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Redox Titrations

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Redox Titrations

Definitions
- Oxidation
- Reduction
- Oxidant
- Reductant
- Potential (E)
  - ↑E° >> strong oxidant
  - ↓E° >> strong reducing
  - To complete redox reaction >> differance in E° between 2 systems > 0.2 V
  - Measure !
    - Electrochemical cell
    - Nernest eq.
      - Effect of common ion
        Zimmermann's reagent
        MnSO4
        H3PO4
        H2SO4
      - Effect of pH
        Ox. contain O
        Ox contain no O
      - Effect of complexing agent
        Det. of Zn+2
      - Effect of ppt agent
- substance act as ox or red
  - ability to gain or loss e-
  - potential of other system
- Eq. weight
- Oxidation no.
- How to balance ?
Detection of E.P.
- Potentiometrically
- Redox titration curve
  - Factors
    - no. of e- transfer
      - Symmetrical
      - Unsymmetrical
    - Potential differance
      - ↑ >> complete
- Indicator method
  - specific
    - starch
      - Blue with I2
    - SCN-
      - Red with Fe+3
  - Self (colored titrant )
    - KMnO4
    - I2
      - Brown in aq.
      - violet in organic
    - K2Cr2O7 can't be used
  - Irreversible
    - destroyed by strong ox
    - ex.
      - M.R.
      - M.O.
  - External
    - outside reaction medium
    - Diphenylcarbazide
      - det. of Fe+2 # st. Cr2O7-2
      - E.P. diphenylcarbazone >> violet
  - Redox
    - Requirements
    - Highly colored + conjugated system
    - Ex.
      - Ferroin
        Det. of Fe+2 # Ce+4
        Ferroin >> Ferrin
        E.P. Pale blue
      - DPA
        Det. of Fe+2 # st. Cr2O7-2
        in presence of H3PO4
        water insol
        so, add H2SO4
        DPA sulphonate
        water sol
        E.P. colorless > violet
      - Methylene blue
        E.P. colorless >> blue
  - transition range
Oxidizing agents
- KMnO4
  - adv
    - self
    - strong
    - low cost
  - disadv
    - oxidize Cl-
      - UseZimmerman's !
    - pH dependent
      - strong acidic
        MnO4- → Mn+2
        Fe+2
        C2O7-2
        AsO3-3
      - strong alkaline
        MnO4- → MnO4-2
        add BaCl
        BaMnO4 ↓
        Filter
      - weak alkaline
        MnO4-→ MnO2 ↓
  - standardization
    - st. oxalix a
  - Reactions
    - strong acidic medium
      - Direct
        Reducing agents
        Fe+2
        add zimmernann's
        Pharmaceutical compounds
        H2O2
        act as Red
        reduce MnO4-
        act as ox
        oxidize I-
      - Indirect
        cations form insol oxalate
        ex. Ca+2
    - strong alkaline medium
      - Back titration
        C → CO3-2
        ex. glycerol
        ex. mixture of CH3COOH + HCOOH
        acid-base >> total
        Redox >> HCOOH
    - weak alkaline medium
      - C → CO3-2 + C2O4-2
        ex. glycerol
- Iodine
  - disadv
    - weak
    - limited solubility
    - limited app
    - volatile
  - preparation
    - I2 + I- → I3-
    - IO3- + I- → I2
  - standardization
    - S2O3--
      - neutral medium
  - Detectio of E.P.
    - self
    - specific (starch)
      - can't be used in acidic medium
    - Organic solvent
  - Reactions
    - Direct ( Iodimetry )
      - reducing agents
        S2O3--
        Sn++
        AsO3-3
      - Pharmaceutical compounds
        Dimercaprol ( BAL)
        Novalgine
        water content
        ex.
        amoxicillin trihydrate
        11.5 -14.5 %
        Amiloride HCl
        11 - 13 %
        Karl-Fisher reagent
        I2
        SO2
        Pyridine
        detected potentiometrically
        Pt. working electrode
        calomel reference electrode
    - Indirect (Iodometry )
      - AsO4-3
      - CuSO4
        add NH4SCN
    - In alkaline medium
      - Det. of Glucose
- KIO3
  - half eq acc to pH
  - prepared as molar sol
  - standardization
    - Iodometrically
  - uses
    - source of I2
    - Andrew's method
      - det. of mix ( I2 & I- )
      - strong acidic medium
      - Ind : CHCl3 or irreversible
    - Lange's method
      - dil. HCL + KCN
      - Ind : starch
      - det. of mix ( I2 & I- )
- BrO3-
  - 1ry st
  - irreversible ind
  - uses
    - source of Br2
    - back titration
      - Phenol
      - Isoniazide (INH)
- NBS
  - strong
  - HCl
    - BrCl
  - determination
    - ascorbic acid
      - irreversible ind
    - sulphonamide
      - irreversible ind
    - Phenothiazine
      - E.P. potentiometrically
Oxygen flask method
- organic sulphur compounds
  - ignition : S → SO2 or SO3
  - absorbing liq : H2O2
    - SO4--
  - # BaCl2
    - ind: sod. rhodizonate
    - E.P. : Ba rhodizonate (red ppt)
- organic phosopho compounds
  - ignition : P → PO4-3
  - absorbing liq : dil HNO3
  - + amm. molybdate
    - canary yellow
      - spectrophotometry
  - Risedronate
- organic halide compounds
  - Cl-
    - Ignition : Cl- → Cl2
    - absorbing lliq : NaOH contain some H2O2
    - determination : Arginometry
      - Mohr's
        ind : K2Cr2O7
        E.P. reddish broen ppt
    - cloxacillin
  - Br-
    - # Hg++
      - [Fe(SCN)6]-3 ind
      - E.P : disappearance of red color
Pre-oxidant and pre-reductant
- adjust oxidation state
- requirements
  - not interfere
  - easily removed
- Pre oxidant
  - gaseous
    - ozone
      - in acidic medium
        Mn++
        Ce+3
      - in alkaline medium
        NO2-
        I-
      - removed by boiling
    - bromine
      - specific for I-
      - removed by addition of phenol
  - Homogenous
    - amm persulphate
      - Ce +3
      - Fe+2
      - Mn+2
      - removed by boiling
    - H2O2
      - In acidic medium
        Sn+2
        Fe+2
      - removed by boiling
    - HClO4
      - hot + conc
      - Fe+2
      - S, As , Sb
      - removed bt dilution or cooling
  - solid
    - sod bismuthate
      - Mn+2
      - Cr+3
      - removed by filtration
- Pre reductant
  - gaseous
    - H2S
      - Fe+3
      - removal by boiling
  - Homogenous
    - SnCl2
      - As+5
      - Fe+3
      - removal by addition of HdCl2
  - Metalic
    - Jones reductor
      - Zn + HgCl2
        zn amalgum
      - non selective
      - uses
        Fe+3
        Ti+4
        MnO4-
        Cr2O7--
      - disadv
        NO3-
        O2
    - Walden's reductor
      - less powerful more selective
      - Ag + HCl
      - uses same jones
      - disadv
        agcl ↓
        dil NH3
        Ag(NH3)+
        sol
        air

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Description: Mind Map on Redox Titrations, created by سمية علي on 28/12/2014.
Created by: سمية علي
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