Potentiometry

Potentiometry

1. Potentiometry Ayesha shafi

2. OBJECTIVES Introduction Principle Theory Types of the electrode Reference electrode

3. Introduction to potentiometry Potentiometry is a branch of electrochemistry which deals with the study and measurement of electrode potential . Electrode potential is the measurement of tendency of an oxidation or a reduction reaction to occur at an electrode. Purpose: to measure the PH and potentiometric titration. Direct and selective measurement of analyte concentration Determination of equilibrium constant. Establishment of end point in various types of potentiometric titration

4. Potentiometry Principle: The principle involved in the Potentiometry is when the pair of electrodes is placed in the sample solution it shows the potential difference by the addition of the titrant or by the change in the concentration of the ions. 4

5. Theory: The main theory involved in the potentiometry is, when the known potential electrode immersed in the sample solution then the potential is given by Nernst equation : E= E +(0.592/n) log c Where E is the potential of the solution; E is the standard electrode potential; n is the valency of the ions; c is the concentration of the sample solution; 0.592 is the value obtained from the RT/F; where R is the gas constant, T is the temperature in Kelvin, F is the faradays constant. Electrodes: These are mainly used to measure the voltages. Mainly two electrodes are used in the potentiometry .They are as follows: Reference electrode Indicator electrode Reference electrode : These are mainly used for the determination of the analyte by maintaining the fixed potential. Ex: Standard hydrogen electrode 5

6. Continue… The reference electrode is the electrode which contains of its own potential value and it is stable when dipped into sample solution. The salt bridge is used to prevent the interference of the analyte solution with that of reference solution. Here analyte solution is the solution whose potential is to be measured. The indicator electrode is the electrode which responds to change in the potential of analyte solution The electromotive force of the complete cell is given by the following equation: E cell = E reference + E indicator + E junction where E reference is the electromotive force of the reference electrode , E indicator is electromotive force of indicator electrode, E junction is the electromotive force at the junction of the liquid.

7. The reference electrodes are classified into two main classes they are as follows : Primary standard electrodes ex: Standard hydrogen electrode Secondary standard electrodes ex: silver-silver chloride electrode saturated calomel electrode Indicator electrode: It is used to measure the potential of the analyte solution comparing with that of reference electrode . Its potential is directly proportional to ion concentration. Ex: Hydrogen electrode. Glass electrode. Antimony –antimony oxide electrode. There are two classes of indicator electrodes: Metal indicator electrodes Ion-selective electrodes 7

8. Properties of ideal reference electrode It should be accurately known and has the constant potential. It should be completely insensitive to the concentration of analyte solution. It should maintained a constant potential irrespective of the current that passes through it. It should be easy to assemble. It must be robust and unaffected by processes.

9. Saturated calomel electrode It contains of an inner jacket and outer sleeve Inner jacket has wire contact with Hg and plugged with a mixture of calomel Hg 2 Cl 2 & KCl. Outer sleeve – crystals of KCl & porous plug of asbestos. Space b/w inner jacket & outer sleeve is filled with either saturated KCl or 1M KCl or 0.1M KCl. Merits of saturated calomel electrode Ease of construction Stability of potential

10. Application of saturated calomel electrode The SCE is used in pH measurement, cyclic voltammetry and general aqueous electrochemistry. This electrode and the silver/silver chloride reference electrode work in the same way. In both electrodes, the activity of the metal ion is fixed by the solubility of the metal salt. The calomel electrode contains mercury, which poses much greater health hazards than the silver metal used in the Ag/ AgCl electrode

11. Silver-silver chloride electrode The silver/silver chloride reference electrode is a widely used reference electrode because it is simple, inexpensive, very stable and non-toxic. it is mainly used with saturated potassium chloride (KCl) electrolyte, but can be used with lower concentrations such as 1 M KCl and even directly in seawater. Silver wire coated electrolytically with silver chloride and dipped into KCl It has the advantage that it is easy to use. Demerit is that it is difficult to prepare.

12. Standard hydrogen electrode Is an important electrode as the potential of other electrode is measured with respect to it. Platinum coil coated with platinum black t o provide the large surface area for adsorption of hydrogen gas. Platinum takes no part in electrochemical reaction. T he electrolyte solution is HCL of unit activity (1.18M HCL at 25◦c) kept saturated with hydrogen gas.

13. Application and Limitation 1) For the determination of electrode potential of metal electrode system . 2) For the determination of pH of the solution. Limitation: It is rather difficult to regulate the pressure of the H2 gas to be at exactly 1atm throughout the experiment. Excess of H2 bubbling out carries little HCl with it and hence the H+ concentration decreases. In such a system, it is difficult to maintain the concentration of HCl at 1M. 4) Platinum foil gets easily poisoned by the impurities present in the gas and HCl. In fact, the attainment of equilibrium is ensured by trial and error.

14. APPLICATIONS OR IMPORTANCE OF REFERENCE ELECTRODES PH sensors Glucose sensors Gas sensor F ield effect transistor

15. INDICATOR ELECTRODE An indictor Electrode is the electrode system having a potential that varies with the variation in the concentration of analyte. Characteristic of the ideal electrode: A n ideal electrode responds rapidly. It can bring changes in the concentration of the analyte group/ ion.

16. INDICATOR ELECTRODE The potential of this electrode is proportional to the concentration of analyte. Two classes of indicator electrodes are used in potentiometry: M etallic electrodes Electrodes of the first kind Electrode of the second kind Redox electrode Membrane electrodes (ion-selective electrodes) G lass pH electrode

17. Metallic electrodes Electrodes of the first kind A metal in contact with a solution containing its cation. The most common ones: Silver electrode ( dipping in a solution of AgNO 3 ) Ag + + e ↔ Ag Copper electrode Cu +2 + 2e ↔ Cu  Zn electrode Zn +2 + 2e ↔ Zn

18. Electrode of the second kind A metal wire that coated with one of its salts precipitate. Respond to changes in ion activity through formation of complex. A common example is silver electrode and AgCl as its salt precipitate. This kind of electrode can be used to measure the activity of chloride ion in a solution. Electrode of the third kind A metal can respond to the concentration of another metal ion in which it is in contact. These electrodes are rarely used. Metallic electrodes

19. Redox electrode An inert metal is in contact with a solution containing the soluble oxidized and reduced forms of the redox half-reaction. The inert metal usually is platinum ( Pt ). The potential of such an inert electrode is determined by the ratio of the reduced and oxidized species in the half-reaction. A very important example of this type is the hydrogen electrode.

20. Ion selective electrode An ion-selective electrode (ISE), also known as a specific ion electrode (SIE), is a transducer (or sensor) that converts the activity of a specific ion dissolved in a solution into an electrical potential, which can be measured by a voltmeter or pH meter. indicator electrode based on determination of cations or anions by the selective absorption of these ions to a membrane surface.

21. TYPES OF ION SELECTIVE ELECTRODE •Glass Membrane Electrode •Solid State Electrode •Liquid Membrane Electrode •Gas Sensing Electrode

22.   GLASS MEMBRANE ELECTRODE • Responsive to univalent cations ( H+ , Na +) • Glass electrodes available for Na+, K+, Li +, Ag +( cations only) by varying glass composition • The selectivity for this cation by varying the composition of a thin ion sensitive glass membrane . Glass membrane manufactured from SiO2 with negatively charged oxygen atom . • Inside the glass bulb, a dilute HCl solution and silver wire coated with a layer of silver chloride . The electrode is immersed in the solution and pH is measured • Example : pH electrode

23. Continue…. Glass pH electrode Advantages over other electrodes for pH measurements: Its potential is essentially not affected by the presence of oxidizing or reducing agents. It operates over a wide pH range. It responds fast and functions well in physiological systems. Selective for monovalent cations only because polyvalent ions can not penetrate the surface of membrane.

24. pH electrode Selective for the detection of hydrogen ions. The measuring or indicator electrode has a “glass membrane” pH is then determined from potential between the pH electrode and a standard reference electrode .

25. SOLID STATE ELECTRODE Solid state electrode are selective primarily to anions . It may be a - homogenous membrane electrode - heterogeneous membrane electrode . Homogenous membrane electrode: ion-selective electrodes in which the membrane is a crystalline material ( AgI /Ag2S).

26. Heterogeneous membrane electrode: ion-selective electrodes prepared of an active substance, or mixture of active substances (silicone rubber or PVC) Example : Fluoride ion selective electrode.

27. LIQUID MEMBRANE ELECTRODE Liquid membrane is a type of ISE based on water- immiscible liquid substances produced in a polymeric membrane used for direct potentiometric measurement . Used for direct measurement of several polyvalent cations ( Ca ion) as well as a certain anions . Inner compartment of electrode contains reference electrode & aqueous reference solution. Outer compartment – organic liquid ion exchanger

28. • The polymeric membrane made of PVC to separate the test solution from its inner compartment which c ontains standard solution of the target ion. • The filling solution contains a chloride salt for establishing the potential of the internal Ag/ AgCl wire electrode.

29. GAS SENSING ELECTRODE Available for the measurement of ammonia, carbon dioxide and nitrogen oxide . This type of electrode consist of permeable membrane and an internal buffer solution . The pH of the buffer changes as the gas react with it . The change is detected by a combination pH sensor. This type of electrode does not require an external reference electrode.

30. Measurement of PCO 2 in routine blood gases A modified pH electrode with a CO 2 permeable membrane covering the glass membrane surface A bicarbonate buffer separates the membranes Change in pH is proportional to the concentration of dissolved CO 2 in the blood pco 2 electrode

31. Application of Potentiometric Measurement Clinical Chemistry Ion-selective electrodes are important sensors for clinical samples because of their selectivity for analytes. The most common analytes are electrolytes, such as Na + , K + , Ca 2+ ,H + , and Cl - , and dissolved gases such as CO 2 . Environmental Chemistry For the analysis of of CN - , F - , NH 3 , and NO 3 - in water and wastewater.

32. Potentiometric Titrations pH electrode used to monitor the change in pH during the titration. For determining the equivalence point of an acid–base titration. Possible for acid–base, redox, and precipitation titrations, as well as for titrations in aqueous and non-aqueous solvents. Agriculture NO 3 , NH 4 , Cl , K, Ca , I, CN in soils, plant material, fertilizers. Detergent Manufacture Ca , Ba, F for studying effects on water quality

33. Food Processing NO 3 , NO 2 in meat preservatives Salt content of meat, fish, dairy products, fruit juices, brewing solutions. F in drinking water and other drinks. Ca in dairy products and beer. K in fruit juices and wine making. Corrosive effect of NO 3 in canned foods.