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 E E O 0.05915 log [ox] 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
Chromatometry In chromatometry, acidified potassium dichromate is used as an oxidizing agent to estimate ferrous (Fe 2+ ) ions in the given solution. Potassium dichromate is not so powerful an oxidizing agent as potassium permanganate but it has the following advantages over the permanganate. i) potassium dichromate is a stable salt and is available in pure state and thus serves as an excellent primary standard. ii) Unlike KMnO 4, it does not oxidize HCl to chlorine gas and thus, the titration can be carried out in the presence of hydrochloric acid also. iii) it does not attack organic compounds and rubber. However, potassium dichromate acts as an oxidizing agent only in the acidic medium, while potassium permanganate can act as an oxidizing agent both in acidic and alkaline media. Also, KMnO 4 acts as a self indicator while a redox indicator is necessarily used in chromatometry, since the orange coloured dichromate ions, Cr(VI), are converted to the green coloured chromium ions,cr(iii). The green colour of chromium (III) ions produced makes it difficult to find the end point without the use of redox indicator. 2 Cr 2 O 7 14H + 6e 2Cr 3+ 7H 2 O (E O = 1.33 V) (Cr in +6 state) (Cr in +3 state) [Fe 2+ Fe 3+ + e - ]6 (E O = 0.77 V) ---------------------------------------------------------------------- Cr 2 O 2 7 6Fe 2 14H + 2Cr 3+ 6Fe 3 +7H 2 O ---------------------------------------------------------------------- Since one mole of dichromate ions react with six moles of Fe 2+ solution, the molarities can be expressed by the following equation. ions in Stoichiometric ratio = Amount of K 2 Cr 2 O 7 Amount of Fe 2+ M Fe 2 V Fe 2 6M Cr2 2V 2 O 7 Cr2 O 7 Thus each mol of potassium dichromate reacts quantitatively with six mol of iron (II). 5
Indicators and the end point detection The titrations involving dichromate can be carried out by using either an internal or external indicator. i) External indicator The external indicator is one, which is not added directly into the solution to be titrated but made to react with the reaction mixture separately to locate the end point. 0.1% solution of potassium ferricyanide acts as an external indicator in the oxidation of ferrous to ferric. Ferrous ions react with potassium ferricyanide to produce a deep blue colour (Turnbull s blue) of ferro-ferricyanide as follows: 3Fe 2+ + 2[Fe(CN) 6 ] 3- Fe 3 [Fe(CN) 6 ] 2 blue complex (Turnbull s blue) Potassium ferricyanide does not give any colour with ferric salts. Potassium ferricyanide cannot be used as internal indicator (cannot be added to the titration mixture) because it will react with ferrous ions to give a blue complex and the colour formation is irreversible. Freshly prepared potassium ferricyanide is used by placing a number of small drops on a dry groove tile (or place a series of drops of the indicator on a glazed tile). Withdraw a drop of titration mixture near the end point and bring the same in contact with the indicator on the tile. As long as the ferrous ions are present in the reaction mixture, a blue colour will be produced on mixing a drop of the titration mixture with the indicator drop. When all the ferrous ions have been converted into ferric ions, a drop of the reaction mixture will not produce any blue colour. The disappearance of any blue colour indicates the end point. ii) Internal indicator The most commonly used internal indicators in chromatometry are: a) 1% solution of diphenylamine in conc. H 2 SO 4 (chemically pure). b) 0.2% aqueous solution of sodium diphenylamine sulphonate. c) N-Phenylanthranilic acid. 6
A redox indicator exhibits two different characteristic colours in oxidized and reduced forms. The diphenylamine indicator is suitable when the potential of the solution is 0.73 to 0.79 V. At potentials below this range, the reduced form, i.e., the diphenylamine form is more predominant and the solution, therefore, is colourless. On the other hand, at a solution potential 0.79 V, the oxidized form diphenyl benzidine violet is predominant and the solution appears blue violet and marks the end point. When Fe 2+ ions get converted to Fe 3+ ions, the potential increases gradually. At equivalence point, all the ferrous ions are oxidized and there is a sudden jump in the potential. This increase in the redox potential of the solution is sufficient to cause the oxidation of the indicator. H 2 N diphenylamine ox H N H N + + 2H + 2e diphenylbenzidine (colourless) N N + + 2H + 2e diphenylbenzidine (violet) 7
Ferric ions, which are generated during the course of reaction, might prematurely oxidize the indicator. Therefore, the liberated ferric ions need to be removed from sphere of action i.e., the premature oxidation of the indicator is avoided by masking ferric ions with syrupy phosphoric acid. H 3 PO 4 forms an undissociated ferric hydrogen phosphate complex [Fe(HPO 4 )] + results in lowering the potential of ferric-ferrous system. As a result, we get a sharp change in the colour at the end-point. When N-phenylanthranilic acid is used as an indicator for dichromate titrations, there is no need to add phosphoric acid as the premature oxidation of the indicator does not occur. At the end point, the colour changes from green to violet red i.e., oxidized form of this indicator is violet-red (purple-red) and reduced form is colourless. 8
EXPERIMENT 4 AIM To determine the strength of given ferrous ammonium sulphate (Mohr s salt) solution by titrating it with potassium dichromate solution ( M/40), using 1% Diphenylamine as an internal indicator. Prepare a standard solution of Mohr s salt. Learning Objectives Preparation of standard ferrous ammonium sulphate solution. Standardization of K 2 Cr 2 O 7 solution Finding the molarity of ferrous ammonium sulphate solution (unknown). Application of chromatometry. Requirements Apparatus Burette (50 cm 3 )----------1 Pipette (10 cm 3 )----------1 Conical flasks (250 cm 3 )----------2 Standard Volumetric flask ----------1 (100 cm 3 ) Wash bottle for distilled water ------1 Weighing bottle -----------1 Chemicals Ferrous ammonium sulphate FeSO 4.(NH 4 ) 2 SO 4.6H 2 O Sulphuric acid (1M) Phosphoric acid (85%) Potassium dichromate solution ( M/40) 1% solution of diphenylamine Burette stand -----------------1 9
Theory 2 Cr 2 O 7 14H + 6e 2Cr 3+ 7H 2 O (E O = 1.33 V) (Cr in +6 state) (Cr in +3 state) [Fe 2+ Fe 3+ + e - ]6 (E O = 0.77 V) ---------------------------------------------------------------------- Cr 2 O 2 7 6Fe 2 14H + 2Cr 3+ 6Fe 3 +7H 2 O ---------------------------------------------------------------------- M 1 V 1 6M 2 V 2 where M 1 and M 2 represent the molarities of ferrous and potassium dichromate solutions and V 1 and V 2 represent their volumes, respectively. Procedure a) Preparation of standard ferrous ammonium sulphate solution (known solution) Transfer a known weight of Mohr s salt (by indirect method) into a 100 cm 3 standard volumetric flask. Add a test tube of dilute sulphuric acid to the volumetric flask to prevent hydrolysis of the Fe 2+ ions. Dissolve the Mohr s salt in distilled water and make up the volume of solution carefully with distilled water. Make the solution homogeneous. Invert the flask, six to ten times, to mix thoroughly. b) Standardization of K 2 Cr 2 O 7 solution Wash the burette with tap water, rinse with distilled water and then with K 2 Cr 2 O 7 solution. Fill up the burette with the given K 2 Cr 2 O 7 solution and mount the burette on a stand. Remove the air bubble, if any, from the nozzle of the burette and note the initial reading of the burette and record it in the observation Table I. Pipette out 10.0 cm 3 of standard solution of Mohr s salt into a clean 250 cm 3 titrating flask. Add one test tube (10-15 cm 3 ) of dil. H 2 SO 4 (1M) to the solution in the titration flask. And then add half test tube of syrupy phosphoric acid (or one test tube of 1:1 mixture of dil. H 2 SO 4 - phosphoric acid). 10
And finally add 6-8 drops of 1% diphenylamine indicator to the titration mixture and place the titration flask over a glazed tile. Run in K 2 Cr 2 O 7 solution slowly (0.5 cm 3 at a time) from the burette, while giving a swirling motion to the titration flask. (Hold the stop-cock of the burette by left hand and the conical flask by right hand.) Continue the addition of K 2 Cr 2 O 7 solution until a persistent violent colour is obtained. Note the final reading of the burette and record it in Table I. The difference between the two readings of burette gives the 2- volume of Cr 2 O 7 required to completely oxidize 10.0 cm 3 of Fe 2+ ions. Repeat the titrations to get at least two concordant readings. Record all the readings in Table I. c) Finding the molarity of the given Mohr s salt solution (unknown) Repeat the above procedure of titration with the given Mohr s salt solution. Record all the readings in Table II. Observations and Calculations a) Preparation of standard ferrous ammonium sulphate solution Mass of the weighing bottle + ferrous ammonium sulphate = m 1 =.g Mass of weighing bottle after transference = m 2 =.g Mass of ferrous ammonium sulphate transferred into 100 cm 3 standard volumetric flask = (m 2 - m 1 ) = m =.g Molar mass of ferrous ammonium sulphate = 392.15 g mol -1 Molarity of ferrous ammonium sulphate solution (M 1 ) mass = molar mass 1000 mol dm 3 V(in cm 3 ) m = 392.15 1000 mol dm 3 100 cm 3 = mol dm 3 Institute of Lifelong Learning, University of Delhi 11
b) Standardization of K 2 Cr 2 O 7 solution Table-I Standard ferrous ammonium sulphate Vs potassium dichromate solution S. No. Volume of standard Mohr s salt solution pipetted (cm 3 ) Burette Readings Initial Final Volume of K 2 Cr 2 O 7 used (cm 3 ) 1 10.0 2 10.0 3 10.0 Molarity of standard Mohr s salt solution = M 1 =. mol dm -3 Volume of standard Mohr s salt solution pipetted = V 1 = 10.0 cm 3 Volume of K 2 Cr 2 O 7 solution used = V 2 = cm 3 (From Table I) Molarity of K 2 Cr 2 O 7 solution = M 2 =? Using Molarity equation, M 1 V 1 6M 2 V 2 Molarity of dichromate solution, M 2 = M 1 V 1 6V 2 =.mol dm -3 12
c) Finding the molarity of given solution of ferrous ammonium sulphate Table II Given ferrous ammonium sulphate Vs potassium dichromate solution S. No. Volume of given ferrous ammonium sulphate solution pipetted (cm 3 ) Burette Readings Initial Final Volume of K 2 Cr 2 O 7 used (cm 3 ) 1 10.0 2 10.0 3 10.0 Molarity of K 2 Cr 2 O 7 solution = M 3 = M 2 = mol dm -3 Volume of K 2 Cr 2 O 7 solution used (from Table - II) = V 3 = cm 3 Volume of given Mohr s salt solution pipetted = V 4 = 10.0 cm 3 Molarity of given Mohr s salt solution = M 4 =? Using the molarity equation, M 4 V 4 6M 3 V 3 6M Molarity of given Mohr s salt solution, M 4 = 3 V 3 =. mol dm -3 V 4 Strength of given ferrous ammonium sulphate solution = (molarity) x (molar mass) = M 4 x (392.15 g mol -1 ) =. g dm -3 Precautions The internal indicator, 1% diphenylamine has to be handed carefully since it has been dissolved in concentrated sulphuric acid. 13
Hazards of chemicals used Potassium dichromate is carcinogenic and should be handled carefully. The compound is also corrosive and exposure may produce severe eye damage. Potassium dichromate is one of the most common culprits in causing chromium dermatitis. Hexavalent chromium compounds are generally more toxic than trivalent chromium compounds. Chromium(VI) compound is a known cancer hazard. Phosphoric acid is corrosive, causes severe irritation and burns to every area of contact. It is harmful if swallowed or inhaled. Wear safety glasses, gloves and work in a ventilated area. Applications of chromatometry Titrations involving acidified potassium dichromate solution can be employed to i) determine the percentage of iron in a sample of iron wire. ii) determine ferrous and ferric iron in a solution. iii) determine the percentage of ferric iron in the given sample of ferric alum. iv) determine the percentage of iron in an iron ore, such as haematite (Fe 3 O 4 ) or spathic iron ore (FeCO 3 ). 14