Lecture 09

Polarography Polarography is an analyzing solutions substances.

electrochemical method of of reducible or oxidizable

It is a technique in which the electric potential is varied in a regular manner between two sets of electrodes (indicator and reference) while the current is monitored. In this technique, the current-potential curve is automatically recorded by means of a recorder.

There are three types of measurements: 1) Constant potential polarography : The potential is kept constant and the resulting current intensity is measured as a function of time. 2) Galvanostatic polarography : The current is kept constant and the variation of potential is measured. 3) Oscillographic polarography : The potential may be varied with time and the corresponding current variation is recorded.

Three variables are available

potential

current

variable potential constant Oscillographic Galvanostatic Constant potential polarography

time

Oscillographic polarography To separate the reaction taking place at the redox electrode, - the counter electrode should be unpolarizable and is made large. - adding an excess of a supporting electrolyte. It prevents the transport of the substances under test to the electrode by migration caused by the electric field - using a non-polarizable reference electrode, generally consists of a calomel electrode.

Current strength , mA

Current-potential curve, (polarogram) for an aerated saturated KCl solution containing MnCl2 K++ + e k (or hydrogen evolution) 2+ + 2 e Mn2+

Mn

H22O22 + 2 e 2OH-- or 4OH-O22 + 2H22O + 4 e O22 + 2H22O + 2 e

H22O22 + 2OH--

Blank solution (free from O22 2+ ions) and Mn2+

E (cell potential), V

Polarograph working electrode inert gas

electrolyte Hg

Potentiometer

reference electrode A

The current potential curve obtained is called the polarogram.

Current , mA

E = E½ + RT ln nF

ilim - i ilim

ilim ilim ≡ concentration

ilim

E½ ≡ type of ion

i

residual current

E½

Potential, V

Current , mA

ilim 2 ilim 1 E½ 1

Tow types of ions < C2 ilim ≡C1concentration E½ ≡ type of ion

E½ 2 Potential, V

I , µA

Polarographic curves for some ions

2+ Ca2+ 2+ Ba2+ 2+ Mn2+ 2+ Zn2+ 2+ Pb2+ 2+ Cu2+

E, V

Advantages of the DME 1- The constant renewal of the Hg-surface eliminates passivity or poisoning effects. 2- The high hydrogen overpotential on mercury makes it possible to deposit all cations whose deposition potentials ranging from + 0.4 to -1.3 V (SHE).

DME potential window +i Hg-dissolution

-E

+E

- 1.3 V + 0.4 V

H22 gas evolution

-i

ILKOVIC Equation For concentration measures of a certain ion Ilkovic equation is used, where:

ilim = K C K is a constant related to the characteristics of the DME, i.e.: - the mass of the Hg-drop m fallen per unit time - the drop life time t - the diffusion coefficient of the specified ion D - the change in the charge of ion n

ilim = 708 n m2/3 t1/6 D½ C

Example Calculate the diffusion coefficient for Ag++ from the data: 10-4-4 M Ag++ produced a diffusion current = 30.8 µA, 2+ produced a while a solution containing 10-5-5 M Cd2+ diffusion current = 20.5 µA on the same polarograph. -5 -5 cm22 s-- 11). = 0.95 × 10 (Dcd 2+ cd2+

Solution ilim = 708 n m2/3 t1/6 D½ C +] (ilim30.8 )Ag+ D½-4Ag+ [Ag 708 n m2/3 t1/6(10 ) = -5) 2+ -5)1/2 1/6 ½ 2/3 (ilim20.5 )Pb2+ (10 (0.95 × 10 2 n m t D Pb2+ [Pb ] 708 DAg+ = 0.86 × 10-6 cm2 s-1

Calibration curve; limiting current ilim. vs. bulk concentration of electro-active species C ilim

C

Crystallization overpotential According to Stranski's theory of crystal growth, each metallic ion deposited must first find its way to a suitable site at the edge of a lattice plane before it can be accepted into the lattice. These sites are called active sites. The ovrpotential when this reaction is inhibited is known as crystallization overpotential.

The transport of metallic ions to a suitable growth site takes place in two ways: 1. thetransition transition 2. IfIfthe of ions to any point and discharge of on the electrode ions can only take surface, they will place at the be transported to growth centers the active sites by (the active sites), surface diffusion. The the overpotential overpotential is then a form of would be a part of concentration one, which istransition referred the to as overpotential. "Lattice inclusion overpotential" .

active sites

Z+

The deposition of metallic ions may thus be resolved into the following stages: 1) Transition 2) Surface diffusion to the growth center 3) Inclusion in the lattice

END of Lecture 09