Analytical Determination by Potentiometric Methods

Potentiometry is an analytical technique used to determine the concentration of an unknown solution by measuring the electrical potential (voltage) of an electrochemical cell without drawing significant current. It involves the use of indicator and reference electrodes, where the potential difference changes with ion concentration. Potentiometric methods are widely used for acid-base, redox, precipitation, and complexometric titrations due to their accuracy, simplicity, and ability to measure without color indicators.

1. PHARMACEUTICAL ANALYSIS III-B COURSE NO. 516

2. Course Outline 1.ELECTROCHEMICAL ANALYSIS Rationale Potentiometry Karl Fischer titration 2.THERMAL ANALYSIS Thermogravimetric analysis Practical and research applications Differential scanning Calorimetry Differential thermal analyzer 3. POLAROGRAPGHY POLAROGRAPGHY theory Instrumentation Application 4. TITRIMETRIC ANALYSIS Acid-base titration Redox Titration Argentometric Titration Complexometric Titration Gravimetric Titration Non-Aqueous Titration 5. RADIOCHEMICAL TECHNIQUES

3. Potentiometry Chapter 4

4. Electrochemistry Electrochemistry is a branch of physical chemistry that studies the relationship between electricity as a measurable and quantitative phenomena and identifiable chemical change. These reactions involve electric charges moving between electrodes and an electrolyte. Thus electrochemistry deals with the interaction between electrical energy and chemical energy.

5. ELECTROCHEMICAL CELL An electrochemical cell are either use chemical reactions to generate electricity or conversely use electricity to provide energy for useful chemical reaction. It is divided into two types Galvanic cell Electrolytic cell

6. GALVANIC CELL (VOLTAIC CELL) ELECTROLYTIC CELL Anode --- - ve charged—oxidation Cathode -- + ve charged --- reduction Anode--- + ve charged – Cathode --- - ve charged Electrical energy is produced by redox chemical reaction It consume electricity for the chemical reaction Spontaneous reaction (no need of current) Non-spontaneous reaction (an external source of electrical energy is used to generate a potential difference between electrodes ) Two compartments Single compartment

7. Basic definitions Potential refers to the electric potential difference between two points in an electric field. In potentiometry, this electric potential (often measured in volts) is related to the chemical potential of ions in a solution. Electric Potential (V): It is the measure of the work needed to move a charge from one point to another in an electric field. Potential Difference is the difference in electric potential between two points in an electric circuit or field. In potentiometry, this is the difference in potential between the reference electrode and the indicator electrode when they are immersed in a solution.

8. Potentiometry Potentiometry is one type of electrochemical analysis methods. Electrochemistry is a part of chemistry which determines electrochemical properties of a substances. An electric circuit is required for measuring current(Ampere A) and potential (voltage-volt) denoted by V, created by movement of charged particles.

9. Introduction to Potentiometry In 1889, Nernst proposed an equation that states that the electrochemical cell potential is directly proportional to the concentration of the sample solution. In 1906, Cremer explained that the potential difference exists between the reference electrode and the indicator electrode. In 1909, pH electrode was developed for the measurement of the potential. Potentiometry is mainly used to determine the potential or the electromotive force of a sample solution. The potential is directly proportional to the concentration of the ions. The potential difference is determined by the electrochemical cell which is composed of a pair of electrodes namely the indicator electrode and the reference electrode.

10. Principle of Potentiometry The principle involved in 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.

11. Reference Electrode and Salt Bridge The reference electrode is the electrode which consists of its own potential value and it is stable when dipped into the sample solution. The salt bridge is used to prevent the interference of the analyte solution with the reference solution.

12. Analyte and Indicator Electrodes Analyte solution is the sample solution whose potential is to be measured. The indicator electrode is the electrode which responds to the changes in the potential of the analyte solution.

13. Electromotive Force 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 the electromotive force of the indicator electrode; E junction is the electromotive force at the junction of the liquid.

14. Theory - Nernst Equation The main theory involved in potentiometry is when the known potential electrode is immersed in the sample solution then the potential is given by the 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; 0.0592 is the value obtained from the RT/F, where R is the gas constant, T is the temperature in Kelvin, F is the Faraday’s constant.

15. Factors Affecting Nernst Equation Factors Affecting Nernst Equation: (a) Temperature dependent. (b) Presence of additional potentials.

16. Charge Transfer Process This can be achieved by the charge transfer process by the following steps: 1. Electrons move from the reference electrode to the indicator electrode. 2. The positive ions move to the cathode and negative ions move to the anode. 3. On the surface of the electrodes, electrons are transferred to ions.

17. Ideal Requirements of Reference Electrodes Stability Reversibility Reproducibility

18. Standard Hydrogen Electrode Usage : Primary reference electrode Composition : Platinum foil coated with platinum black, immersed in HCl solution Process : Hydrogen gas passed over platinum foil through side tube Applications : Entire pH range Disadvantages : Instability due to oxidizing/reducing agents, purity of hydrogen ​

19. Saturated Calomel Electrode Composition : Porous disc at the base Glass tube filled with potassium chloride crystals Calomel paste (mercury chloride with pure mercury and potassium chloride solution) Pure mercury Advantages : Easy to construct and highly stable Reaction of Saturated Calomel Electrode Hg2Cl2+2e−→2Hg(l)+2Cl2

20. Silver-Silver Chloride Electrode Similar to Saturated calomel electrode Composition : Silver wire coated with silver chloride solution Dipped in saturated potassium chloride solution Ag/AgCl,  KCl AgCl(s)+e−→Ag(s)+Cl−

21. Indicator Electrode Function : Measures the potential of the analyte solution comparing with the reference electrode Proportional to : Ion concentration Examples : Hydrogen electrode Glass electrode Antimony-antimony oxide electrode

22. Classes of Indicator Electrodes Metal indicator electrodes Ion-selective electrodes Metal Indicator Electrodes Function : Develop electric potential in response to redox reactions on the metal surface Common Materials : Platinum or Au Disadvantage : Not very selective

23. Types of Metal Indicator Electrodes First Kind Electrodes Metal rod immersed in its metal solution Examples : Silver electrode in silver nitrate solution: Ag++e−→Ag Copper electrode in copper sulphate solution: Cu2++2e−→Cu

24. Second and Third Kind Electrodes Second Kind Electrodes Metal wires coated with salt precipitate Respond to changes in ionic activity through complex formation Examples : Ag/AgCl/ KCl , Hg/ Hg₂Cl ₂/ KCl Third Kind Electrodes Known as inert electrodes and redox electrodes inert metal electrode immersed in redox solution Example : Pt-H₂ electrode

25. Ion-Selective Indicator electrode Glass Membrane Electrode Function : Composed of ion-selective membrane, creates imbalance, opposes further ion movement Examples : Glass membrane electrode, Antimony-antimony oxide electrode Glass Membrane Electrode: Most common ion-selective electrode Composition : Glass tube with a thin glass bulb Silver-silver chloride wire immersed in 0.1N HCl Setup immersed in solution to measure potential ​

26. Type of Electrode Material/Composition Application Advantages Disadvantages Reference Electrode Silver/Silver Chloride (Ag/AgCl), Calomel (Hg/Hg2Cl2) Provides a stable reference potential Stable, Reliable Sensitive to temperature changes, Requires maintenance Indicator Electrode Glass Electrode, Metal Electrodes (Platinum, Gold), Ion-Selective Electrodes (ISE) Measures the potential of the analyte solution High selectivity, Versatile Can be expensive, Fragile (glass electrode) Ion-Selective Electrode (ISE) Specific membranes (e.g., glass for H+, PVC for other ions) Selectively measures specific ions (e.g., pH electrode for H+, fluoride electrode for F-) High selectivity, Can be used in complex matrices Requires calibration, Limited by ion interference Auxiliary Electrode (Counter Electrode) Platinum, Graphite Completes the circuit in potentiometric measurements Chemically inert, Good conductivity Can be expensive, Requires maintenance Redox Electrode Platinum, Gold, Glassy Carbon Measures redox potentials in solutions High sensitivity, Wide range of applications Sensitive to fouling, Requires clean surface