Redox Titrations

 Chapter 11

 Redox Titrations 

Introduction 

• It is well known that in volumetric analysis or titrimetric analysis the strength of the analyte in solution is determined from the volume of standard solution by reaction of titrant with titrate completely. The chemical reaction involved in titrimetric is of different reactions. One such reaction is oxidation-reduction reaction. Thus, the titrations which employ oxidation-reduction reactions are called as redox titration or oxidation-reduction titration.

Concept of Oxidation and Reduction 

• Generally, Oxidation reaction is defined as, "addition of oxygen or loss of electron or removal of hydrogen during a reaction". Similarly, Reduction reaction is defined as, "gain of electron, or removal of oxygen or addition of hydrogen during the reaction" In oxidation- reduction reactions, there occurs change in the valency of reacting elements or ions.

          The concept of oxidation-reduction can be categorized as:

(a) Classical concept

(b) Electronic concept

Classical Concept 

• n classical concept when addition of oxygen or electronegative radical/element is involved in any reaction, the reaction is said to be oxidation. Further, if the reaction involves the removal of hydrogen or electropositive radical/element then the reaction is said to be oxidation.

Example:

Addition of oxygen

2MgO + O/ —+ 2MgO

Addition of electronegative element

3

FeCI + 2 CI + FeCI

Removal of hydrogen

H2S + C!2 + HCI + S

Removal of electropositive element

HgmC!2 + HgC!2 +

Electronic Concept 

• According to this concept, oxidation-reduction reaction is considered as a process in which electrons are transferred from one element or ion to another. This leads to change in the valency of reacting atoms or ions. An atom or element consists of electrons in the outer shell which involve in the reaction directly when the element reacts with the other by transferring or accepting electrons from the other elements.

e.g., conversion of FeCl3 to FeCI2.

                                                    Fe*** + e Fe**

• Iron in reduction accept one electron and result in decrease in the positive valency of ironfrom +3 to +2.

Redox Potential

• It can be calculated by measuring the potential difference of a cell in which oxidation-reduction half-cell is coupled with standard reference cell, i.e., standard hydrogen electrode.

• Oxidizing agents gain electrons and get reduced while reducing agents lose electrons and get oxidized. This transfer of electrons leads to the changes in the valency of the atoms or ions. The positive valency of oxidized atom or ion is increased while that of reduced atom or ion is decreased.

• Oxidizing agents gain electrons and get reduced while reducing agents lose electrons and get oxidized. This transfer of electrons leads to the changes in the valency of the atoms or ions. The positive valency of oxidized atom or ion is increased while that of reduced atom or ion is decreased.

Oxidation - Reduction Reaction Writing Methods 

Oxidation-reduction reactions can be written by two methods:

(A) Electron balance method

(B) Ion-electron balance method.

Electron Balance Method

• In course of the reaction, electrons are transferred. Oxidizing agent has the ability to gain electrons while reducing agent has the ability to lose electrons. The number of electrons lost by reducing agent must be equal to the number of electrons gained by oxidizing agent. This is done by multiplying the reaction by suitable coefficient required to balance the number of electrons.

         This method involves the following steps:

1. Ascertain the composition and formulae of the reactants and the products of the reaction.
2. Determine the valency of the element participating in the H reaction in the reactantsand the products of the reaction.
3. Determine the valency of the element participating in the H reaction in the reactantsand the products of the reaction.

Ion-Electron Balance Method

• In case of oxidation-reduction reaction, the electrons are transferred in the process. In addition to electrons, the number of ions is also balanced.

         This method involves the following steps:

1. Ascertain the composition and formulae of the reactants and products of the reactions
2. Determine the oxidizing agent. Write down partial equation for oxidizing agent.
3.  Find out the coefficient required to balance the number of electrons in both reactions andmultiply each partial equation by this coefficient.

Detection of End - Point 

(A) Internal or Redox Indicator

• As seen in acid-base titrations, acid-base indicators are employed to mark the sudden change in pH during titrations. Similarly, an oxidation-reduction indicator should mark the sudden change in the oxidation potential in the neighborhood of the equivalence point in an oxidation-reduction titration.
• An oxidation-reduction indicator is a compound which exhibits different colours in the oxidized and reduced forms:

Ing, + ne         Inned

• Another indicator as used is 1% solution of diphenylamine in concentrated sulphonic acid. It is used for titration of ferrous ion with K/ Cr/ O7 solution. An intense blue violet colourationis produced at the end point. Addition of phosphoric acid is desirable; it lowers the formal potential of ferric-ferrous system so that equivalence potential coincides more nearly with that of the indicator.

Equivalent Weights

• The equivalent weight of an oxidizing or reducing agent is defined as, that weight of the reagent which reacts with or contains 1.008 g of available hydrogen or 8 g of available oxygen. By available is meant capable of being utilized in oxidation or reduction.

meant capable of being utilized in oxidation or reduction.

2 KMnO4           K2O + 2 MnO + 5 0

Ion-Electron Balance Method:

• As discussed in the balancing of Redox reaction by ion electron balance method, the Redox reactions are written. Equivalent weight is calculated by knowing the number of electrons transferred by one molecule.

• Thus, 5 electrons are gained by MnO4 in reduction and hence equivalent weight of KM nO4 is MW/5 in this case.

Oxidation Number Method:

• This is a development of the view that oxidation and reduction reactions are attended by the changes in valency. The oxidation number of an element is a number which is applied to that element in a particular compound indicates the amount of oxidation or reduction whichis required to convert one atom of the element from free-state to that in compound. If oxidation is necessary to effect the change, the oxidation number is positive and if reduction is necessary, oxidation number is negative.

1. Oxidation Number of free or uncombined element is zero.

2. Oxidation Number of hydrogen has a value + 1

3. Oxidation Number of oxygen (except peroxides) is —2

4. Oxidation Number of metal in combination is usually positive

5. Oxidation Number of a radical or ion is that of its electro-valency with correct sign

 Types of Redox Titrations

• We have already discussed redox titration or oxidation-reduction titrations are the volumetric methods of analysis or titrations in which oxidation-reduction reaction is involved. In these titrations, various oxidizing agents such as potassium permanganate, potassium dichromate, iodine, ceric sulfate, potassium iodate, potassium bromate etc. are used for titration with reducing agents. Depending on the use of oxidizing agents, Redox titrations can be classified as:

1. Permanganometry 

• This titration method involves the usage of potassium permanganate as oxidizing agent. The ability of KMnO4 solution to oxidize is due to the conversion of the MnO4 ion to Mn°° in acidic solution and to MnO/ in alkaline, neutral or very feeble acidic solution Ceriometry.

    2. Ceriornetry

• Oxidation-reduction titrations involving cerci sulfate as an oxidizing agent are called as Ceriometry titrations. Cerci sulphate is a powerful oxidant and can be used only in acidic solution. In neutral solution, cerci hydroxide (hydrous cerci oxide) or basic salts precipitate. Cerci salts have intense yellow colour and end point detection can be possible without any indicator in hot solutions.

lodimetry 

• lodimetry covers the titrations with a standard solution of iodine. lodimetry deals with the titration of iodine liberated in chemical reaction.

lodometry 

• This type of titration uses a standardized sodium thiosulfate solution as the titrant. lodometry is used to determine the concentration of oxidizing agents through an indirect process involving iodine as the intermediary. In the presence of iodine, the thiosulphate ions oxidized quantitatively to the tetrathionate ions.

Bromometry 

• This titration involves the use of Bromine in place of iodine as an oxidizing agent in redox titrations. During the reaction either a water-insoluble bromine-substitution products or Water-insoluble bromine-addition products are formed.

Dichrometry 

• Dichromate titrations or titrations with potassium dichromate in acidic solution are based on conversion of dichromate ion containing hexavalent chromium into trivalent chromium ions.

Titration with Potassium iodate

• Potassium iodate AR is used as a primary standard. Titrations in which potassium iodate is used, involve the reaction with KI in acid solution to liberate iodine.

• Oxidation-reduction titrations are used for the assay of various drugs as per pharmacopoeia of India. Following is the list of drugs assayed by redox titration method.