Question

QUESTION 3 Answer ALL parts. [Marks: (a) 50% (b) 25% (c) 2596] Q 3(a) i Using the Nernst equation, explain why a silver/silver chloride electrode can function as either a chloride, Cr, potentiometric sensor (lon Selective Electrode, ISE) OR as a reference electrode. i) Draw a labelled diagram of an electrochemical cell where the silver/silver chloride electrode acts as an lon Selective Electrode Q 3(b) A potential of 0.37 V was observed when a chloride, Cl, lon Selective Electrode was immersed in 25 cm3 of a solution containing an unknown concentration of CI . After addition of 0.25 cm3 of a 0.200 M standard solution of CI, the potential shifted to 0.180 V. Calculate the chloride ion concentration in the unknown solution. Q 3(c) Explain what is meant by an interfering ion in an ISE experiment and give one example of both an ISE and an interfering ion(s).

Answer 1

Explanation:

Nernst equation is the most important equation int he fields of electrochemistry which relates the reduction potential of an electrochemical reaction (half-cell or full cell reaction) to the standard electrode potential.

The equation can be expressed as

E=E⁰-(RT/zF)Ln(Red/Ox)

Where,

Ecell is the half-cell potential difference
Eθcell is the standard half-cell potential
R is the universal gas constant; R = 8.314471 J K-1 mol-1
T is the thermodynamic temperature, in Kelvin; 0 K = -273.15oC
z is the number of moles of electrons transferred between cells (defined by the valency of ions)
F is the Faraday's constant; F = 96,485.3415 C mol-1
[red] is the concentration of ion that gained electrons (reduction)
[oxi] is the concentration of ion that lost electrons (oxidation)

Reference electrodes generally have a stable and known electrode potential. It is simplest is that when the reference electrode is used as a half cell to build an electrochemical cell. Using RE the potential of another half-cell can be estimated.

Apply the Nernst equation to the Ag/AgCl electrode.

E=E⁰-(RT/F)Ln[Cl^-]

The above Nernst equation shows the dependence on the activity or effective concentration of chloride-ions

The standard electrode potential E⁰ against the standard hydrogen electrode (SHE) is 0.230V ± 10mV. The potential is however very sensitive to traces of other bromide ions which make it more negative.

Ag/AgCl can also be considered as an ion-selective electrode (ISE). According to the Nernst equation, the voltage is theoretically dependent on the logarithm of the ionic activity. Alos electrode has been inbuild with an ion-selective membrane.

Theoretical diagram of an electrochemical cell

Explanation: Nernst equation: is the most important equation int he fields of electrochemistry which relates the reduction potential of an electrochemical reaction (half-cell or full cell reaction) to the standard electrode potential. The equation can be expressed as E = E degree - (R T/z F)L n(Red/Ox) Where, Ecell is the half-cell potential difference E theta cell is the standard half-cell potential R is the universal gas constant: R = 8.314471 J K-1 mol-1 T is the thermodynamic temperature, in Kelvin: 0 K = -273.15oC z is the number of moles of electrons transferred between cells (defined by the valency of ions) F is the Faraday's constant: F = 96,485.3415 C mol-1 [red] is the concentration of ion that gained electrons (reduction) [oxi] is the concentration of ion that lost electrons (oxidation) Reference electrodes generally have a stable and known electrode potential. It is simplest is that when the reference electrode is used as a half cell to build an electrochemical cell. Using RE the potential of another half-cell can be estimated.