CONDUCTOMETRIC METHODS OF ANALYSIS Ashraf M. Mahmoud, Associate professor

Contents Introduction Ohm’s law. Conductometric measurements. Factor affecting conductivity. Application of conductometry. 2.Conductometric titration-: Introduction. Types of conductometric tiration. Advantages of conductometric tiration. 3.Recent devlopement 4.References .

Introduction Conductometry: is the simplest of the electroanalytical techniques; by Kolthoff in 1929. Conductivity is: “the ability of the medium to carry electric current”.

Introduction Conductors are: either metallic (flow of electrons) or electrolytic (movemenmt of ions). Conductance of electricity: migration of positively charged ions towards the cathode and negatively charged ones towards the anode (i.e.) current is carried by all ions present in solution. Conductance depends on the number of ions in solun.

Introduction  

Factors affecting conductance: 1- Temperature: (1C increase in temperature causes 2 % increase in conductance). 2- Nature of ions Size, molecular weight and number of charges. 3- Concentration of ions: As the number of ions increases, the conductance increases. 4- Size of electrodes Conductance is directly proportional to the cross sectional area (A).

G  A/l so G =KA/L Specific conductance (K) Conductance is directly proportional to the cross section area A and is inversely proportional to the length of a uniform conductor, Thus, G  A/l so G =KA/L where K is the specific conductance (the conductance when A and L are numerically equal). When unit of A and L is centimeter, K is: “the conductance of a cube of liquid one centimeter on a side” its unit is Ohm-1 Cm-1

Equivalent conductance (o ): “It is the conductance of a solution of one-gram equivalent of solute (with no respect of its volume) contained between two electrodes placed 1 cm apart.” Due to the interionic effects, the equivalent conductance (o ) is concentration dependant. The value of (o) (equivalent conductance at infinite dilution) is used for comparison purposes.

The magnitude of (o) is determined by the charge, size and degree of hydration of the ion. (o) is also known as limiting ionic conductance or ionic mobility. Table 6, shows limiting ionic conductances or ionic mobilities (o) in water at 25C for many ions.

Conductivity measurements Electrodes Two parallel platinized Pt. foil electrodes or Pt. black with electrodeposited a porous Pt. film which increases the surface area of the electrodes and further reduces faradaic polarization. 2.Primary standard solutions Primary standard KCl solution ,at 25℃, 7.419g of KCl in 1000g of solution has a specific conductivity of 0.01286Ω-1/cm.

Conductivity Cell and Wheat stone Bridge Avoid the change of temperature during determination 4.Wheat stone bridge :

Factors affecting conductivity Size of ions Temperature Number of ions Charge of ions Specific conductivity:-It is conductivity offered by a substance of 1cm length and 1sq.cm surface area. units are mhos/cm. Equivalent conductivity:-it is conductivity offered by a solution containing equivalent weight of solute in it.

Molar conductance of various ions at infinite dilution at 25℃

the largest equivalent conductances. H2O has a very low conductivity, Molar conductance of various ions at infinite dilution at 25℃ H+ and OH- ions have by far the largest equivalent conductances. H2O has a very low conductivity, So, acid-base titrations yield the most clearly defined equivalence points by conductometry.

Calculate the limiting equivalent conductance (ionic mobility) of: Molar conductance of various ions at infinite dilution at 25℃ Solved Example: Calculate the limiting equivalent conductance (ionic mobility) of: 1- H2SO4 2- Propionic acid 3- Propionic acid from (Ao) of HCl, sodium propionate and NaCl. Solution: 1. Limiting equivalent conductance (ionic mobility) (Ao) of H2SO4 = (2 x 350) + (2 x 80) = 860

(Ao) for propionic acid = = 350 + 36 = 386 Molar conductance of various ions at infinite dilution at 25℃ (Ao) for propionic acid = = 350 + 36 = 386 Na-propionate + HCl = NaCl + propionic acid (85.9) (426) (126.4) (?) 3. (Ao) for propionic acid = [(Ao) for HCl + (Ao) for Na-propionate - (Ao) for NaCl] = = 426 + 85.9 – 126.4 = 385.5 No significant difference between results in 2 & 3.

It can be used for the determination of:- APPLICATIONS OF CONDUCTOMETRY It can be used for the determination of:- Solubility of sparingly soluble salts Ionic product of water Basicity of organic acids Salinity of sea water (oceanographic work) Chemical equilibrium in ionic reactions Conductometric titration

CONDUCTOMETRIC TITRATIONS The determination of end point of a titration by means of conductivity measurements are known as conductometric titrations.

Types of conductometric titrations Acid-base titration Precipitation titration Replacement titration Redox (oxidation-reduction) titration Complexometric titration

Titration of strong acid ACID-BASE TITRATIONS Titration of strong acid (a) with strong base e.g. HCl with NaOH (b) with weak base e.g. HCl with NH4OH

Titration of weak acid (c) with strong base e.g. CH3COOH with NaOH (d) with weak base e.g. CH3COOH with NH4OH

PRECIPITATION TITRATIONS [K+ + Cl-] + [Ag+ + No3_]

REPLACEMENT TITRATIONS Salt of strong acid and weak base vs. strong base Ex: ammonium chloride vs. sodium hydroxide NH4Cl+NaOH→NH4OH+NaCl

REPLACEMENT TITRATIONS B. Salt of strong base and weak acid vs. strong acid eg. sodium acetate vs. hydrochloric acid CH3COONa + HCl → CH3COOH + NaCl

REDOX TITRATION Titration of ferrous ions with dichromate ions: 6 Fe2+ + Cr2O72- + 14H+→ 6Fe3+ + 2Cr3+ +7H2O

COMPLEXOMETRIC TITRATION Eample:- KCl vs. Hg(ClO4)2 Non-aqueous titrations can also be measured using conductometry. Eample:- a)titration of weak bases vs. perchloric acid in dioxan-formic acid. b)Titration of weak organic acids in methanol vs. tetra methyl ammonium hydroxide in methanol-benzene.

ADVANTAGES OF CONDUCTOMETRIC TITRATIONS No need of indicator Colored or dilute solutions or turbid suspensions can be used for titrations. Temperature is maintained constant throughout the titration. End point can be determined accurately and errors are minimized as the end point is being determined graphically. DISADVANTAGES OF CONDUCTOMETRIC TITRATIONS - non specificity - interference of high conc. of other electrolytes.

POLAROGRAPHY Ashraf M. Mahmoud, Associate professor

Introduction The earliest voltammetric technique Heyrovsky invented the original polarographic method in 1922, conventional direct current polarography (DCP). It employs a dropping mercury electrode (DME) to continuously renew the electrode surface. Diffusion is the mechanism of mass transport.

Introduction

Theory of polarography When an external potential is applied to a cell containing a reducing substance such as CdCl2, The following reaction will occur: Cd2+ + 2e + Hg = Cd(Hg) The technique depends on increasing the applied voltage at a steady rate and simultaneously record photographically the current-voltage curve (polarogram) The apparatus used is called a polarograph .

A typical polarograph Instrumentation – Three electrodes in solution containing analyte Working electrode: microelectrode whose potential is varied with time Reference electrode: potential remains constant (Ag/AgCl electrode or calomel) Counter electrode: Hg or Pt that completes circuit, conducts e- from signal source through solution to the working electrode Supporting electrolyte:excess of nonreactive electrolyte (alkali metal) to conduct current Inlet of inert gas (H2 or N2 to expel dissolved oxygen Outlet of inert gas

A typical polarograph

Dropping Mercury Electrode (Working electrode) Capillary tube about 10-15cm Int. diameter of 0.05mm A vertical distance being maintained betwwen DME and the solution Drop time of 1-5 seconds Drop diameter 0.5mm

Supporting electrolyte The supporting electrolyte is a solution of (KNO3, NaCl, Na3PO4) in which the sample (which must be electroactive) is dissolved. Function of the supporting electrolyte It raises the conductivity of the solution. It carries the bulk of the current so prevent the migration of electroactive materials to working electrode. It may control pH It may associate with the electroactive solute as in the complexing of the metal ions by ligands.

Polarographic measurements Polarography measurement is governed by ilkovic equation: id= 708 nD1/2m2/3t1/6C n= no. of electrons t= droptime(second/drop) D= diffusion coefficient of analyte (cm2/s) m= rate of flow of Hg through capillary (mg/s) C= analyte’s concentration in mM

Applied potential, V vs SCE Polarogram 0.001 M Cd2+ in 0.1 M KNO3 supporting electrolyte All Cd2+ around the electrode has already been reduced. Limiting Current at the electrode is reached by limiting the diffusion rate of Cd2+ from the solution to the electrode. Thus, current stops rising and levels off at a plateau Electrode become more and more reducing and capable of reducing Cd2+ Cd2+ + 2e- Cd Current starts to be registered at the electrode (decomposition potential) Current at the working electrode continue to rise as the electrode become more reducing and more Cd2+ around the electrode are being reduced. Diffusion current of Cd2+ is not limited id Current i (A) Working electrode is no yet capable of reducing Cd2+  only small residual current flow through the electrode Base line of residual current E½ Half –wave potential -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 Applied potential, V vs SCE

Polarogram The Polarogram is characterized by the following parameters: Residual current Limiting current Diffusion current Half wave potential (E1/2)

A typical Polarogram

Factors affecting electrode reaction rate and current A- Mass transfer The movement of sample from one location in solution to another, it arises from either: 1. Migration (under the influence of electric field difference), 2. Diffusion (under influence of concentration difference), 3. Convection (under the influence of stirring), Diffusion Chemical reaction at the electrode surface

Factors affecting electrode reaction rate and current Pb2+ K+ -1.0 V vs SCE Pb2+ + 2e- Pb Layers of K+ build up around the electrode stop the migration of Pb2+ via coulombic attraction Concentration gradient created between the surrounding of the electrode and the bulk solution Pb2+ migrate to the electrode via diffusion

Advantages and disadvantages of DME Its surface is reproducible, smooth and continuously renewed, this eliminates the poisoning effect. Mercury forms amalgams (solid solution) with many metals. The diffusion current assumed a steady value immediately after each change of applied potential and is reproducible. The large hydrogen over-potential of mercury renders possible deposition of substance that difficult to reduce. The surface area can be calculated from the weight of drop. Disadvantages of DME At potential more positive than + 0.4 V vs SCE, mercury dissolves producing anodic polarographic wave which masks other waves, therefore DME can be used only for the analysis of reducible or easily oxidizable substances. The capillary is very small so easy to be blocked→ malfunction of the electrode Mercury is very toxic and easily oxidized

Application of Polarography A. Qualitative: by using the half wave potential which is characteristic to each substance B. Qualitative INORGANIC ANALYSIS Analysis of metals Zn Cd Analysis of anions as dromate, iodate, etc. ORGANIC ANALYSIS Analysis of carbonyl,peroxide, nitro, azo group, etc. Biochemical analysis

Application of Polarography Advantages of using polarography in pharmaceutical analysis 1- Only small volume of sample is required. 2- Turbid and coloured solutions can be analyzed. 3- It can be used for the determination of substances, which are not electrochemicall active (indirect). 4- Prior separation of excepients is not required. 5- Its sensitivity is sufficient for the determination trace elements and toxic impurities. 6- Samples of natural origin 7- high speed analysis which is important for QC

Good Luck Ashraf M. Mahmoud

RECENT DEVLOPEMNTS In refinary industries. Estimation of polyelectrolytic solution. Biotechnology. Microbiosensors for enviromental monitoring.

References Gurdeep.R. chatwal,sham k.anand,instrumental method of chemical analysis,himalaya publishing house,2008,p.no.2.482-2.497. Hovert H.willard,lynne L.merritt,john A.dean,frank A.settle,jr.,instrumental method of analysis CBS publishers 1986,p.no.732-750. Kenneth A. connors,e textbook of pharmaceutical analysis,third edition,wiley india,p.no. 334. Danniel christein,analytical chemistry,2nd edition,wiley india,p.no. 274. www.pharmapaedia.com

Continued…. www.authorstream.com Kissinger, P. T., AND W. W. Heineman, eds., Laboratory Techniques in Electroanalytical Chemistry, Dekker, New York, 1984. A.H.beckett ,J.B. stenlake,practical pharmaceutical chemistry,fourth edition ,part –two,p.no-91. Lingane, J.J., Electroanalytical Chemistry, 2nd ed., Wiley- Interscience, New York, 1958