Yodimetria, Yodometria Yodatometria_2014-II

8/10/2019 Yodimetria, Yodometria Yodatometria_2014-II

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YODIMETRIA, YODOMETRIA

y YODATOMETRIA


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There are a lot of redox titrations classified according to the

titrant used.

1) Permanganimetric: Titrant KMnO4

2) Dichromatometric: Titrant K2Cr2O7

3) Titrations involving iodine (I2)Iodimetry

Iodometry

Titrations that create or consume I2 are widely used in

quantitative analysis.


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A reducing agent is the element or compound in a redox

reaction that reduces anotherspecies. In doing so, itbecomesoxidized, and is therefore the electron donorin the

redox.

Examples of reducing agents:

The active metals sodium, magnesium, aluminum, andzinc,

NaH, CaH2, and LiAlH4, which formally contain the H-

ion.
http://en.wikipedia.org/wiki/Redoxhttp://en.wikipedia.org/wiki/Chemical_specieshttp://en.wikipedia.org/wiki/Electron_donorhttp://en.wikipedia.org/wiki/Electron_donorhttp://en.wikipedia.org/wiki/Electron_donorhttp://en.wikipedia.org/wiki/Electron_donorhttp://en.wikipedia.org/wiki/Chemical_specieshttp://en.wikipedia.org/wiki/Redox


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An oxidaizing agent is the element or compound in a redox

reaction that oxidaizes anotherspecies. In doing so, it

becomesreduced, and is therefore the element or compundthat gain electrons.

Examples:

permanganate (MnO4-), chromate (CrO42-), and dichromate(Cr2O7)

2- ions, sodium hypochlorite (bleach) as well as nitric

acid (HNO3), perchloric acid (HClO4), and sulfuric acid (H2SO4)
http://en.wikipedia.org/wiki/Redoxhttp://en.wikipedia.org/wiki/Chemical_specieshttp://en.wikipedia.org/wiki/Chemical_specieshttp://en.wikipedia.org/wiki/Redox


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Iodimetry: A direct titration with only 1 reaction:

Analyte + titrant (iodine I2) product (iodide I-)

unknown known

When a reducing analyte is titrated with iodine (the titrant),

the method is called iodimetry.


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When a reducing analyte is titrated with iodine (the titrant),

the method is called iodimetry.

Example: Quantification of Ascorbic Acid (Vitamin C)

C6H8O6 + I2 CH6O6 + 2I- + 2H+

Iodine rapidly oxidizes ascorbic acid, C6H8O6 , to produce

dehydroascorbic acid, C6H6O6 .

Ascorbic acid Dehydroascorbic acid

Pictures taken from: http://en.wikipedia.org
http://en.wikipedia.org/http://en.wikipedia.org/


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Iodometry is the titration of iodine (I2) produced when an

oxidizing analyte is added to excess I- (iodide).

Then the iodine (I2) is usually titrated with standardthiosulfate solution.

Iodometry: Not a direct titration because there are 2 reactions:

analyte + I-

I2unknown

I2 + titrant (standard thiosulfate) product

Known


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Iodimetric titrations:

a) A reducing analyteb) One reaction

c) Standard solution: Iodine (I2)

Iodometric titrations:

a) An oxidizing analyte

b) Two reactions

c) Standard solution: Sodium thisoufate


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Analytical applications:

Iodimetric titrations:

Species analyzed (reducing analytes)

SO2

H2S

Zn2+ , Cd2+ , Hg2+ , Pb2+

Cysteine, glutathione, mercaptoethanol

Glucose (and other reducing sugars)

Section of a protein structure

Source: http://en.wikipedia.org


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Analytical applications:

Iodometric titrations:

Species analyzed (oxidizing analytes)

HOCl

Br2

IO3- , IO4

-

O2, H2O2, O3

NO2-

Cu 2+

MnO4-, MnO2


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Example: Quantification of Copper

Analyte of

unknown

concentration

Titrant

-standrard solutions: sodiumthiosulfate

-known concentration

2 Cu 2+ + 4I- 2CuI + I2

I2 + 2S2O32- 2I- + S4O6

2-


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Fact File 1: Introduction to iodometric and iodimetric titrations

Second: Pre-treatment of the sample

In this lesson: Iodometric titration of copper

Sample: Copper wire (solid)

First: Dissolve the sample

copper wire Cu0 dissolution Cu 2+

Copper ion: oxidizing agent


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Fact File 1: Introduction to iodometric and iodimetric titrations

Third: Iodometric titration

Analyte of

unknown

concentration

Titrant

-standrard solutions: sodiumthiosulfate

-known concentration

2 Cu 2+ + 4I- 2CuI + I2

I2 + 2S2O32- 2I- + S4O6

2-

APLICACIONES PARA LAS


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APLICACIONES PARA LAS

VALORACIONES

YODOMETRICAS


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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 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)

920310 15http:\\asadipour.kmu.ac.ir 33 slides

Prereduction : Stannous chloride ( SnCl2)

Chromous chloride

Jones reductor (zinc coated with zinc amalgam)

Walden reductor ( solid Ag and 1M HCl)


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Reagents used in redox titration

Reducing agents

1) ammonium iron(II) sulfate hexahydrate (Mohrs salt) FeSO4(NH4)2SO46H2O

2) iron(II) ethylene diamine sulfate (Oespers salt) FeC2H4(NH3)2(SO4)24H2O

3) Sodium thiosulfate pentahydrate Na2S2O35H2O

4) Arsenic trioxide: arsenious oxide As2O3

5) Sodium oxalate and oxalic acid dihydarte Na2(COO)2 , (COOH)22H2O



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Standardization of thiosul fate solution:

Primary standard :potassium iodate (KIO3), K2Cr2O7, KBrO3

Titration reactions:

KIO3 + 5KI + 6HCl 3I2 + 6KCl + 3 H2O

I2 + 2Na2S2O3 2NaI + Na2S4O6

KIO3 3I2 6Na2S2O35H2O 6 Equivalent

S2O32- +H+ HSO3

- +S(s)

pH, Microorganisms, Concentration, Cu2+, Sunlight

Stabilizer for sodium thiosulfate solution : Na2CO3

Na2S2O3 + H2O + CO2 Na2CO3 + H2S2O3

H2S2O3 H2SO3 + S



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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



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Starch is the indicator of choice for those procedures

involving iodine because it forms an intense blue colour

with 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.



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Arsenious oxide, As4O6

As4O6 + 6H2O = 4H3AsO3

H3AsO3 + I3+ H2O = H3AsO4 + 3I

+ 2H+



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Reagents used in redox titration

Oxidizing agents

1) Potassium permanganate KMnO4 : Permanganometry

2) Ceric sulfate / Ceric ammonium sulfate Ce(SO4)22(NH4)2SO44H2O : Cerimetry3) Potassium dichromate K2Cr2O7 : Dichrometry

4) Iodine I2 : Iodimetry, Iodometry

5) Potassium iodate KIO3 : Iodatimetry

6) Potassium bromate KBrO3 : Bromatimetry



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Permanganate titration

Oxidation with permanganate : Reduction of permanaganate

KMnO4 Powerful oxidant that the most widely used.

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

MnO4 + 8H+ + 5e = Mn2 + + 4H2 O E

o = 1.51 V

KMnO4 is a self-indicator.

2) In feebly acidic, neutral, or alkaline solutions

MnO4 + 4H+ + 3e = MnO2 (s) + 2H2 O E

o = 1.695 V

3) In very strongly alkaline solution (2M NaOH)

MnO4 + e = MnO4

2 Eo = 0.558 V



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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

-



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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



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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.



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Applications of permanganometry


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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 = CaCl2 + 2CH2OH9CHOH)4COOH

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

CaCl2 + H2SO4 = H2C2O4 + CaSO4

2KMnO4 + 5 H2C2O4 + 3H2SO4 = 2MnSO4 + K2SO4 + 10CO2 + 8H2O920310 33http:\\asadipour.kmu.ac.ir 33 slides


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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 H2SO4

Indicator : ferroin, diphenylamine

Preparation and standardization:

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

Sodium oxalate.



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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



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Oxidation with potassium dichromate

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

K2Cr2O7 is a primary standard.

Indicator : diphenylamine sulphonic acid



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Ex. Redox titration ( hydroquinone vs dichromate standard solution )

HO OH O O + 2H+ + 2e Eo= 0.700

Cr2O72 + 14H+ + 6e 2 Cr3+ + 7 H2O E

o= 1.33

3

3 HO OH + Cr 2O72 + 8H+ 3 O O + 2 Cr3+ + 7 H2O

E

o

= E

o

cathodeE

o

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 produceiodine, which is then titrated with standard thiosulfatesolution.


I2 + V.C 2I- +

I- + Cu2+ I2 + Cu+

I2 + S2O32- 2I- + S4O6

2-


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1) Iodine only dissolves slightly in water. Its solubility isenhanced by interacting with I-

2) An excellent way to prepare standard I3- is to add a weighedquantity of potassium iodate to a small excess of KI. Thenadd excess strong acid (giving pH 1) to produce I3

- byquantitative 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 hypoiodousacid (HOI), iodate, and iodide.


I2 + OH- IO- + I- + H+ 3IO- IO3

- + 2I-


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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 :

C5H

5NI

2+ C

5H

5NSO

2+ C

5H

5N + H

2O 2 C

5H

5NH I + C

5H

5NSO

3

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 sidereactions 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.920310 45http:\\asadipour.kmu.ac.ir 33 slides


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BROMATOMETRIA

Es el mtodo de las oxidoreductimetrias donde se empleanlas reacciones de in bromato, (BrO3)

1-

(BrO3)1- + 6 H+ + 6e-

Br1- + 3 H2O

Pot +1,45 v

Por tanto, el equivalente gramo ser 1/6 del Peso Formula.

(167,01 / 6) = 27,84

Puede observarse que el medio tiene que sernecesariamente cido. Como el bromato necesita realizarse

en caliente y en medio fuertemente cido.


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Durante la titulacin los iones bromato se reducen a bromuro, que

al aparecer en el medio, en presencia de iones bromato,

reaccionan con este oxidndose.

(BrO3)1- + 6 H+ + 5 Br1-

3 Br2 + 3 H2O

El bromo libre que se forma colorea la solucin de amarillo plido.

Este color es imposible de observar con precisin en el punto de

equivalencia. Pero existen colorantes organicos que se oxidanirreversiblemente con el bromo y la solucin coloreada con ellos

se decolora. Algunos de estos colorantes son el anaranjado de

metilo y el rojo de metilo.

La solucin patrn que se emplea en bromatometra es la 0,10N,

preparada por pesada directa de la sal seca. Pero puede usarse

la solucin 0,010N (contiene 2,7835 g de sal por litro)

Se emplea para valorar, por ejemplo, fenoles


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Yodimetria, Yodometria Yodatometria_2014-II