Paper No: 2 Analytical Chemistry Module: 23 Potentiometry Development Team Principal Investigator Prof. R.K. Kohli & Prof. V.K. Garg & Prof. Ashok Dhawan Co- Principal Investigator Central University of Punjab, Bathinda Dr. J. N. Babu, Paper Coordinator Central University of Punjab, Bathinda Dr. Heena Rekhi Content Writer Department of Chemistry, G.S.S.D.G.S. Khalsa College Patiala Content Reviewer Prof. Ashok Kumar Punjabi University, Patiala Anchor Institute Central University of Punjab 1 Analytical Chemistry Environmental Sciences Potentiometry Description of Module Subject Environmental Sciences Name Paper Name Analytical Chemistry Module Potentiometry Name/Title Module Id EVS/AC-II/23 Pre- requisites T 1. What is Potentiometry? 2. Why is it required? 3. Where is it used? Objectives 4. How is it used? 5. How is it used in reallife? 6. What is its importance? Keywords 2 Analytical Chemistry Environmental Sciences Potentiometry Module 23: Potentiometry Objectives: To study the basics of Potentiometry and know the following about self generated questions. 1. What is Potentiometry? 2. Why is it required? 3. Where is it used? 4. How is it used? 5. How is it used in reallife? 6. What is its importance? 3 Analytical Chemistry Environmental Sciences Potentiometry MODULE 23: POTENTIOMETRY 1. Description Potentiometry is a classical analytical technique with roots before the 20th century. It is based on the measurement of potential of an electrode system. In the presence of multitude of other substances it enables the selective detection of ions. It is one of the electrochemical analytical methods. An electric circuit is used to measure the current and potential created by the flow of charged particles. For complete electrochemical cell from which the negligible current is drawn the EMF is given by: Ecell = Eind- Eref + Ej where, Eind, Eref and Ej are the potentials of the indicator electrode, reference electrode and liquid junction potential respectively. The potentiometric methods cuddle two major type of an analysis. One is direct measurement which involves the direct measurement of an electrode potential from which the activity of an active ion may be derived. Other kind is indirect potentiometric method which measures the variation in EMF brought by the addition of a titrant to the sample. Description of charge transfer process is as follows: 1. Movement of electrons from zinc electrode to copper electrode. 2. In the solution zinc ions move away from the electrode and sulfate ions move towards. 3. Positive ions move towards right and negative towards left in salt bridge. 4. On the surface of electrodes electrons are transferred to ions. 4 Analytical Chemistry Environmental Sciences Potentiometry 5. Zinc dissolves and metallic copper deposits. Fundamentals of Potentiometry The electrode potential is developed and given by Nernst equation whenever metal ion immersed in asolution containing its own ions Mn+. n+ E = E° + (RT/nF) ln aM The value of an electrode potential E can be established by linking the calomel electrode and measuring the EMF of the resultant cell. It can be deduced form the reference electrode. It is also possible to measure the potential directly via direct potentiometer. This involves the use of an electrode of second kind. For example Ag-AgCl electrode formed by coating a silver wire with AgCl. This measures the concentration of chloride ion in the solution. The basic functions of potentiometer are as follows: • To measure the electric potential or voltage. Step 1 • By rotating the potentiometer wheel we are changing the voltage applied to the resistor, which results in more or less light coming out Step 2 from the LED. • To control electrical devices such as volume controls on audio Step 3 equipment. • It works as a rheostat and provides varying resistance depending on the Step 4 flow. Figure 1: Various functions of potentiometer 5 Analytical Chemistry Environmental Sciences Potentiometry Furthermore, the requirements and methodologies used to accomplish the analysis are modified day to day by the researchers working in this field to improve the routine analytical methods. The equipment required for direct potentiometric measurements includes an ion-selective electrode, a reference electrode, and a potential-measuring device. The reference electrode should provide a highly stable potential for an extended period of time. The ion-selective electrode is an indicator electrode capable of selectively measuring the activity of a particular ionic species. Ion-selective electrodes are mainly membrane-based devices, consisting of selective ion-conducting materials, Potentiometer which separate the sample from the inside of the electrode. It is necessary to select an appropriate electrode both indicator and reference depending on chemically reacting components in various titrations. Reference electrode is the electrode with a potential which is an independent of concentration and temperature. It must be reversible and obeys the Nernst equation. It gives the stable potential with time and always returns to its original position after the passage. The common reference electrode used in potentiometer is calomel electrode Hg/Hg2Cl2 (satd), KCl (xM) and the half cell reaction is as follows: - - Hg2Cl2 (s) + 2e ↔ 2Hg + 2Cl A reference electrode, Eref, is a half-cell having a known potential that remains constant at constant temperature and independent of the composition of the analyte solution. Another is an indicator electrode having a potential that varies with variations in the concentration of an analyte. Metallic indicator electrode and membrane electrodes are types of indicator electrodes. 6 Analytical Chemistry Environmental Sciences Potentiometry 2. Instrumentation The measurements of potential are generally employed from the non-electronic, electronic and automatic instruments. Every kind of potentiometric measurement requires a suitable galvanic cell consisting of an indicator electrode and a stable reference electrode. The reference electrode may be immersed directly in the sample solution or brought in contact with it through a salt bridge. The type of an indicator electrodes are either noble metal ions or ion selective electrodes. The commonly used electrodes systems in potentiometry are described as follows: Saturated Calomel Electrode Reference electrodes: The reference electrode has a potential that is known accurately, constant and completely insensitive to the composition of an analyte solution. When the demand of current is made upon it, the value of a potential must not depart from its equilibrium value. This kind of an electrode consists of an internal element, salt bridge electrolyte, setting up the fluid junction potential. Potentials are quoted with reference to standard hydrogen electrode termed as a primary reference electrode. In addition, this electrode should be rugged and easy to assemble and maintain a constant potential while passing minimal currents. Reproducibility factor involves the two aspects one is reference electrode should respond according to the Nernst equation and the second is feasibility of establishing an easy and standard method of electrode preparation. For example calomel electrode can be represented schematically as Hg|Hg2Cl2(satd),KCl(xM)|| Where, x represents the molar concentration of potassium chloride in the solution. It consists of metallic mercury and calomel (mercury chloride) in equilibrium with solution of KCl solution of definite concentration. The most commonly used electrode in saturated calomel electrode (SCE) because of the suppressive effect of saturated KCl solution on liquid junction potentials. Other kind of 7 Analytical Chemistry Environmental Sciences Potentiometry electrodes decimolar and molar electrodes are preferred in cases where accurate values of electrode potentials are required. A calomel half cell is represented as: ||Hg2Cl2 (satd), KCl (M) ||Hg All the electrodes are same differ only in their potassium chloride concentrations, all are saturated with calomel electrode. In the modern electrodes an ion exchange membrane is incorporated in lower part of the electrode which prevents migration of mercury ions to sintered disc and hence to the test solution. Here the main function of this double junction is to prevent the ingress of ions from the test solution which may interfere with the electrode. Mercury mercuric oxide electrode: It consists of mercury pool in contact with a solution of NaOH or KOH saturated with HgO. - Hg + 2OH- HgO + H2O + 2e All the calomel electrodes are subjected problems arising from accelerated disproportionation of mercury (I) to mercury (II) at higher temperatures. The silver-silver chloride electrode: An electrode is immersed in a solution that is saturated in both potassium chloride and silver chloride Ag|AgCl (satd), KCl (satd)|| It consists of a silver wire or a silver plated platinum wire coated electrolytically with a thin layer of AgCl, dipping into a KCl solution of known concentration. Ion exchange membranes and double junctions are used to prevent the clogging of sintered disc in Ag-AgCl electrode. The main part of drawback of this kind of an electrode includes not applicable for the determination of proteins, sulphide, bromide and iodide. The strong reducing agents should also be avoided because they can reduce the Ag+ ions to Ag metal at liquid junction potential. Indicator electrodes: The potential depends on the activity of a particular ionic species which is desired to quantify in an indicator electrodes.