0.05 M solution of I2 for titrations is prepared by dissolving 0.12 mol of KI plus 0.05 mol of I2 in 1 L of water. When we speak of using iodine as a titrant, we almost always mean that we are using a solution of I2 plus excess I−.901013 26http:\\asadipour.kmu.ac.ir
Preparation and Standardization of Solutions
• 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.
• 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 I2 by quantitative reverse disproportionation:
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.
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Pyridine free Karl Fisher reagent
In recent years, pyridine, and its objectionable odor, have been replaced in the Karl Fisher reagent by other amines, particularly imidazole.