Explain the role of redox reactions in titrimetre processes and galvanic cells.

Explanation

(a) Redox reactions in titrimetric quantitative analysis : In titrimetric analysis the substance of known concentration is called the titrant and the substance being titrated is called the titrand. The standard solution is generally added from a long graduated tube called burette. The process of adding the stamdard solution until the reaction is just complete is called titration. The substance to be estimated is titrated. The point at which the titrand just completely. reacts is called the equivalence point (or) the theoretical point (or) stoichiometric end point. in the redox reactions the completion of the titration is detected by a suitable method like
(i) Observing a physical change,for example, the light pink colour of `KMnO_(4)` titrations.
By using a reagent known as indicator which gives a clear visual change like colour change, formation of turbidity etc. The Point at which this is observed is called the end point of the titration which should coincide with theoretical end point.
Examples:
(1) In `Cr_(2)O_(7)^(2-)` titrations dippenyl amine is used as indicator and at the end point it produces intense blue colour due to oxidation by `Cr_(2)O_(7)^(2-)`
In the titration of `Cu^(2+)"With"I^(-).`
`2Cu^(2+)(aq)+4I^(-)(aq)to Cu_(2)I_(2)(s)-: I_(2)(aq)`
A redox reaction `I_(2)` gives a deep blue colour with starch solution .
(3) In the titration of `I_(2)(aq)S_(2)O_(3)^(-2)` as per the stoichiometric reduce equation .
In this way redox reactions are taken as the basis for titrimetric analysis with `MnO_(4)^(-).Cr_(2)O_(7)^(-2)` etc.,as oxidising agents and `S_(2)O_(3)^(-)` etc., as a reducing agents.
(b) Redox Reactions - Galvanic cells : Redox reaction (i.e.) cell reaction that takes place in gavamic cell is

The process of transfer of electrons from Zn(s) to `Cu^(+2)` take place directly. To make this transfer Indirectly, Zn rod is kept in `ZnSO_(4)` solution in one beaker and in the other beaker `CuSO_(4)` solution is taken and a copper rod is dipped in it. Now the redox reaction takes place in either of the beakers. Each beakers contains both oxidised and reduced forms of the respective species in the beakers containing `CuSO_(4)` solution & Cu rod at the interface Cu and `Cu^(+2)` and in the other beaker at the interface Zn and `Zn^(+2)` The two forms i.e., oxidised and reduced forms of a species participating in oxidation and reduction half reactions is called redox couple. Both the beakers contains each a redox couple. The oxidised form and reduced form are Separated by a vertical line. (or) a slash that represent an interface. e.g.: `Zn(s)//Zn^(+2).(aq)`
In the above arrangement, the two redox couples are represented by `Zn^(+2)//Zn`and `Cu^(+2)//Cu.`
halvamic cell is represented as : `Zn//Zn^(2+)////Cu^(+2)//Cu.`