REDOX TITRATIONS Titrations involving oxidizing and reducing agents are termed as oxidationreduction or redox titrations. The phenomenon of oxidation and reduction plays an important role in our day-to-day life. For example, digestion of food in our body, the fading of the colour of our clothes, the rusting of iron and the functioning of a battery involve oxidation reactions. Many of the industrial processes like extraction of metals are based on reduction process. A reducing agent is an electron donor Fe 2 Fe 3 e An oxidizing agent is an electron acceptor Cl 2 2e - 2Cl The oxidation-reduction reaction is Cl 2 2Fe 2+ 2Fe 3+ 2Cl Such electron transfer reactions are commonly called oxidation-reduction or redox reactions. These reactions involve change in oxidation state or transfer of electrons among reacting substances. The electrons are transferred from a reducing agent to an oxidizing agent. The reducing agents are generally sodium oxalate, ferrous sulphate, ferrous ammonium sulphate, oxalic acid etc. The commonly employed oxidizing agents are acidified potassium permanganate, acidified potassium dichromate, iodine solutions etc. Different types of redox titrations are named on the basis of oxidizing/reducing agents involved. Redox titrations involving KMnO 4 and K 2 Cr 2 O 7 are referred to as PERMANGANATOMETRY and CHROMATOMETRY respectively. Other types of redox titrations using KI (Source of I 2 ) or Iodine directly are called IODOMETRY and IODIMETRY respectively. Oxidation-Reduction : Redox Potential Strength of an oxidizing agent or reducing agent is expressed in terms of Normal or Standard potential. oxidising agent (Oxidant) + ne - OR ox + ne - red reducing agent (reductant) 1
For such a system, the Nernst equation takes the form E E O 2.303RT nf log 10 [ox] [red] Where E is the formal potential at the specified concentration, n is the number of electrons involved in the half reaction, R is a gas constant (8.314 J mol -1 K -1 ) T is the absolute temperature and F is the Faraday constant (96,500 C) E O is the standard electrode potential and is characteristic of a particular system. RT At 25 C, (2.303 F ) = 0.05915 0.05915 [ox] E E O log n [red] The solution potential can be calculated if we know the concentrations of the two forms, i.e. [ox] and [red]. Also, knowing the chemical reaction involved and the potential of the solution, we can use Nernst equation to evaluate the relative concentrations of oxidized and reduced forms. Some of the redox systems with their standard reduction potentials are given in the following table 2.1. Table 2.1 Standard reduction potentials of some redox systems. Oxidized form Reduced form E O (Volts) - MnO 4 Mn 2+ 1.51 2- Cr 2 O 7 Cr 3+ 1.33 Fe 3+ Fe 2+ 0.76 Sn 4+ Sn 2+ 0.15 Table 2.1 indicates that MnO 4 - is the strongest oxidizing agent and Sn 4+ is the weakest one among those listed above. 2
Titration Curves According to Nernst equation, E E O 0.05915 log [ox] n [red] The potential of a given reaction depends upon the relative concentrations of oxidized/reduced forms. In the course of a redox titration, the solution potential also changes, since the concentration of oxidized and reduced forms goes on changing. At the completion of a reaction i.e., when either of the forms gets exhausted, there is a sharp change in potential. Using the Nernst equation, it is possible to calculate theoretically variation of potential during the course of titration, which is called redox titration curve. A representative redox titration curve for the titration of iron (II) ammonium sulfate solution with 0.02 M potassium permanganate solution is shown in figure 1. Figure 1. Titration of a iron (II) ammonium sulfate solution with 0.02 M potassium permanganate solution 3
Redox titrations can also be followed by measuring the potential of the solution with the help of a potentiometer. Redox Indicators In the redox titrations, we need a chemical species that can change colour in the potential range corresponding to the sharp change at the end point. A chemical substance, which changes colour when the potential of the solution reaches a definite value, is termed as an oxidation-reduction or redox indicator. It is necessary, while choosing a redox indicator for a particular titration to ensure that its redox potential lies within that of the system. In ox + ne - In red colour A colour B A redox indicator may be defined as a substance whose oxidized form is of different colour from that of its reduced form. The oxidation and reduction of the indicator is readily reversible. Indicators used in redox titrations are of three types: i) Self-Indicating: In permanganatometric titrations, one of the reacting species (KMnO 4 ) changes colour at the end point and is called self-indicator. ii) External-Indicators: These indicators are not added internally to the reaction medium but are used externally in the form of small droplets on a white tile. For example, potassium ferricyanide, [K 3 Fe(CN) 6 ] is used as an external indicator in the chromatometry. However, the method of using external indicator has become obsolete as it introduces errors in quantitative volumetric analysis. It was employed when no suitable internal indicators were available for redox titrations. iii) Internal-Indicators: Internal indicators are the substances which are added to the titration flask in which titration is carried out. In case of chromatometry, the indicator such as 1% diphenylamine needs to be added to the titrating mixture. Such indicators are called internal indicators. 4
Permanganatometry Potassium permanganate is a very strong oxidizing agent and is employed in the estimation of reducing agents like ferrous salts, oxalic acid, arsenious oxide, etc. The permanganate ion, MnO 4 -, gets reduced to Mn 2+ ion in acidic medium and to MnO 2 in neutral and alkaline media. +7 MnO 4 8H 5e +7 MnO 4 2H2 O 3e 2 Mn 2 4H 2 O, E O = 1.51 V +4 MnO 2 4OH -, E O = 0.57 V Titrations involving potassium permanganate are usually carried out in acidic medium. This is due to higher oxidizing power of permanganate ion in acidic medium than in neutral or alkaline medium; secondly, the formation of brown coloured, MnO 2 in alkaline medium interferes with the detection of the end point. For acidification of KMnO 4 solution, only H 2 SO 4 is suitable whereas the other mineral acids like HCl and HNO 3 are not. HCl is not used because some of the KMnO 4 will oxidize Cl - ions to chlorine gas according to the following equation and thus interferes in the quantitative estimations of reducing agents. 2MnO 4 16H 10Cl 2Mn 2 8H 2 O5Cl 2 Nitric acid cannot be used because it is itself a strong oxidizing agent and may oxidize the reducing agent, thereby introducing error. Potassium permanganate is not a primary standard i.e., a standard solution of KMnO 4 cannot be prepared by weighing because (i) it can never be obtained in the purest form (99.99%) and is always associated with organic impurities (ii) its normality changes on standing (iii) KMnO 4 may react with organic matter present in water in which it is dissolved. KMnO 4 solution can be standardized by titrating with a suitable primary standard solution such as Mohr s salt, oxalic acid or arsenious oxide etc. KMnO 4 solution needs to be added to a known volume of reducing agent containing dilute H 2 SO 4, gradually in small amounts. Rapid addition of KMnO 4 results in the formation of hydrated manganese dioxide, MnO 2.H 2 O, which is brown in colour. 2KMnO 4 3MnSO 4 7H 2 O K 2 SO 4 5MnO 2.H 2 O 2H 2 SO 4 The above reaction also occurs if the medium is not sufficiently acidic. Potassium permanganate is such a powerful oxidizing agent that it oxidizes even water, according to the following equation, 4MnO 4 - + 2H 2 O 4MnO 2 + 4OH - + 3O 2 5
An aqueous solution of potassium permanganate, therefore, should be unstable. However, this reaction is extremely slow; hence the permanganate solution attains reasonable stability in the absence of light. Thus, potassium permanganate solution is stored in dark coloured bottles since the above reaction is catalyzed by light. Expressing concentrations As already mentioned in the acid-base titrations that the concentrations of standard solutions are expressed in terms of normality or molarity. The molarity is being accepted more and more as the way of expressing concentrations. The molar mass of a substance is an inherent property. It is independent of the nature of the chemical reaction it may be undergoing. Hence, a given solution containing a known amount of the solute will have the same molarity under-all conditions. Normality, on the other hand, can change as the gram equivalent of a substance depends on the chemical reaction involved in the titration. For example, KMnO 4 can have a gram equivalent of 158.04, 52.68 or 31.6 depending on the reaction conditions. In view of the above, the use of the term, normality has now been abandoned. Let us consider the oxidation reaction of KMnO 4 in acidic medium, MnO 4 8H 5e Mn 2 4H 2 O Molecular weight of KMnO Gram equivalent weight of KMnO 4 = 4 5 158 = 5 31.6 Potassium permanganate can also act as an oxidizing agent in alkaline medium according to the following reaction, MnO 4 2H 2 O3e MnO 2 4OH - Gram equivalent weight of KMnO 4 in alkaline medium Molecular weight of KMnO = 4 3 = 156/3 = 52.6 In view of the above, the use of the term, normality has now been abandoned. Normality =(molarity) x (factor relating molecular wt. and equivalent wt.) Number of equivalents = {number of mols} x {factor relating mol. wt. and eq. wt.} 6
Factor relating to mol. wt. and eq. wt. is called the stoichiometric ratio. It is the change in oxidation number per mole. (volume of oxidizing agent) x (conc. of oxidizing agent)= [(vol. of reducing agent) x (conc. of reducing agent)] x (stoichiometric ratio) stoichiometric ratio = Amount of oxidising agent Amount of reducing agent In general, for titration between two substances A and B, yielding P and Q, as per the stoichiometric ratio equation, ma + nb xp + yq Where m, n, x and y are the amounts of species involved in the reaction. Stoichiometric ratio = Amount of A Amount of B = (M A V A /1000) / (M B V B /1000) = (M A V A )/(M B V B ) = m/n While applying the volumetric titration correlation, one should keep in mind that in acid-base titrations, the stoichiometric ratio (= amount of acid / amount of base) is a multiplier with the volume and concentration of the base and not the acid while in oxidation reduction titrations, the stoichiometric ratio (= Amount of oxidising agentamount of reducing agent) is a multiplier with the volume and concentration of the reducing agent and not the oxidizing agent. For example, MnO 4 5Fe 2+ +8H Mn 2 5Fe 3+ + 4H 2 O Stoichiometric ratio = 1 mol KMnO 4 5 mol Fe 2+ 1 5 V KMnO4 M KMnO4 [V Fe 2 M Fe 2 ] x (stoichiometric ratio) 7
EXPERIMENT 1 AIM : To determine the strength of given solution of Mohr s salt (ferrous ammonium sulphate) by titrating it with KMnO 4 solution ( M/40). Prepare a standard solution of Mohr s salt. Learning Objectives Preparation of standard Mohr s salt solution Finding the molarity of KMnO 4 solution by titrating it with standard Mohr s salt solution Finding the molarity of given Mohr s salt solution Applications of permanganatometry Theory Ferrous ammonium sulphate is a stable salt and is a primary standard. Fe 2+ ions when titrated with permanganate get oxidized to Fe 3+ ions. Fe 2+ Fe 3+ + e - E O = 0.77 V The overall ionic equation of the titration can be obtained by adding two half reactions (oxidation half reaction and reduction half reaction). [Fe 2+ Fe 3+ + e - ]5 MnO - 4 + 8H + + 5e - Mn 2+ + 4H 2 O ---------------------------------------------------------- MnO - 4 + 5Fe 2+ + 8H + 5Fe 3+ + Mn 2+ + 4H 2 O --------------------------------------------------------- It is clear from the above equation that one mol of potassium permanganate reacts with 5 mol of ferrous ions. Recalling the equation, Amount of oxidising agent M KMnO4 V KMnO4 [M Fe 2 V Fe 2 ] Amount of reducing agent M KMnO4 V KMnO4 M Fe 2 V Fe 2 1 5 1 M V Fe 2 Fe 2 i.e., 5 M KMnO4 V KMnO4 8
Self Indicator Since KMnO 4 is pink in colour and Mn(II) ions are almost colourless, KMnO 4 itself functions as an indicator. As soon as the end point is reached, even the slightest excess of KMnO 4 turns the solution pink and this is why KMnO 4 titrations are called self-indicator titrations. Requirements Apparatus Burette (50 cm 3 ) 1 Pipette (10 cm 3 ).1 Conical flasks (250 cm 3 )..2 Volumetric flask (100 cm 3 ).1 Chemicals Ferrous ammonium sulphate FeSO 4.(NH 4 ) 2 SO 4.6H 2 O Potassium permanganate ( M/40) Sulphuric acid (1M) Beaker (250 cm 3 )..1 Weighing bottle..1 Solutions provided A solution of potassium permanganate ( M/40). Potassium permanganate solution is stored in dark or amber coloured bottle because light accelerates decomposition of KMnO 4 by the reaction given below: 4KMnO 4 2H 2 4MnO 2 4KOH 3O 2 Procedure (a) Preparation of standard ferrous ammonium sulphate Transfer (by the indirect method) a known weight of Mohr s salt into a 100 cm 3 standard volumetric flask. Add a test tube of dil. H 2 SO 4 to the volumetric flask to prevent hydrolysis of the Fe 2+ ions. Dissolve the transferred salt in distilled water and make up the volume of solution carefully upto the mark with distilled water. Make the solution homogeneous. Invert the flask, six to ten times, to mix thoroughly. 9
(b) Standardization of KMnO 4 solution. Wash the burette with tap water, rinse with distilled water and then with KMnO 4 solution. Fill up the burette with the given KMnO 4 solution and mount the burette on a stand. Remove the air bubble from the nozzle of the burette and note the reading of the burette and record it in the observation Table I. Pipette out 10.0 cm 3 of standard ferrous ammonium sulphate solution into a 250 cm 3 conical flask. Add one test tube (10-15 cm 3 ) of dil. H 2 SO 4 (1M) to the solution in the titration flask and place it over a glazed tile. Hold the stop-cock of the burette by left hand and the conical flask by right hand. Open the stop-cock slightly so that the solution from the burette flows slowly and steadily into the conical flask. Stir the contents of the conical flask with right hand by swirling motion. The pink colour, obtained on addition of KMnO 4 solution, disappears on shaking. Continue the titration until a permanent pale pink colour appears. This indicates the end point of the titration. Note the final reading of the burette and record it in Table I. The difference between the two readings of burette gives the volume of MnO - 4 required to completely oxidize 10.0 cm 3 of Fe 2+ ions. Repeat the titrations till at least two concordant readings are obtained (Table-I) c) Finding the molarity of given Mohr s salt solution Repeat the titrations with the given Mohr s salt solution. Record all the readings in Table-II. 10
Observations and calculations: (a) Preparation of standard Mohr s salt solution Mass of weighing bottle + ferrous ammonium sulphate = m 1 =..g Mass of the weighing bottle after transference = m 2 =...g Mass of ferrous ammonium sulphate transferred = (m 2 - m 1 ) = m =..g Molar mass of ferrous ammonium sulphate = 392.15 g mol -1 Molarity of ferrous ammonium sulphate (M 1 ) mass = molar mass 1000 mol dm 3 V(in cm 3 ) m = 392.15 1000 mol dm 3 100 cm 3 Table I = mol dm 3 Standard ferrous ammonium sulphate solution Vs potassium permanganate solution S. No. Volume of standard Mohr s salt solution pipetted (cm 3 ) Burette Readings Initial Final Volume of KMnO 4 used (cm 3 ) 1 10.0 2 10.0 3 10.0 11
(b) Standardization of KMnO 4 solution Molarity of Mohr s solution = M 1 =..mol dm -3 Volume of Standard Mohr s salt solution = V 1 = 10.0 cm 3 Volume of KMnO 4 solution used = V 2 =..cm 3 (from Table-I) Molarity of KMnO 4 solution = M 2 (?) Using Molarity equation, M 1 V 1 5M 2 V 2 Molarity of KMnO 4 solution, M 2 = M 1 V 1 5V 2 Table II Given solution of Mohr s salt Vs standardized potassium permanganate solution S. No. Volume of standard Mohr s salt solution pipetted (cm 3 ) Burette Readings Initial Final Volume of KMnO 4 solution used (cm 3 ) 1 10.0 2 10.0 3 10.0 12
(c) Finding the molarity of given Mohr s salt solution Molarity of given Mohr s salt solution = M 3 = (?).mol dm -3 Volume of given Mohr s salt solution = V 3 = 10.0 cm 3 Molarity of KMnO 4 solution = M 2 = M 4. mol dm -3 Volume of KMnO 4 solution used = V 4 =. cm 3 (from Table II) Using the molarity equation, M 3 V 3 5M 4 V 4 M 3 5M 4 V 4 V 3 Strength of given Mohr s salt solution= (molarity) x (molar mass) = M 3 x (392.15 g mol -1 ) =. g dm -3 Precautions All the apparatus used should be properly cleaned and rinsed. Always place the permanganate solution in the burette and read the upper surface of its meniscus, as the lower one is not clearly visible. Titration flask should be rinsed only with the distilled water but not with the given solution. Add sufficient (10-15 cm 3 ) of dil H 2 SO 4 to a solution of Mohr s salt in the titration flask before titrating it with KMnO 4. If a brown precipitate of hydrated MnO 2 forms, reject the liquid and start a fresh. The KMnO 4 solution from the burette should be added in small installments. For instance, the final reading in the burette in the first titration is 10.0 ml, then in the second attempt it is not advisable to add 9.0 ml of KMnO 4 at one stroke and remaining amount dropwise. In that case instead of colourless Mn 2+, brown turbidity of MnO 2 is formed. Chemically pure sulphuric acid should be used in permanganate titrations. The commercial acid often contains oxides of nitrogen, which reduce the permanganate. Wash the burette and pipette with water after use. 13
Hazards of chemicals used Dilute solutions of potassium permanganate are mildly irritating, and high concentrations are caustic. Potassium permanganate stains the hand and clothing and should be handled with care. Skin stains go off within 48 hours. Mixing the solid KMnO 4 with the concentrated hydrochloric acid generates lethal chlorine gas. Iron (I) ammonium sulphate hexahydrate, FeSO 4.(NH 4 ) 2 SO 4.6H 2 O (Mohr s salt) is moderately toxic by ingestion and is an irritant through inhalation and eye and skin contact. Wear safety glasses and keep the solid or solution of KMnO 4 away from contact with skin. Applications of Oxidation-Reduction Titrations Permanatometry can be used for the determination of percentage of iron in the given iron fillings solution. Determinations using instruments Potentiometry For the reaction, oxidised form + n electrons reduced form, the potential E acquired by the electrode at 25 o C is given by E E O 0.0591 log [ox] n [red] During the oxidation of reducing agent or the reduction of an oxidizing agent, the ratio, and therefore the potential, changes more rapidly in the vicinity of the end point of reactions. Thus titrations involving these reaction (e.g. iron (II) with potassium permanganate or potassium dichromate) may be followed potentiometrically and produce titration curves characterized by a sudden change of potential at the equivalence point. 14