CONDUCTIMETRY D2

GROUP MEMBERS

Amponsah Kodua AngelaEssilfie SamuelOnyekaba Adaobi BlessingAgyei Gyimah Dominic

Introduction InstrumentationPrinciple of OperationConductimetric TitrationsModern AdvancementsApplications in Pharmacy

INTRODUCTION

What is CONDUCTIMETRY?

It is the measurement of the conductance of a solution OR

The study of how ions move in solution based on its electrical properties

INTRODUCTION The conductance of an electrolytic

solution depends on i. Concentration of ions ii. Degree of Solvation iii. Temperature iv. Potential difference of the solution v. Viscosity of the medium

Units of conductivity are Siemens/cm or mhos/cm

INTRODUCTIONOhm’s law can be written symbolically as

V=IR where V=Potential difference in Volts. I=Current in Amperes.R=Resistance of the conductor in Ohms.The reciprocal of the resistance (R)is known

as the conductance (G) of the solution Conductance (G)=1/R

INTRODUCTION conductance

Reciprocal of resistance R and is measured in siemens or mho (ohm-1)

Specific conductance Reciprocal of the specific resistance and is measured in (ohm-1cm-1 or siemenscm-1)

Depends on the number of ions

INTRODUCTION Equivalent conductance is the conductance of a

solution containing 1g equivalent of the electrolyte, where it dissolved in a volume, Vml, of solution, and measured in a cell with electrodes 1cm apart and of sufficient cross-sectional area to contain all the electrolyte

Λ=1000Kc C Equivalent conductance is used to determine the

conductivity of two solutions of the same electrolyte at different concentration

INTRODUCTION The molar conductivity Λm is the product of the

conductivity k and the volume in m3 (litres) containing 1 molecular weight of the solute.

Λm=kv= k Cell constant

Given by the ratio l/a which can be calculated from dimensions of electrodes or measurement of the conductance of a known concentration of KCl solution of known resistivity

INTRODUCTIONThe conduction of ions in electrolyte

solutions obey Ohm’s law. Ohm’s law states that the current

passing through a conductor is directly proportional to the potential difference applied and inversely proportional to the resistance of the conductor, provided temperature and other factors remain constant.

INSTRUMENTATIONThe conductivity

apparatus/conductimeter is the instrument employed in conductimetry.

This is used to measure the conductance of an electrolyte solution.

There are different types of conductimeters but our emphasis for this presentation will be on the Mullard Conductimeter

INSTRUMENTATION

An ac sourceProbe Platinum ElectrodesDisplay screen

INSTRUMENTATIONAn oscillator – this is made to measure

the frequency of the ac.The Wheatstone bridge- the operation

of the conductimeter depends on it.The amplifier – this increases the

signals coming from the Wheatstone bridge

INSTRUMENTATION

The electrodes are always kept in conductivity water.

Ordinary water or distilled laboratory water is not used because it may contain dissolved salts, gases such as ammonia and carbon dioxide.

Conductivity water has a conductivity of less than 0.1×10-6mho.

PRINCIPLE OF OPERATION OF THE MULLARD CONDUCTIMETERThe principle of operation is based

on the mechanism of the wheatstone bridge.

The wheatstone bridge mainly deals with resistances of two legs of a bridge circuit

THE WHEATSTONE BRIDGE

THE WHEATSTONE BRIDGE - OPERATIONThe two resistances R1 and R3 are of fixed

resistances while R2 and Rx are variable.Rx represents the substance whose

resistance is to be measured.R2 is a variable resistor (rheostat)When the ratio of the resistances at the left

leg (R1/R2) equals the ratio of the resistances at the right leg (R3/Rx), there will be no voltage, no current will be generated hence the galvanometer shows no deflection.

OPERATIONMullard Conductimeter converts restance

values to conductance by C= 1/RThe set-up measures electroconductivity

by measuring the ionic conductances of the individual components and summing to give the conductance of the whole solution.

PRINCIPLE OF OPERATION The meter is equipped with a probe, usually

handheld, for field or on-site measurements. After the probe is placed in the liquid to be measured, the meter applies voltage between two electrodes inside the probe. Electrical resistance from the solution causes a drop in voltage, which is read by the meter. The meter converts this reading to milli- or micromhos or milli- or microSiemens per centimeter. This value indicates the total dissolved solids.

CONDUCTIMETRIC TITRATION CURVESThe basic principle underlying

these titrations is the substitutions of ions of one conductivity by ions of another conductivity.

The addition of an electrolyte to a solution of another electrolyte under conditions producing no appreciable change in volume will affect the

CONDUCTIMETRIC TITRATION CURVESconductance of the solution

according to whether or not ionic reactions occur.

A graph of Conductance of the solution is plotted against the Volume of solution and the equivalence end point is determined.

STRONG ACID – STRONG BASE TITRATION

CONDUCTANCE

Endpoint VOLUME

STRONG ACID – STRONG BASE TITRATIONAcid dissociates completely to

release more hydrogen ions in solution. Hydrogen ions are strong electrolytes and so have high conductance.

Addition of strong base which has OH ions (also a strong electrolyte) causes the conductance to drop sharply.

STRONG ACID – STRONG BASE TITRATIONThe conductance starts to rise

sharply again after the end point is passed since the base is in excess.

Endpoint is reached when the two plots of conductance intersect.

WEAK ACID – STRONG BASE TITRATION

CONDUCTANCE

Endpoint VOLUME

WEAK ACID – STRONG BASEWeak acid dissociates partially to

release less hydrogen ions in solution and so have less ability to conduct.

Addition of strong base which is a strong electrolyte exerts its conducting action in the solution and causes the conductance to rise gradually.

WEAK ACID – STRONG BASEThere is a sharp rise in the

conductance after the end point since all the weak base is neutralized and the strong base is in excess.

The endpoint is found at the point where both phases meet.

STROND ACID – WEAK BASE TITRATIONCONDUCTANCE

Endpoint VOLUME

STRONG ACID – WEAK BASEAcid dissociates completely

releasing more and powerful H+ ions in solution.

Addition of the base causes the conductance to reduce until all the acid is neutralized.

After the endpoint is reached, the ions of the weak base cannot strongly ionize and so there is no change in the levels of conductance.

WEAK ACID – WEAK BASE TITRATION

CONDUCTANCE

Endpoint VOLUME

WEAK ACID – WEAK BASE

The H+ ions from the weak acid are few and are a result of partial hydrolysis but the ions from the base which is causing the neutralization is also from a weak base.

This causes the conductance to rise considerably until the endpoint is reached where all the acid has been neutralized.

The conductance remains constant after the base is in excess.

MIXTURE OF STRONG AND WEAK ACIDS AGAINST STRONG BASE

CONDUCTANCE a = 1 st endpoint b = 2nd endpoint

a b VOLUME

MIXTURE OF STRONG AND WEAK ACIDS AGAINST STRONG BASEStrong acid is dissociated

completely to release more hydrogen ions. This causes the strong acid to react fast and exert its conductance in the solution.

A first endpoint is produced when all the acid is neutralized.

MIXTURE OF STRONG AND WEAK ACIDS AGAINST STRONG BASE The ions of the weak acid then undergo partial

dissociation to release less hydrogen ions in the solution. The ions from the strong base neutralize the weak acid and cause the conductance to rise since they hydrogen ions here are few and partially dissociated.

After this, another endpoint is arrived at where all the weak acid has been neutralized.

The conductance then begins to rise again when the base is in excess.

MODERN ADVANCEMENTS

MODEL SC72Features• Waterproof (IP67), sealed case and connector cover to keepout moisture• Autoranging allows 0 to 2000 mS/cm capability• Large, easy-to-read display• Automatic Temperature Compensation• Self-Diagnostics warn of fault conditions• Auto power off extends battery life• 300 Points of Data Memory

MODEL ES-51 Automatic data-logging function.

Self diagnostic function assures reliable measurement.

User-friendly features and portable

LCD with large display.

Quick connection to PC allows easy and fast data evaluation.

MODEL B-173A pocket size conductimeter.Can accurately measure minute samples in its unique built-in sampling chamber.Auto ranging and a measurement range from 1 µS/cm to 19.9mS/cm. Auto-calibration and automatic temperature conversion.Can be operated manually too.Salinity conversion function (from 0 to 1.1%).

A modern conductivity meter with cell and standard KCl solution fordetermination of cell constant

DRY CELL BATTERY OPERATED METER

• Long data logging system

• Measurement at depths as low as 60 meters.

• pH & conductance

MODEL CAU-04

APPLICATIONS IN PHARMACYSolubility of sparingly soluble saltsIonic product of waterBasicity of organic acidsPurity of water and solutions

needed for pharmaceutical products

APPLICATIONS IN PHARMACYTo check drug delivery where

charged molecules are transported into and through the skin by application of a weak direct electrical current

Determination of end point in volumetric analysis

REFERENCES

Olaniyi A. A. and Ogungbamila F. O (1991) Experimental Pharmacy Chemistry (1st Edition) Shaneson C. I. Limited Ibadan Pages 162 – 166