Module 12: Potentiometry

• Potentiometry


  used to find the concentration of solute in a solution.
• Potentiometry


  potential is measured under the conditions of no current flow.
• Potentiometry


  these electrodes are relatively free from interferences and provide a rapid, convenient, and nondestructive means for quantitativelv determining numerous important anions and cations.
• Potentiometry


  the most widely used electroanalytical technique.
• Potentiometry


  based on measuring the potential of electrochemical cells without drawing appreciable current.
• Potentiometry

  Use: pH and pCO2 tests
• Potentiometry


  Applications: Quantitative determination and monitoring of many species in solution over a wide range of concentrations (10^-7 - 1 M); relative precision 0.1-5%.
• Potentiometry


  Disadvantage: titrations are slow and time-consuming unless automated.
• Ion Selective Electrode (ISE)


  an electrochemical tranducer capable of responding to one given ion. They are selective and sensitive but they are not specific.
• ISE

  • Direct: without sample dilution.
  • Indirect: with sample dilution.
• Reference Electrode


  is a half-cell having a known electrode potential that remains constant at constant temperature and is independent of the composition of the analyte solution.
• Reference Electrode

  • Saturated Calomel Electrode
  • Silver—silver Hydrogen Electrode
  • pH electrode
  • Standard Hydrogen Electrode (SHE)
• Indicator Electrode


  has a potential that varies in a known way with variations in the concentration of an analyte. Responds rapidly and reproducibly to changes in the concentration of an analyte ion.
• Indicator Electrode 3 types: metallic, membrane, and ion-sensitive field effect transistors.
• Clinical Chemistry

  lon-selective electrodes are important sensors for clinical samples because of their selectivity for analytes in complex matricies. • The most common analytes are electrolytes, such as Na+, K+, Ca2+,H+, and Cl-, and dissolved gases such as CO2.
• Environmental Chemistry


  One potential advantage of an ion-selective electrode is the ability to incorporate it into a flow cell for the continuous monitoring of wastewater streams.
• Potentiometric Titrations 


  Use a pH electrode to monitor the change in pH during the titration.
• Glass Electrode for Measuring pH


  The liquid inside the glass electrode usually has a pH of 7.
• Direct Potentiometry


  measurements provide a rapid and convenient method for determining the activity of a variety of cations and anions.
• True or False

  True: If the response of the electrode is specific for the analyte, as it often is, no preliminary separation steps are required.
• Glass/Calomel Electrode System


  is remarkably versatile tool for the measurement of pH under many conditions. 
• Alkaline Error


  also known as the Sodium Error; the ordinary glass electrode tend to be somewhat sensitive to alkali metal ions and gives low readings at ph values greater than 9.
• Acid Error


  values registered by the glass electrode tend to be somewhat high when the pH is less than about 0.5.
• Dehydration


  dehydration may cause erratic electrode performance.
• True or False

  True: It has been found that significant errors may occur when the pH of samples of low ionic strength, such as lake or stream water, is measured with a glass/calomel electrode system.
• Potentiometric Titrations


  provide data that are more reliable than data from titrations that use chemical indicators and are particularly useful with colored or turbid solutions and for detecting the presence of unsuspected species.
• Potentiometry


  uses NERNST EQUATION
• Relative Precision

  • Potentiometry: 0.1-5%
  • Voltammetry: 2-3%
  • Polarography: 2-3%
  • Amperometry: 2-3%
  • Coulometry: 0.2-5%
  • Gravimetry: 0.1-1%
  • Titrimetry: 0.1-1%
• Trace Levels

  • Potentiometry: 10^-7 to 1 M
  • Polarography: 10^-4 to 10^-8 M