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Titrasi redoks 2

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  • 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.