Kurva Titrasi Redoks

Pendahuluan

1.) Titrasi Redoks Berdasarkan reaksi reduksi oksidasi antara analit dan titrant Banyak analit dalam lingkup kimia, biologi, lingkungan dan ilmu material dapat

diukur menggunakan titrasi redoks.

Measurement of redox potentials permit detailed

analysis of complex enzyme mechanism

Electron path in multi-heme active site of P460

PR

1. Buat kurva titrasi 25 ml Sn2+ 0,1 M dengan Ce4+ 0,1 M. Reaksi:

Sn2+ + 2Ce4+ Sn4+ + 2Ce3+

2. Tunjukkan bahwa potensial pada saat titik ekivalen untuk titrasi Fe2+ dengan MnO4

- adalah:

E = (EFe3+/Fe2+ + 5EMnO4-/Mn2+)/6 – 0,08pH

Titrasi Redoks Bentuk kurva titrasi redoks

1.) Perubahan voltase sebagai fungsi penambahan titran Perhatikan reaksi titrasi (biasanya satu arah/sempurna). Misalnya:

Ce4+ di dalam buret diteteskan ke larutan Fe2+

Elektrode Pt mendeteksi konsentrasi relatif dariFe3+/Fe2+ & Ce4+/Ce3+

Elektrode calomel/SHE/dll digunakan sebagai reference

Eo = 0,68 V

Setengah reaksi pada elektrode Pt (reduksi – oksidasi):

K ≈

Eo = 1.44 V

Titrasi Redoks

Bentuk kurva titrasi redoks

2.) kurva titrasi memiliki tiga wilayah Sebelum titik ekivalen Pada titik ekivalen (TE) Setelah titik ekivalen

3.) Wilayah 1: sebelum titik ekivalen Tiap aliquot Ce4+ menghasilkan mol Ce3+

dan Fe3+ yang ekivalen

Kelebihan Fe2+ yang belum bereaksi berada dalam larutan

Jumlah Fe2+ dan Fe3+ dapat diketahui, digunakan untuk menghitung potensial sel.

Sisa Ce4+ tidak diketahui

Titrasi Redoks

Bentuk kurva titrasi redoks

3.) Wilayah 1: sebelum TE

Eo = 0.68 V

Use iron half-reaction relative to calomel reference electrode:

][

][log05916.068.0

3

2

Fe

FeE

Redox Titrations Bentuk kurva titrasi redoks

4.) Daerah 2: Pada titik ekivalen Enough Ce4+ has been added to react with all Fe2+

- Primarily only Ce3+ and Fe3+ present- Tiny amounts of Ce4+ and Fe2+ from equilibrium

From Reaction:

- [Ce3+] = [Fe3+]- [Ce4+] = [Fe2+]

Both Reactions are in Equilibrium at the electrode

][

][log05916.068.0

3

2

Fe

FeE

][

][log05916.044.1

4

3

Ce

CeE

Redox Titrations Shape of a Redox Titration Curve

4.) Region 2: At the Equivalence Point Don’t Know the Concentration of either Fe2+ or Ce4+

Can’t solve either equation independently to determine E+

Instead Add both equations together

][

][log05916.068.0

3

2

Fe

FeE

][

][log05916.044.1

4

3

Ce

CeE

][

][log05916.0

][

][log05916.044.168.02

4

3

3

2

Ce

Ce

Fe

FeE

Rearrange

][

][

][

][log05916.012.22

4

3

3

2

Ce

Ce

Fe

FeE

Add

Redox Titrations Shape of a Redox Titration Curve

4.) Region 2: At the Equivalence Point Instead Add both equations together

][

][

][

][log05916.012.22

4

3

3

2

Ce

Ce

Fe

FeE

][][

][][

24

33

FeCe

FeCeLog term is zero

VEVE 06.112.22

Equivalence-point voltage is independent of the concentrations and volumes of the reactants

Redox Titrations Shape of a Redox Titration Curve

5.) Region 3: After the Equivalence Point Opposite Situation Compared to Before the Equivalence Point

Equal number of moles of Ce3+ and Fe3+

Excess unreacted Ce4+ remains in solution

Amounts of Ce3+ and Ce4+ are known, use to determine cell voltage.

Residual amount of Fe2+ is unknown

Redox Titrations Shape of a Redox Titration Curve

5.) Region 3: After the Equivalence Point

Eo = 1.44 V

Use iron half-reaction relative to calomel reference electrode:

][

][log05916.044.1

4

3

Ce

CeE

Redox Titrations Shape of a Redox Titration Curve

7.) Asymmetric Titration Curves Reaction Stoichiometry is not 1:1 Equivalence point is not the center of the steep part of the titration curve

Titration curve for 2:1 Stoichiometry

2/3 height

Redox Titrations Finding the End Point

1.) Indicators or Electrodes

Electrochemical measurements (current or potential) can be used to determine the endpoint of a redox titration

Redox Indicator is a chemical compound that undergoes a color change as it goes from its oxidized form to its reduced form

Redox Titrations Finding the End Point

2.) Redox Indicators Color Change for a Redox Indicator occurs mostly over the range:

where Eo is the standard reduction potential for the indicator and n is the number of electrons involved in the reduction

voltsn

.EE o

059160

V.to.volts.

.E 206108811

0591601471

V.to.).(V.to.)calomel(E.

.E 965084702410206108811

0591601471

For Ferroin with Eo = 1.147V, the range of color change relative to SHE:

Relative to SCE is:

Redox Titrations Finding the End Point

2.) Redox Indicators In order to be useful in endpoint detection, a redox indicator’s range of color

change should match the potential range expected at the end of the titration.

Relative to calomel electrode (-0.241V)

Redox Titrations Common Redox Reagents

1.) Adjustment of Analyte Oxidation State Before many compounds can be determined by Redox Titrations, must be

converted into a known oxidation state- This step in the procedure is known as prereduction or preoxidation

Reagents for prereduction or preoxidation must:- Totally convert analyte into desired form- Be easy to remove from the reaction mixture- Avoid interfering in the titration

Potassium Permanganate (KMnO4)- Strong oxidant- Own indicator Titration of VO2+ with KMnO4

Before Near AfterEquivalence point

Eo = 1.507 VViolet colorless

pH ≤ 1

Eo = 1.692 VpH neutral or alkaline

Violet brown

pH strolngly alkalineEo = 0.56 V

Violet green

Redox Titrations Common Redox Reagents

2.) ExampleA 50.00 mL sample containing La3+ was titrated with sodium oxalate to precipitate La2(C2O4)3, which was washed, dissolved in acid, and titrated with 18.0 mL of 0.006363 M KMnO4.

Calculate the molarity of La3+ in the unknown.