Redox titration. KMnO 4 H 2 SO 4

Transcription

1 Redox titration KMnO 4 H SO 4

2 In acid-base titrations, the change in ph during titration may be calculated, and the titration curves thus obtained can be used to ascertain the most suitable indicator to be used in a given titration, and to determine the titration error. Similar procedures may be carried out for oxidation-reduction titrations. Consider first a simple case which involves only change in ionic charge. A suitable example, for purposes of illustration, is the titration of 1 ml of,1 M iron (II sulfate with,1 M potassium permanganate in the presence of dilute sulfuric acid.

3 A redox titration is based on an oxidation-reduction reaction between analyte and titrant.

4 Oxidation-reduction titrations can be classified according to the titrant Permanganatometric titration (titrant is KMnO 4 Iodometric titration (titrant is I Dichromatometric titration (titrant is K Cr O 7 Bromatometric titration (titrant is KBrO 3 Nitritometric titration (titrant is NaNO etc

5 The equivalent is the amount of a substance needed to react with or supply one mole of electrons in a redox reaction. 3 5Fe MnO4 8H 5Fe Mn 4HO Mn 7 5e Mn z = 5 Fe 1e Fe 3 z = 1 n(1/5 KMnO 4 = n(fe +

6 Redox titration curve Consider the titration of 1 ml of,1 M iron(ii sulfate FeSO 4 solution with,1 M potassium permanganate solution, monitored potentiometrically. 3 5Fe MnO4 8H 5Fe Mn 4HO. The quantity corresponding to c(h+ in acid-base titrtion is the ratio [Ox]/[Red]. We are concerned here with two systems: 1 the Fe 3 / Fe ion electrode, and the MnO / Mn 4 ion electrode

7 H SO 4 KMnO 4

8 There are three distinct regions in the titration of iron(ii with potassium permanganate solution, monitored potentiometrically. 1 Before the equivalence point, where the potential is dominated by the analyte redox pair. At the equivalence point, where the potential at the indicator electrode is the average of their conditional potential. 3 After the equivalence point, where the potential was determined by the titratant redox pair.

9 1 Before the equivalence point: During the addition of the KMnO 4 solution up to the equivalence point, its only effect will be to oxidize the Fe +, and consequently change the ratio [Fe 3+ ]/[Fe + ]. Prior to the equivalence point, the half-reaction involving analyze is used to find the voltage because the concentrations of both the oxidized and the reduced forms of analyte are known. After adding X ml of KMnO 4 the voltage can be calculated using Nernst equation as follows Fe 3 / Fe.59 [ Fe lg 1 [ Fe 3 ] ] formed leftover

10 ,77.59 lg 1 с( Fe с( V ( Fe 1 5 KMnO4 V ( KMnO с( 1 KMnO initial V ( KMnO 4 а After adding 1 ml of,1 M KMnO 4 : ,77 lg. 65V =,65 Volts b After adding 5 ml of,1 M KMnO 4 : ,77 lg, 77 V Е =,77 Volts c After adding 91 ml of,1 M KMnO 4 : ,77 lg, 83V Е =,83 Volts

11 d After adding 99 ml of,1 M KMnO 4 : ,77 lg, 89 V =,89 Volts e After adding 99,9 ml of,1 M KMnO 4 : ,9,77 lg, ,9 В =,994 Volts

12 At the equivalence point At the equivalence point, both half-reactions are used simultaneously to find the voltage. aox 1 b Re d a Re d 1 box The electrode potential is given by: b Ox b a a Red 1 Fe 3 / Fe MnO 4 / Mn, ,51 1,39 V Е = 1,39 Volts

13 3 After the equivalence point: The subsequent addition of the KMnO 4 solution will merely increase the ratio [MnO 4- ]/[Mn + ]. MnO.59 c( MnO4 c( H lg 4 / Mn 5 c( Mn 8 MnO.59 с( MnO4 added V ( MnO4 added lg 4 / Mn 5 c( Fe V ( Fe c( Fe V ( Fe

14 MnO.59 с( MnO4 V ( MnO4 added c( Fe lg 4 / Mn 5 c( Fe V ( Fe V ( Fe а After adding 1,1 ml of,1 M KMnO 4 : lg MnO / Mn 4 V = 1,475 Volts b After adding 11 ml of,1 M KMnO 4 : lg MnO / Mn 4 V = 1,487 Volts c After adding 11 ml of,1 M KMnO 4 : lg MnO / Mn 4 V = 1,498 Volts

15 Redox titration curve is the plot of potential (, Volts versus the volume (ml of titrant Thus (Volt changes from,994 to 1,475 between,1 ml before and,1 ml after the stoichiometric endpoint. These quantities are of importance in connection with the use of indicators for the detection of the equivalence point. V ( KMnO4, V 1 1,65 5, , , ,9, ,39 7 1,1 1, , ,498

16 Conclusions: It is evident that the abrupt change of the potental in the neighbourhood of the equivalence point is dependent upon the standard potentials of the two oxidationreduction systems thet are involved. The greater the difference in reduction potential between analyze and titrant, the sharper will be the end point. The voltage at any point in this titration is independent of dilution and of the concentrations unless these are extremely small.

17 Detection of ndpoint in Redox Titration Self indicator (no indicator. When the titrant solution is coloured (e.g. permanganatometric titration. xternal Indicators (e.g. starch in Iodometric titration. Irreversible Indicators (e.g. methyl orange in Bromatometric Titration. Redox Indicators compounds which have different colours in the oxidized and reduced forms.

18 Redox Indicators The ideal oxidation-reduction indicator will be one with an oxidation potential intermediate between that of the solution titrated (analyte and that of the titrant, and which exibits a sharp, readily detectible colour change. An redox indicator is a compound which exhibits different colours in the oxidized and reduced forms Ind Ox ne Ind Red

19 The oxidation and reduction should be reversible. At a potential the ratio of the concentrations of the two forms is given by the Nernst equation.59 n lg c( Ind c( Ind Ox Red They change colour when the oxidation potential of the titrated solution reaches a definite value Indicator colours may be detected when hence, indicator range: c( Ind c( Ind Ox Red 1/1 or 1/1,58 n

20 XAMPLS Diphenylamine, 76 V,58,76 (,73,79, 73 V colourless (Reduced form, 79 V bluish violet (Oxidized form В (рн =

21 Ferroin 1, 147 V,58 1,6 (1,88 1,6 1 V 1, 88 V red (Reduced form 1, 6 V Pale blue (Oxidized form

22 Phenylanthranilic acid 1, 8 V,58 1,8 (1,5 1,11 V 1, 5 V uncoloured (Reduced form 1, 11V Red violet (Oxidized form

23 A redox titration is feasible if the difference between analyte and titrant is >. V. If the difference in the formal potential is >.4 V, then a redox indicator usually gives a satisfactory end point.

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