POTENTIOMETRY Flashcards
Based on measuring the potential of electrochemical cell without drawing appreciable current
It is used for locating end points in titrations
Ion concentrations are measured directly from the potential of ion-selective membrane electrodes
It is also used to determine thermodynamic equilibrium constant
It simple and inexpensive and includes a reference electrode, an indicator electrode, and a potentialmeasuring device
POTENTIOMETRIC METHODS
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
It has a potential that varies in a known way with variations in the concentration of an analyte - Immersed in a solution of the analyte and develops potential
Indicator Electrode
prevents the components of the analyte solution from mixing with those of the reference electrode
Salt Bridge
Consist of mercury in contact with a solution saturated with mercury chloride that also contain known concentration of Potassium Chloride.
Calomel Reference Electrode
most widely marketed reference electrode system
consists of a silver electrode immersed in a solution of potassium chloride that has been saturated with silver chloride
Silver/silver Chloride Reference Electrode
produced when two electrolyte of different composition are in contact with one another
It is the result of unequal distribution of cations and anions across the boundary
Junction Potential
- ideal indicator electrode responds rapidly and reproducibly to changes in the concentration of an analyte ion
Indicator Electrode
Three Types of Indicator Electrode
Metallic
Membrane
Ion Selective Field Effect Transistors
pure metal electrode that is in direct equilibrium with its cation in the solution
not widely used for potentiometric determinations
Electrodes of the First Kind
This are metal electrodes assembly with the equilibrium potential being a function of the concentration of an anion in the solution.
The electrode of the Second Kind
Inert conductors respond to redox system.
Inert Redox Indicators
is a convenient indicator electrode for titrations involving standard cerium(IV) solutions
Platinum electrode
Sometimes called p-ion electrodes
It use the glass electrode for pH measurements
presented as p-functions, such as pH, pCa, or pNO3
Membrane Indicator Electrodes
It is a type of ion-selective electrode made of a doped glass membrane that is sensitive to a specific ion.
Most common application of ion-selective glass electrodes is for the measurement of pH.
pH electrode is an example of a glass electrode that is sensitive to hydrogen ions.
Glass Electrode
It is developed for the direct potentiometric measurement of numerous polyvalent cations as well as certain anions. - uses a hydrophobic membrane that contains a liquid organic complexing agent that reacts selectively with the analyte.
Liquid Membrane Electrodes -
This are solid-state ion-selective electrode has a membrane that consists of either a polycrystalline inorganic salt or a single crystal of an inorganic salt
Crystalline – Membrane Electrodes
provide a rapid and convenient method for determining the activity of a variety of cations and anions
requires only a comparison of the potential developed in a cell containing the indicator electrode in the analyte solution with its potential when immersed in one or more standard solutions of known analyte concentration
DIRECT POTENTIOMETRY
is unquestionably the most important indicator electrode for hydrogen ion.
It is convenient to use and subject to few of the interferences that affect other pH-sensing electrodes.
Glass electrode
6 Errors Affecting pH Measurements
- Alkaline Error
- Acid Error
- Dehydration
- Errors in Low Ionic Strength Solutions
- Variation in Junction Potential
- Error in the pH of the Standard Buffer
measures the potential of a suitable indicator electrode as a function of titrant volume
POTENTIOMETRIC TITRATIONS
are related methods that are based on electrolysis carried out long enough to ensure complete oxidation or reduction of the analyte to a product of known composition.
Electrogravimetry and coulometry
must be considered when current is present because in these phenomena, potentials larger than the thermodynamic potential are needed to operate an electrolytic cell.
IR drop and polarization
IR Drop - Electrochemical cells, like metallic conductors, resist the flow of charge
Ohmic Potential
describes the effect of this resistance on the magnitude of the current in the cell.
Ohm’s law
It is the departure of the electrode potential from its theoretical Nernst equation value on the passage of current.
Polarization
Is the potential difference between the theoretical cell potential from Equation 22-2 and the actual cell potential at a given level of current.
Overvoltage
occurs because of the finite rate of mass transfer from the solution to the electrode surface.
Concentration polarization
Mechanisms in Transporting Reactants in the Surface of an Electrode
- Diffusion
- Migration
- Convection
is the movement of a species under the influence of a concentration gradient.
It is the process that causes ions or molecules to move from a more concentrated part of a solution to a more dilute.
Diffusion
It is an electrostatic process by which ions move under the influence of an electric field.
Migration
is the transport of ions or molecules through a solution as a result of stirring, vibration, or temperature gradients.
Convection
such as stirring or agitation, tends to decrease the thickness of the diffusion layer at the surface of an electrode and thus decrease concentration polarization.
Forced convection
resulting from temperature or density differences also contributes to the transport of molecules and ions to and from an electrode
Natural convection
The magnitude of the current is limited by the rate of one or both of the electrode reactions, that is, the rate of electron transfer between the reactants and the electrodes.
is most pronounced for electrode processes that yield gaseous products because the kinetics of the gas evolution process are complicated and often slow.
Kinetic Polarization
the potential of the working electrode is maintained at a constant level versus a reference electrode
potentiostatic method
The quantity of electrical charge required to convert a sample of an analyte quantitatively to a different oxidation state is measured.
COULOMETRIC METHODS
The potential of the working electrode is maintained at a constant level such that only the analyte is responsible for conducting charge across the electrode/solution interface.
Controlled – Potential (Potentiostatic)
Performed with a constant-current source, sometimes called a galvanostat.
Controlled – Current Coulometry (Coulometric Titrimetry)
senses decrease in current cell and responds by increasing thee potential applied to the cell unit in the current restored to its original level
Galvanostat
refers to a group of electroanalytical methods in which we acquire information about the analyte by measuring current in an electrochemical cell
widely used fundamental studies of oxidation and reduction processes in various media adsorption processes on surfaces, and electron transfer mechanisms at chemically modified electrode surfaces
Voltammetry
used to monitor current proportional to analyte at fixed potential
Amperometry
is a particular type of voltammetry that was invented by the Czechoslovakian chemist Jaroslav Heyrovsky in the early 1920s
it differs from other types of voltammetry in that the working electrode is the unique dropping mercury electrode
important tool used in the determination of inorganic ions and certain organic species in aqueous solutions
Polarography
Four Common Excitation Signals (Waveforms)
Linear Scan
Square Wave
Differential Pulse
Triangular
The cell is made up of three electrodes immersed in a solution containing the analyte and also an excess of a nonreactive electrolyte called a ____
supporting electrolyte.
is the electrode at which the analyte is oxidized or reduced
dimensions of the working electrode are kept small to enhance its tendency to become polarized
Working Electrode
has a potential that remains constant throughout the experiment
Reference Electrode
it is often a coil of platinum wire or a pool of mercury
Counter Electrode
is a salt added in excess to the analyte solution
commonly, it is an alkali metal salt that does not react at the working electrode at the potentials being used
the salt reduces the effects of migration and lowers the resistance of the solution
Supporting Electrolyte
they are small flat disks of a conductor that are press fitted into a rod of an inert material, such as Teflon or Kel-F that has imbedded in it a wire contact
Working Electrodes
widely used in voltammetry
they have a relatively large negative potential range
many metal ions are reversibly reduced to amalgams at the surface of a mercury electrode
Mercury Working Electrode
Example of Mercury Working Electrode
Hanging Mercury Drop Electrode (HMDE)
Microelectrode
Sandwich Type
Dropping Mercury Electrode
Electrodes have been tailored to accomplish a broad range of functions.
MODIFIED ELECTRODES
In this application, electrodes capable of reducing oxygen to water have been sought for use in fuel cells and batteries.
Electrocatalysis
APPLICATIONS OF VOLTAMMETRY
Inorganic Application
Organic Application
Most metallic cations are reduced at common working electrodes.
Successful voltammetric determination of cations frequently depends on the supporting electrolyte that is used.
Inorganic Application
Several organic functional groups are reduced at common working electrodes, thus making possible the determination of a wide variety of organic compounds.
Oxidizable organic functional groups can be studied voltammetrically with platinum, gold, carbon, or various modified electrodes.
Organic Application
