Nothing Special   »   [go: up one dir, main page]
Scribd Logo
Upload

Welcome to Scribd!

  • 45 views

    Redox Titration

    Uploaded by

    Mohammad Russell
    The document discusses redox titration and the factors that determine the potential profile during titration. It provides examples of: 1) Titrating Mohr's salt solution (Fe2+) with potassium manganate (KMnO4) in acid medium. The potential is calculated using the Fe3+/Fe2+ couple before equivalence point and the MnO4-/Mn2+ couple after. 2) Common redox indicators used to detect the equivalence point, including their oxidized/reduced colors and transition potentials. An indicator's transition potential should be close to the equivalence point potential. 3) Choosing diphenylamine sulfonic acid as the indicator for titrating Fe2+ with

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd

    Redox Titration

    Uploaded by

    Mohammad Russell
    45 views6 pages
    The document discusses redox titration and the factors that determine the potential profile during titration. It provides examples of: 1) Titrating Mohr's salt solution (Fe2+) with potassium manganate (KMnO4) in acid medium. The potential is calculated using the Fe3+/Fe2+ couple before equivalence point and the MnO4-/Mn2+ couple after. 2) Common redox indicators used to detect the equivalence point, including their oxidized/reduced colors and transition potentials. An indicator's transition potential should be close to the equivalence point potential. 3) Choosing diphenylamine sulfonic acid as the indicator for titrating Fe2+ with

    Original Description:

    very good

    Copyright

    © © All Rights Reserved

    Available Formats

    PDF, TXT or read online from Scribd

    Did you find this document useful?

    Is this content inappropriate?

    The document discusses redox titration and the factors that determine the potential profile during titration. It provides examples of: 1) Titrating Mohr's salt solution (Fe2+) with potassium manganate (KMnO4) in acid medium. The potential is calculated using the Fe3+/Fe2+ couple before equivalence point and the MnO4-/Mn2+ couple after. 2) Common redox indicators used to detect the equivalence point, including their oxidized/reduced colors and transition potentials. An indicator's transition potential should be close to the equivalence point potential. 3) Choosing diphenylamine sulfonic acid as the indicator for titrating Fe2+ with

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    45 views6 pages

    Redox Titration

    Uploaded by

    Mohammad Russell
    The document discusses redox titration and the factors that determine the potential profile during titration. It provides examples of: 1) Titrating Mohr's salt solution (Fe2+) with potassium manganate (KMnO4) in acid medium. The potential is calculated using the Fe3+/Fe2+ couple before equivalence point and the MnO4-/Mn2+ couple after. 2) Common redox indicators used to detect the equivalence point, including their oxidized/reduced colors and transition potentials. An indicator's transition potential should be close to the equivalence point potential. 3) Choosing diphenylamine sulfonic acid as the indicator for titrating Fe2+ with

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    of 6

    Dr.

    Piya Seth, Assistant Professor in Chemistry Page 1


    Redox Titration
    During any titration process the relative amount of the titrant increases and the analyte decreases in
    the reaction mixture.. When redox couples are involved in such titrations, it acquires different
    potentials at different stages of the titration. Before the equivalence point, the potential of the system
    is governed by the potential of the analyte and after the equivalence pointnt is reached it gets controlled
    by the titrant. At equivalence point the system remains in equilibrium, so the potential of both the
    analyte and titrant system will be same.

    Let us consider the potential profile during titration of Mohr’s salt solution [i.e. Fe(II)] by KMnO4 in
    acid medium.

    Overall Reaction: MnO4- + 5Fe2+ + 8H+ Mn2+ + 5 Fe3+ + 4 H2O

    E°Fe3+/Fe2+= 0.77V and E°MnO4-/Mn2+


    /Mn2+= 1.51V

    Fig. 1. Colour change during the titration of Fe(II) by KMnO4

    Let us titrate 100 ml 0.1 N Mohr’s salt solution ((Fe2+) with 0.1 N KMnO4 solution and the medium is
    maintained with 1(M) acidity (H+). Before the equivalence point the potential is calculated by using
    Fe3+/Fe2+ (analyte) couple and after the equivalence point it is calculated by using MnO4-/Mn2+
    (Titrant) couple.

    1. When 1 ml KMnO4 added: After addition of 1 ml KMnO4 equivalent amount of Fe2+ will be
    oxidized to Fe3+. So using Nernst Equation for Fe 3+/Fe2+ couple we get,
    Fe2++
    E = E° - 0.059 log
    1 Fe3++
    When 0 ml KMnO4 added, [Fe3+]: [Fe2+] = 1:99, Hence, E= +0.65 V

    Dr. Piya Seth, Assistant Professor in Chemistry Page 2


    2. When 50ml KMnO4 added: After addition of 50ml KMnO4 equivalent amount of Fe2+ will be
    oxidized to Fe3+. So [Fe3+]: [Fe2+] = 50:50
    As a result E= +0.77 V
    3. When 90ml KMnO4 added: After addition of 90ml KMnO4 equivalent amount of Fe2+ will be
    oxidized to Fe3+. So [Fe3+]: [Fe2+] = 90:10
    As a result E= +0.83 V
    4. When 99ml KMnO4 added: After addition of 99ml KMnO4 equivalent amount of Fe2+ will be
    oxidized to Fe3+. So [Fe3+]: [Fe2+] = 99:1
    As a result E= +89 V
    5. When 99.9ml KMnO4 added: After addition of 99.9ml KMnO4 equivalent amount of Fe2+ will be
    oxidized to Fe3+. So [Fe3+]: [Fe2+] = 99.9:0.1
    As a result E= +95 V
    6. When 100ml KMnO4 added: At equivalence point the system is at equilibrium,
    So, Eeq = n1E1°+n2E2°/(n1+n2) V
    = 5×1.51+1×0.77/6 V = 1.39 V
    7. When 100.1 ml KMnO4 added: After the equivalence point is reached the potential of the system
    will be calculated by MnO4-/Mn2+ couple. So addition of 100.1 ml KMnO4 gives
    [MnO4-]:[Mn2+ ]= 0.1:100. By using Nernst equation we get
    Mn2+
    E = E° - 0.059 log
    5 MnO4-
    E= 1.47 V
    8. When 101 ml KMnO4 added: After addition of 101 ml KMnO4, the [MnO4-]:[Mn2+ ] becomes
    1:100.
    Thus, E= 1.49 V
    9. When 110 ml KMnO4 added: After addition of 110 ml KMnO4, the [MnO4-]:[Mn2+ ] becomes
    10:100.
    Thus, E= 1.50 V
    When these calculated potentials (E in Volt) are plotted against the volume of KMnO 4
    (Titrant) added the following curve is obtained.

    Fig. 2. Change of Potential during the progress of titration of Fe2+ by MnO4- in acid medium.

    Dr. Piya Seth, Assistant Professor in Chemistry Page 3


    In this titration no redox indicator is required as KMnO4 acts as a self indicator and the equivalence
    point can be detected by the pale pink colouration (Stable for 30 Sec). But that is not the case with
    other titrants e.g. K2Cr2O7, Na2S2O3 etc.

    Redox Indicators are mostly organic dyes and are themselves redox couples having specific
    standard reduction potentials. They do not participate in the redox titration, and have distinctly
    different colours in their oxidized and reduced forms.
    Indox + ne IndRed where, EInd = EInd - 0.059 log [IndRed]
    n [InOx]
    To detect the colour of any of the form it should have 10 times more concentration. i.e. to detect
    [InOx]
    10
    [InRed]
    colour of the oxidized form it must have and to detect the colour of reduced form it
    [InRed]
    10
    [InOx]
    must have . Hence the colour change can be detected at the transition potential (Colour
    change potential) given by
    Etp = EInd0 +
    - 0.059
    n
    To get the best result, Etp should lie at the potential close to that attained at the equivalence point. i.e.
    Eeq ≈ Etp.

    Some common redox indicators:


    Diphenylamine (insoluble in water) undergoes irreversible oxidation to colourless diphenylbenzidine
    which in turn suffers a reversible oxidation to form diphenylbenzidine violet.

    Fig. 3. Mechanism of colour change of Diphenylamine indicator

    Due to low solubility of free Diphenylamine it is replaced by its sulfonic acid derivative. The sodium
    or Ba salt of Diphenylamine-4-sulphonic acid is used as indicator in the form of 0.2% aq soln. The
    formal potential EInd = 0.85 V (0.5M H2SO4)

    Fig. 4. Barium Diphenylamine 4-Sulphonate (BDS) indicator

    Ferroin Indicator has the chemical formula [Fe(O-phen)3]SO4, where O-phen is 1,10-phenanthroline,
    a bidentate ligand. The redox couple is [Fe-(O-phen)3]3+/[Fe-(O-phen)3]2+, E0Ind = 1.14 V having
    colour pale blue and red respectively.
    Dr. Piya Seth, Assistant Professor in Chemistry Page 4
    Fig. 5. Structure of Ferroin indicator.

    Table for some common redox indicators with their transition potentials (Colour change potentials)

    Indicator Colour in Colour in E°Ind at pH=0


    Oxidised form Reduced form
    Methylene Blue Blue colourless 0.53
    Diphenylamine Blue-violet colourless 0.76
    Diphenylamine- Red-violet colourless 0.85
    sulfonic acid
    N-phenyl anthranilic Red-violet colourless 1.08
    acid
    Ferroin [Fe(II)- Pale blue Red 1.14
    phenantroline]
    Tris (5-nitro-1,10- Pale blue Red violet 1.25
    phenanthroline)iron

    Choice of Redox Indicator:

    1) Let us consider the redox titration of Mohr’s salt solution with Potassium Dichromate.
    Dichromate ion Cr2O72- oxidizes Fe(II) to Fe(III) itself being reduced to chromium(III) ions.

    Cr2O72-+ 14H+ + 6 Fe2+ 2Cr3+ + 6 Fe3+ + 7H2O


    E°Fe3+/Fe2+= 0.77V and E° Cr2O72-/Cr3+ = 1.33V

    The reduction potential at equivalence point for this system


    Eeq = n1E1°+n2E2°/(n1+n2) V
    = (6×1.33 + 1×0.77) /7 V = 1.25 V
    In fact the potential break at the equivalence point lies in the range 0.95 V- 1.30 V. The indicator
    must have its colour changing potential (Etp) in this range.
    Now the reduction potential value of di phenylaminesulfonic acid system is E° Ind = +0.0.85V which is
    near to that of ferrous-ferric system (E°Fe3+/Fe2+= 0.77V) and also far away from the potential at
    equivalence point Eeq = 1.25V. Thus phosphoric acid is required in this titration as it reacts with the

    Dr. Piya Seth, Assistant Professor in Chemistry Page 5


    product Fe3+ ion to form the complex ion [Fe(HPO4)]+, thus lowering the formal potential of the
    Fe(III)/Fe(II) system thereby increasing its reducing power. Thus the end point is sharper.

    Fig. 6. Colour change during the titration of Fe2+ by Cr2O72- in acid medium with BDS indicator.

    2) Another example is oxidation of Fe2+ by Ce4+ in acid medium.

    Ce4+ + Fe2+ Ce3+ + Fe3+


    E°Fe3+/Fe2+ = 0.77V and E° Ce4+/Ce3+ = 1.44V

    The reduction potential at equivalence point for this system


    Eeq = n1E1°+n2E2°/(n1+n2) V
    = (1.44+0.77)/2 = 1.10 V

    In fact the equivalence point potential lies in the range 1.0 to 1.20 V in H 2SO4 medium. So, the
    suitable indicator for this titration is Ferroin (Etp= 1.14+0.059 V). When all the Fe2+ ions are titrated a
    slight excess of Ce4+ ion will bring about the oxidation of the indicator to change the colour. The
    indicators having transition potential (Colour change potential) slightly less than 1.10 V are not
    suitable for this titration. Because they will change their colour before the arrival of actual
    equivalence point.

    Fig. 7. Colour change during the titration of Fe2+ by Ce4+ in acid medium with Ferroin indicator.

    Dr. Piya Seth, Assistant Professor in Chemistry Page 6

    You might also like