Potentiometry in Instrumental Analytical Chemistry

This presentation explores potentiometry as a key analytical technique in instrumental chemistry. It covers fundamental principles, instrumentation, and applications in pH measurement, environmental monitoring, clinical diagnostics, food analysis, and industrial processes. Emphasizing accuracy and sensitivity, potentiometry remains crucial for ion detection, ensuring quality control in pharmaceuticals, agriculture, and research.

1. POTENTIOMETRY GROUP 6 KIWANUKA JOBIC 2301200487 ANGUYII HOLY PIUS 2301201028 NAGUJJA BISIRIKIRWA STELLAH 2301200480 THWARIK DORUGA 2301200294 1

2. INTRODUCTION TO POTENTIOMETRY This technique involves the measurement of the potential of an electrochemical cell under static conditions because no current flows while measuring a solution’s potential its composition remains constant. Information on the composition of the sample is obtained through the potential appearing between two electrodes Analytical use of this method is used in two ways Direct potentiometry Potentiometric titration ( https://www.egyankosh.ac.in/bitstream/123456789/82041/3/Unit-9.pdf ) 2

3. PRINCIPLE OF OPERATION Potentiometry deals with the measurement of difference in potential between two electrodes which have been combined to form an electrochemical cell. The difference in the potential between the two electrodes in the electrochemical cell is known as cell potential The cell potential depends on the composition of the electrodes, concentration of the solution/activity of the species in solution and temperature The potentiometric methods of analysis are based on measurements of the potential of the electrochemical cells under conditions of zero current, where the NERNST EQUATION governs the operation of potentiometry 3

4. NERNST EQUATION E cell =E cell – ( ) InQ Where; E cell = cell potential under nonstandard conditions E cell = cell potential under standard conditions R = gas constant , which is 8.31(volt-coulomb)/(mol-K) n = number of moles of electrons exchanged in the electrochemical reaction (mol) F = Faradays constant, 96500 coulombs/mol Q = reaction quotient   4

5. The cell formed by two electrodes (half cell)and the solution is known as an electrochemical cell/ a galvanic cell . The net cell reaction can be considered as the sum of the two half-cell reactions in which each half-cell reaction is representation of the actual chemical process at each individual electrode of the electrochemical cell. A half-cell reaction always include the electrons transferred Usually; One of the electrodes(half-cell) is chosen so that its potential is invariant and its called reference electrode . The potential of the other is a function of the concentration (more correctly activity)of the species involved in the electron transfer process(a redox reaction),through the Nernst equation. This electrode is termed as the indicator electrode 5

6. Under these conditions the cell electromotive force (emf) is given by E cell = ( E ind - E ref ) + E j Where; E cell = emf of the electrochemical cell E ind = half-cell potential of the indicator electrode E ref = half-cell potential of the reference electrode E j = liquid-junction potential developed at the interface between two electrolytes The concentration of the solution is determined by a single measurement of cell potential applying the Nernst equation 6

7. INSTRUMENTATION The equipment required for direct potentiometric measurements includes an ion-selective electrode, a reference electrode, and a potential measuring device. 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, 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. 7

8. An indicator electrode has a potential that varies with variations in the concentration of an analyte. Metallic indicator electrode and membrane electrodes are types of indicator electrodes. 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/Hg 2 Cl 2 (satd), KCl (xM) and the half cell reaction is as follows: Hg 2 Cl 2 (s) + 2e - ↔ 2Hg + 2Cl 8

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10. The commonly used potential measuring device is usually the voltmeter and potentiometer which measure potential difference between two points in a circuit. 10

11. Potentiometric titrations The measurement of an analyte can be done by the titration in which the potential of an indicator electrode is measured as a function of the volume of titrant added. This is known as potentiometric titration. Sudden change in potential in the plots of emf against volume of titrating solution reveals the end point. A typical potentiometric curve is represented as below: 11

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13. Automatics titrations The Automatic titrator is provided with two-point auto calibration and standardization. The instrument is capable of displaying pH and mV of the sample, with temperature compensation. The Automatic titrator can accept a variety of electrodes to cater to various applications in different fields. This kind of instrumentation is ideal for performing the multiple analysis in which the fundamental analytical procedure remains fixed as in quality control situation. The entire titration can be performed automatically by the titrators equipped with microcomputers, digital convertors and using dedicated software. 13

14. APPLICATIONS OF POTENTIOMETRY Potentiometry is a widely used electroanalytical technique with numerous applications across scientific, industrial, medical, and environmental fields. The technique’s high sensitivity, accuracy, and ability to measure specific ion concentrations make it a valuable tool for both research and practical applications 14

15. pH Measurement One of the most common applications of potentiometry is pH measurement, essential in chemistry, biology, medicine, and industry. The glass electrode is the most widely used indicator electrode for pH determination. Accurate pH measurement is crucial in: Water treatment plants for maintaining safe water quality. Agriculture and soil testing to optimize soil conditions for crop growth. Pharmaceutical and biochemical industries to ensure drug stability and effectiveness . 15

16. Environmental Analysis Potentiometry plays a significant role in environmental monitoring, especially in detecting and quantifying ions in natural waters and wastewater. Ion-selective electrodes (ISEs) enable the detection of toxic heavy metals such as lead (Pb²⁺), cadmium (Cd²⁺), and mercury (Hg²⁺), which are major pollutants. Other applications include: Monitoring nitrate (NO₃⁻) and fluoride (F⁻) concentrations in drinking water. Assessing acid rain impact on soil and aquatic systems. Tracking industrial waste discharge to prevent environmental contamination. 16

17. Clinical and Biomedical Applications Potentiometry is widely used in medical and clinical diagnostics for measuring electrolytes in blood, urine, and plasma. Common applications include: Sodium (Na⁺), potassium (K⁺), chloride ( Cl ⁻), and calcium (Ca²⁺) analysis in patient samples. Blood gas analysis, which is critical in monitoring respiratory and metabolic conditions. pH measurement in biological fluids to diagnose conditions such as acidosis and alkalosis 17

18. Food and Beverage Industry Potentiometric methods are essential in ensuring food safety and quality by measuring: Salt content (NaCl) in processed foods. Acidity levels in beverages such as soft drinks, wine, and dairy products. Preservative concentration, ensuring compliance with food safety regulations . 18

19. Pharmaceutical Industry In pharmaceutical applications, potentiometry is used for: Quality control of drugs by monitoring pH and ion concentrations. Ensuring drug stability by measuring electrolyte levels in formulations. Analyzing active ingredients in medications using ion-selective electrodes . 19

20. Industrial Applications Potentiometry has numerous applications in chemical manufacturing and industrial processes: Corrosion monitoring by detecting metal ions in industrial fluids. Electroplating and battery manufacturing, where ion concentration affects efficiency. Oil and fuel quality control, ensuring optimal chemical composition . 20

21. Agriculture and Soil Science Potentiometry has important applications in agriculture and soil science: Soil pH measurement to determine soil suitability for crop growth. Monitoring nutrient levels, such as nitrate and potassium, in soil and fertilizers. Detecting contaminants in agricultural runoff. 21

22. Research and Electrochemical Studies Potentiometry is a valuable tool in academic and industrial research: Electrochemical studies involving redox reactions. Developing new ion-selective electrodes for various chemical and biological applications. Investigating enzyme activity and biomolecule interactions . 22

23. ADVANTAGES OF POTENTIOMETRY 1.High Accuracy and Precision It provides accurate and precise measurements of ion concentrations and PH Levels. 2. Non destructive Analysis Unlike methods that involves chemical reactions with the sample, Potentiometry does not chemically alter or consume the sample or chemical during measurement . This feature is essential for tests requiring sample preservations

24. contn 3.Wide range off applications Potentiometry is versatile and can be used for various analytical purpose such as Acid base titrations, Redox titrations, Precipitation titrations and Complexometric titrations 4.Small sample volume requirement Only small sample volume of solution is needed for potentiometric analysis which is beneficiary when working with expensive or limited samples

25. contn 5.Minimal interference These method is less affected by color, turbidity of the sample compared to spectrophotometric methods 6.Real time Monitoring It allows continuous monitoring and and measurement of changes in potential during titration 7.Cost effective The equipment used e.g. Electrodes and pH meters is relatively inexpensive and easy to maintain

26. DISADVANTGES OF POTENTIOMETRY Selectivity Issue Indicator electrodes, especially ion selective electrodes (ISEs), may respond to interfering ions other than the target analytes, leading to in accurate measurements . Examples, a fluoride ion-selective electron (used to measure fluoride ion in water) can also respond to hydroxide ions at PH levels. If the PH is not controlled, the electrode may give a false reading for fluoride concentration due to hydroxide interference. 26

27. Calibration Requirements Potentiometric require frequent calibration with standard solutions to ensure accuracy. Without proper calibration, the results can be unreliable. Example, a PH electrode used to measure acidity of soft drink must be calibrated using buffer solutions of known PH (e.g.; PH of 4.0% and PH of 7.0). if the electrode is not calibrated regularly the PH of soft drink maybe in correct leading quality control issues . 27

28. Limited Sensitivity Potentiometric is less sensitive compared to other analytical techniques, making it unsuitable for detecting very low concentration analytes. Example, inability to detect trace of lead ions in water at ppb levels. 28

29. Electrode Drift The potential of the indicator electrode may drift over time due to factors like aging, contamination or changes electrode services. Example; a potassium ion-selective electrode (used to measure potassium in blood samples) may show a gradual change in potential overtime, even when measuring the same sample. This drift can lead to incorrect potassium concentration readings, which is critical in clinical diagnostics . 29

30. Limited Dynamic Range Potentiometric electrodes have a limited range of concentrations over which they can provide accurate measurements. Example, a chloride ion-selective electrodes may accurately measure chloride concentrations in the range of 1*10-1 to 1*10-5 M. however, if the sample has a chloride concentration outside the range (e.g., very dilute or highly concentrated), the electrode may not provide a reliable result. 30

31. Temperature Sensitivity The Nernst equation is temperature-dependent, so changes in temperature can affect the measured potential and lead to errors if not corrected. Example, in industrial processes, if the temperature of solution containing sodium ions fluctuates during measurements, the potential reading from sodium ion-selective electrode will also change. Without temperature concentration, the calculated sodium concentration will not be accurate . 31

32. REFERENCES Douglas A. Skoog, F. James Holler and Stanley. R. Crouch (2014). Fundamentals of Analytical Chemistry. 9 th edition, Mary Finch, London. Douglas A. Skoog, F. James Holler and Stanley. R. Crouch (2018). Principles of Instrumental Analysis, seventh edition. Saunders College Publishing, New York. https://www.egyankosh.ac.in/bitstream/123456789/82041/3/Unit-9.pdf 32