1) A sample containing La3+ was precipitated with sodium oxalate to isolate the La3+. 2) The precipitate was dissolved in acid and titrated with potassium permanganate. 3) From the volume of permanganate used and its molarity, the molarity of the original La3+ sample was calculated.

1. 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.
Electron path in multi-heme active site of P460
Measurement of redox
potentials permit detailed
analysis of complex
enzyme mechanism

2. 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 MnO4adalah:
E = (EFe3+/Fe2+ + 5EMnO4-/Mn2+)/6 – 0,08pH

3. Titrasi Redoks
Bentuk kurva titrasi redoks
1.) Perubahan voltase sebagai fungsi penambahan titran

Perhatikan reaksi titrasi (biasanya satu arah/sempurna). Misalnya:
K≈

Ce4+ di dalam buret diteteskan ke larutan Fe2+

Elektrode Pt mendeteksi konsentrasi relatif dari
Fe3+/Fe2+ & Ce4+/Ce3+
Elektrode calomel/SHE/dll digunakan
sebagai reference
Setengah reaksi pada elektrode Pt (reduksi – oksidasi):

Eo = 0,68 V
Eo = 1.44 V

4. 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

5. Titrasi Redoks
Bentuk kurva titrasi redoks
3.) Wilayah 1: sebelum TE
Use iron half-reaction relative to calomel reference electrode:
Eo = 0.68 V

[ Fe 2+ ] 
E = 0.68 − 0.05916 log

[ Fe 3+ ] 


6. Redox Titrations
Bentuk kurva titrasi redoks
4.) Daerah 2: Pada titik ekivalen

Enough Ce4+ has been added to react with all Fe2+
-

From Reaction:
-

Primarily only Ce3+ and Fe3+ present
Tiny amounts of Ce4+ and Fe2+ from equilibrium
[Ce3+] = [Fe3+]
[Ce4+] = [Fe2+]
Both Reactions are in Equilibrium at the electrode
 [ Fe 2+ ] 
E+ = 0.68 − 0.05916 log
 [ Fe3+ ] 



 [Ce 3+ ] 
E+ = 1.44 − 0.05916 log
 [Ce 4+ ] 




7. 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
 [ Fe 2+ ] 
E+ = 0.68 − 0.05916 log
 [ Fe3+ ] 



 [Ce 3+ ] 
E+ = 1.44 − 0.05916 log
 [Ce 4+ ] 



Add
 [ Fe 2+ ] 
 [Ce 3+ ] 
2 E+ = 0.68 + 1.44 − 0.05916 log
 [ Fe3+ ]  − 0.05916 log [Ce 4+ ] 







Rearrange
 [ Fe 2+ ] [Ce 3+ ] 
2 E+ = 2.12 − 0.05916 log
 [ Fe3+ ] [Ce 4+ ] 




8. Redox Titrations
Shape of a Redox Titration Curve
4.) Region 2: At the Equivalence Point

Instead Add both equations together
 [ Fe 2+ ] [Ce 3+ ] 
2 E+ = 2.12 − 0.05916 log
 [ Fe 3+ ] [Ce 4+ ] 



[Ce 3 + ] = [ Fe 3 + ]
[Ce 4 + ] = [ Fe 2 + ]
Log term is zero
2 E+ = 2.12V ⇒ E+ = 1.06V
Equivalence-point voltage is independent of the
concentrations and volumes of the reactants

9. 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

10. Redox Titrations
Shape of a Redox Titration Curve
5.) Region 3: After the Equivalence Point
Use iron half-reaction relative to calomel reference electrode:
Eo = 1.44 V
 [Ce 3+ ] 
E = 1.44 − 0.05916 log
 [Ce 4+ ] 




11. 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

12. 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

13. Redox Titrations
Finding the End Point
2.) Redox Indicators

Color Change for a Redox Indicator occurs mostly over the range:
0.05916 

E =  Eo ±
volts
n


where Eo is the standard reduction potential for the indicator
and n is the number of electrons involved in the reduction
For Ferroin with Eo = 1.147V, the range of color change relative to SHE:
0.05916 

E =  1.147 ±
volts = 1.088 to 1.206 V
1


elative to SCE is:
0.05916 

E =  1.147 ±
 − E ( calomel ) = ( 1.088 to 1.206 V ) − ( 0.241 ) = 0.847 to 0.965V
1



14. 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)

15. 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
pH ≤ 1
Eo = 1.507 V
Violet
colorless
pH neutral or alkaline
Eo = 1.692 V
Violet
brown
pH strolngly alkaline
Eo = 0.56 V
Violet
green
Before
Near
After
Equivalence point

16. Redox Titrations
Common Redox Reagents
2.) Example
A 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.