preadjustment of analyte oxidation state

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

Jones reductor :

2Zn (s) + Hg2+ Zn2+ + Zn(Hg) (s)

901013 2http:\\asadipour.kmu.ac.ir

Reagents used in redox titration

Reducing agents

Ferrous salts :

ammonium iron(II) sulfate hexahydrate (Mohr’s salt) FeSO4(NH4)2SO4· 6H2O

iron(II) ethylene diamine sulfate (Oesper’s salt) FeC2H4(NH3)2(SO4)2· 4H2O

Sodium thiosulfate pentahydrate Na2S2O3·5H2O

Arsenic trioxide: arsenious oxide As2O3

Sodium oxalate and oxalic acid dihydarte Na2(COO)2 , (COOH)2·2H2O

Titanium trichloride TiCl3

Potassium ferrocyanide K4Fe(CN)6 · 3H2O


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]:

OCl– + 2I– + 2H+ Cl– + I2 + H2O (unmeasured excess KI)

I2 + 2 S2O3 2– 2I– + S4O6

2–

Indicator: soluble starch (-amylose)901013 5http:\\asadipour.kmu.ac.ir

Standardization of thiosulfate solution:

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


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


Molecular model of thiosulfate ion.


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.


• 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.


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


Reagents used in redox titration

Oxidizing agents

Potassium permanganate KMnO4 : Permanganometry

Ceric sulfate / Ceric ammonium sulfate Ce(SO4)2·2(NH4)2SO4· 4H2O : Cerimetry

Potassium dichromate K2Cr2O7 : Dichrometry

Iodine I2 : Iodimetry, Iodometry

Potassium iodate KIO3 : Iodatimetry

Potassium bromate KBrO3 : Bromatimetry

Sodium nitrite NaNO2 :

Calcium hypochlorite Ca(ClO)2 :


Permanganate titration

Oxidation with permanganate : Reduction of permanaganate

KMnO4 Powerful oxidant that the most widely used.

In strongly acidic solutions (1M H2SO4 or HCl, pH 1)

MnO4– + 8H+ + 5e = Mn2 + + 4H2 O Eo = 1.51 V

violet color colorless manganous

KMnO4 is a self-indicator.

In feebly acidic, neutral, or alkaline solutions

MnO4– + 4H+ + 3e = MnO2 (s) + 2H2 O Eo = 1.695 V

brown manganese dioxide solid

In very strongly alkaline solution (2M NaOH)

MnO4– + e = MnO4

2 – Eo = 0.558 V

green manganate


Permanganate titration

Duration of colour in end point (30 seconds)

MnO4– + 3Mn2+ + 2H2O 5MnO2 + 4H+ K=1*1047

Stability of aqoues solution of MnO4-

MnO4– + 2H2O 4MnO2 (s) + 3O2 (g) +4OH-


Standardization of KMnO4 solution

Potassium permanganate is not primary standard, because traces of MnO2

are invariably present.

Standardization by titration of sodium oxalate (primary standard) :

2KMnO4 + 5 Na2(COO)2 + 8H2SO4 = 2MnSO4 + K2SO4 + 5Na2SO4 + 10 CO2 + 8H2O

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.


Applications of permanganometry

(1) H2O2

2KMnO4 + 5 H2O2 + 3H2SO4 = 2MnSO4 + K2SO4 + 5O2 + 8H2O

(2) NaNO2

2NaNO2 + H2SO4 = Na2SO4 + HNO2

2KMnO4 + 5 HNO2 + 3H2SO4 = 2MnSO4 + K2SO4 + 5HNO3 + 3H2O

(3) FeSO4

2KMnO4 + 510 FeSO4 + 8H2SO4 = 2MnSO4 + K2SO4 + 5Fe2(SO4)3 + 8H2O

(4) CaO

CaO + 2HCl = CaCl2 + H2O

CaCl2 + H2C2O4 = CaC2O4 + 2HCl (excess oxalic acid)

2KMnO4 + 5 H2C2O4 + 3H2SO4 = 2MnSO4 + K2SO4 + 10CO2 + 8H2O (back tit)

(5) Calcium gluconate

[CH2OH(CHOH)4COO]2Ca + 2HCl = CaCl + 2CH2OH9CHOH)4COOH

(NH4)2C2O4 + CaCl2 = CaC2O4 + 2 NH4Cl

CaCl2 + H2SO4 = H2C2O4 + CaSO4

2KMnO4 + 5 H2C2O4 + 3H2SO4 = 2MnSO4 + K2SO4 + 10CO2 + 8H2O


Oxidation with Ce4+

Ce4+ + e = Ce3+ 1.7 V in 1 N HClO4

yellow colorless 1.61 V in 1N HNO3

1.47 V in 1N HCl

1.44 V in 1M HSO4

Indicator : ferroin, diphenylamine

Preparation and standardization:

Ammonium hexanitratocerate, (NH4)2Ce(NO3)6, (primary standard grade)

Ce(HSO4)4, (NH4)4Ce(SO4)4·2H2O

Standardized with Sodium oxalate.


Applications of cerimetry

(1) Menadione (2-methylnaphthoquinon: vitamin K3)

O

O

CH3

OH

OH

CH3

2 Ce(SO4)2

HCl, Zn

Reduction

(2) Iron

2FeSO4 + 2 (NH4)4Ce(SO4)4 = Fe2(SO4)3 + Ce2(SO4)3 + 4 (NH4)2SO4


Oxidation with potassium dichromate

Cr2O72– + 14H+ + 6e = 2Cr3+ + 7H2O Eo = 1.36 V

K2Cr2O7 is a primary standard.

Indicator : diphenylamine sulphonic acid


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


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:


• Cu2++4I- 2CUI + I2


Bromatimetry

KBrO3 BrO3– + 5Br– + 6H+ 3Br2 + H2O

2I– + Br2 I2 + 2Br–

I2 + 2 S2O32– 2I– + 2S4O6

2–

Substitution reactions BrO3– + 5Br– + 6H+ 3Br2 + H2O

2I– + Br2 I2 + 2Br– I2 + 2 S2O3

2– 2I– + S4O62–


pH 4-9

Al3+ + 3HOC9H6N Al(OC9H6N)3 (s) + 3H+

hot 4M HCl

Al(OC9H6N)3 (s) 3HOC9H6N + Al3+

3HOC9H6N + 6 Br2 3HOC9H4NBr2 + 6HBr

1 mol Al3+ 3 mol HOC9H6N 6 mol Br2 2 mol KBrO3 901013 34http:\\asadipour.kmu.ac.ir

Addition reactions


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 :

C5H5N•I2 + C5H5N•SO2 + C5H5N + H2O 2 C5H5N•HI + C5H5N•SO3

C5H5N+•SO3–

+ CH3OH C5H5N(H)SO4CH3

Pyridinium sulfite can also consume water.

C5H5N+•SO3–

+ H2O C5H5NH+SO4H–

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.


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.

(1) Solvolysis 2ROH + SO2 RSO3– + ROH2

+

(2) Buffering B + RSO3– + ROH2

+ BH+SO3R– + ROH

(3) Redox B•I2 + BH+SO3R– + B + H2O BH+SO4R– + 2 BH+I–


preadjustment of analyte oxidation state

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