Conductometry (Pharmaceutical analysis)

Introduction
Instrumentation
Conductometric titration
Applications
Advantages & disadvantages

1. DISCOVER . LEARN . EMPOWER Mr. Yunes Alsayadi Assistant Professor of Pharmaceutical Analysis E 10695 UNIVERSITY INSTITUTE OF PHARMA SCIENCES B. Pharm Conductometry

2. Topics to covered Introduction Instrumentation Conductometric titration Applications Advantages & disadvantages

3. Introduction Conductometry  is used to analyze ionic species and to monitor a chemical reaction by studying the electrolytic conductivity of the reacting species or the resultant products . It has notable applications in analytical chemistry. Conductivity measurement can be performed directly by using a conductivity meter or by performing conductometric titration . Conductometric analysis of electrolytes is a long-time practice.

4. Definition It is an electrochemical method of analysis used for the determination or measurement of the electrical conductance of an electrolyte solution by means of a conductometer . Electric conductivity of an electrolyte solution depends on : Type of ions (cations, anions, singly or doubly charged Concentration of ions Temperature Mobility of ions

5. Principle The main principle involved in this method is that the movement of the ions creates the electrical conductivity. The movement of the ions is mainly depended on the concentration of the ions. The electric conductance in accordance with ohms law which states that the strength of current ( i ) passing through conductor is directly proportional to potential difference & inversely to resistance. i =V/R

6. Important Definitions and Relations Conductance (G): ease with which current flows per unit area of conductor per unit potential applied & is reciprocal to resistance (R) , its unit is Siemens (ohm -1 ) G = 1⁄ R Resistance (R): is a measure of the conductors opposition to the flow of electric charge, its unit is ohm. R =1/G Specific resistance (ρ): resistance offered by a substance of 1 cm length (l) and 1 sq.cm surface area (A), its unit is ohm cm ρ = aR /l Specific conductivity ( k v ): conductivity offered by a substance of 1 cm length (l) and 1 sq. cm surface area, its unit is siemens cm -1 k v =1 ⁄ ρ

7. Equivalent conductivity ( λ v ): conductivity of a solution containing equivalent weight of the solute between electrodes 1 cm apart and 1 sq. cm surface area, its unit is siemens cm -1 Equivalent conductivity = specific conductivity ( k v ) X volume of solution containing 1 gram equivalent weight of electrolyte Molar conductivity (µ v ): conductivity of a solution containing molecular weight of the solute between electrodes 1 cm apart and 1 sq. cm surface area Molar conductivity = specific conductivity ( kv ) X volume of solution containing 1 molecular weight of electrolyte

8. The conductance of the solution depends on:  Temperature: It is increased by increase of temperature. Nature of ions: size, molecular weight, number of charges the ion carries and other factors The concentration of ions: As the number of ions increases the conductance of the solution increases. The size of the electrodes.

9. Instrumentation  The instrument used for measurement of conductance are known as conductometers . It consists of : Current source Mechanical high frequency AC generator. DC current is not employed in conductance measurement because electrodes becomes polarised leading to high cell resistance. Conductivity cells Made of pyrex or quartz and are fitted with two platinum electrodes. Should be placed in vessel containing water to maintain constant temperature Types : 1 . Wide mouthed cell 2 . Cell for reactions producing precipitation 3 . Dip type cells Instrumentation

10. Electrodes: Platinum sheets, each of 1 cm 2 are fixed at distance of 1 cm The surface is coated with platinum to avoid polarization effects and increase effective surface area. Platinisation of electrodes is done by coating solution of 3% chlorplatinic acid and lead acetate on it to get uniform coating. Electrodes usage depends on conductivity and concentration. If concentration is low then electrodes should be largely and closely packed.

11. Measurment  The instrument used to measure conductance is called conductance bridge or conductometer . Classical circuit employed for measurement is wheatstone bridge. Various types are: Kohlrausch conductance bridge Direct reading conductance bridge Phillips conductance bridge Mullard’s conductance bridge Pye’s conductance bridge Measurement

12. The measured conductivity is not always equal to the specific conductivity of solution, because the physical configuration of platinum electrode i.e , length and area of electrodes varies from one another Hence conductivity of solution is obtained by calculating a factor called “cell constant”. Cell constant: Defined as ratio of distance between the two electrodes(l) to the area of electrodes(A) Therefore, θ =1 ⁄ A

13. Conductometric Titrations Principle The determination of end point of a titration by means of conductivity measurements is known as conductometric titration. During the course of titration, the conductivity of the solution changes, since there is change in the number and the mobility of ions. At the end point of the titration, there is a sharp change in the conductivity of a solution shown by the intersection of the lines in the graph of conductivity Vs volume of titrant added.

14. Types of conductometric titrations Acid –base or neutral titrations Strong Acid with a Strong Base, e.g. HCl with NaOH When the base is added in HCl solution, the conductance falls due to the replacement of hydrogen ions by the added cation as H + ions react with OH − ions to form undissociated water. After the equivalence point, the conductance increases due to the large c onductivity of OH - ions

15. Weak Acid with a Strong Base, e.g. acetic acid with NaOH Initially the conductance is low due to the feeble ionization of acetic acid, followed by increase due to NaOH Beyond the equivalence point, steep rise due to excess of NaOH .

16. Strong Acid with a Weak Base, e.g. sulphuric acid with dilute ammonia Initially the conductance is high and then it decreases due to the replacement of H +. But after the endpoint has been reached the graph becomes almost horizontal , since the excess aqueous ammonia is not appreciably ionised in the presence of ammonium sulphate

17. Weak Acid with a Weak Base, e.g. sulphuric acid with dilute ammonia The nature of curve before the equivalence point is similar to the curve obtained by titrating weak acid against strong base. After the equivalence point, conductance virtually remains same as the weak base which is being added is feebly ionized and, therefore, is not much conducting

18. Precipitation Titrations   eg Potassium chloride Vs Silver nitrate KCl + AgNO 3 AgCl + KNO 3 When silver nitrate added, the first part of the curve shows no increase in conductivity as there is only replacement of chloride ions with nitrate ions. After end point conductivity increases because of increase in the concentration of silver as well as nitrate ions.

19. Application Check water pollution in rivers and lakes. Solubility of sparingly soluble salts like AgCl , BaSO4 can be detected Determination of atmosphericSO2,etimation of vanillin in vanilla flavour Applications Alkalinity of fresh water. Salinity of sea water (oceanography) U sed to trace antibiotics. Deuterium ion concentration in water- deuterium mixture Food microbiology- for tracing micro organisms Tracing antibiotics Estimate ash content in sugar juices Purity of distilled and de - ionised water can determined

20. Advantages Determination of specific conductivity is not required. No indicator is necessary. Suitable for coloured or dilute solutions. Since end point is determined by graphical means accurate results are obtained with minimum error. Used for analysis of turbid suspensions, weak acids, weak bases, mix of weak & strong acids.

21. Disadvantages  Increased level of salts in solution masks the conductivity changes , in such cases it does not give accurate results. Application of conductometric titrations to redox systems is limited because, high concentrations of hydronium ions in the solution tends to mask the changes in conductance

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