Ch 21 Potentiometry Flashcards
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.
A reference electrode
Dictate the representation of cell
reference electrode|salt bridge|analyte solution|indicator electrode
An _______ has a potential
that varies in a known way with variations in the concentration of an analyte. It is immersed in a solution of the analyte
indicator electrode
__________ is a nearly ideal electrolyte for the salt
bridge because ________
Potassium chloride; the mobilities of the K1 ion and the Cl2 ion are nearly equal.
Formula for the potential of the cell
Ecell = Eind - Eref + Ej
Strictly, the potential of a galvanic cell is related to the
activity of the analyte.
An electrode that has a potential that is accurately known, constant, and completely insensitive to the composition of the analyte solution. In addition, this electrode should be rugged, easy to assemble, and should maintain a constant potential while passing minimal currents.
reference electrode
consist of mercury in contact with a solution that is saturated with mercury(I) chloride (calomel) and that also contains a known concentration of potassium chloride
Calomel reference electrodes
All calomel electrodes are saturated
with
Hg2Cl2 (calomel).
, which is available as translucent
flakes, is a heteropolysaccharide that
is extracted from certain East Indian
seaweed.
Agar
The 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 Chloride Reference Electrodes
Advantage of Silver Chloride Reference Electrodes from Calomel Reference Electrodes
Silver–silver chloride electrodes can be used at temperatures greater than 60°C, while calomel electrodes cannot.
When two electrolyte solutions of different composition are in contact with one another, the ____________ potential difference across the interface is created due to the unequal distribution of cations and anions across the boundary
Liquid-Junction Potentials
The magnitude of the Liquid-Junction Potentials can be minimize by placing a ______ between the two solutions
salt bride
three types of Indicator electrodes
metallic Indicator electrodes
membrane Indicator electrodes
ion-sensitive field effect transistors.
Classification of metallic indicator electrodes
electrodes of the first kind
electrodes of the second kind
inert redox electrodes
is a pure metal electrode that is in direct equilibrium
with its cation in the solution. A single reaction is involved.
electrode of the first kind
a metal electrode that does not only serves as indicator electrodes for its own cations, but also respond
to the activities of anions that form sparingly soluble precipitates or stable complexes with such cations.
Electrodes of the Second Kind
relatively inert conductors such as platinum, gold, palladium, and carbon, can be used to monitor
redox systems.
inert metallic redox electrodes
membrane electrode that separates two solutions with different hydrogen ion concentrations is called
p-ion electrodes
What part of the glass/ calomel cell responds to the pH
the thin glass membrane bulb at the tip of the electrode
Glasses that has a pH function and absorb water are said to be
hygroscopic
Glasses that show no pH function and does not absorb water are said to be
Nonhygroscopic
Why do we need to hydrate the glass membrane of an electrode before it will function as a pH electrode?
Because hygroscopic glasses are pH-sensitive.
The hydration of a pH-sensitive glass membrane involves an ion-exchange reaction between singly charged cations in the interstices of the glass lattice and hydrogen ions
from the solution.
What are the four membrane potentials that develop in a cell when pH is being determined with a glass electrode?
The 2 reference electrode potentials from the solution
The junction potential, Ej, across the salt bridge
Lastly, the boundary potential, Eb, which varies with the pH of the analyte solution.
Three components of the potential of a glass indicator electrode, Eind
(1) the boundary potential Eb
(2) the potential of the internal Ag/ AgCl reference electrode
(3) the small asymmetry potential, Easy
Formula for the glass indicator electrode, Eind
Eind = Eb + EAg/AgCl + Easy
is a measure of the response of an ionselective electrode to other ions.
The selectivity coefficient
Errors that glass electrode exhibit
Alkaline error
Acid error
is a saturation effect that occurs when all the surface sites on the glass are occupied with H+ ions. Under these conditions, the electrode no longer responds to further
increases in the H1 concentration, and the pH readings are too high.
acid error
The ________ disk is porous
toward organic liquids but repels
water.
hydrophobic
ISFETs stands for
ion-sensitive
field effect transistors.
is a tiny solid-state semiconductor device that is widely used in computers and other
electronic circuits as a switch to control current flow in circuits
field effect transistor or metal oxide field effect transistor (MOSFET),
offer a number of significant advantages over membrane electrodes
including ruggedness, small size, inertness toward harsh environments, rapid
response, and low electrical impedance.
ISFETs
is a galvanic cell whose potential is related to the
concentration of a gas in a solution.
gas-sensing probe
is fabricated from a hydrophobic polymer. The membrane is highly porous (the average pore size is less than 1 mm) and allows the free passage of gases
microporous membrane
pH meters are frequently used for the measurement of concentrations of other ions, it is also called
p-Ion meters or ion meters
The technique 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
For direct potentiometric measurements, the potential of a cell can then be expressed in terms of the potentials
developed by the indicator electrode, the reference electrode, and a junction
potential,
Ecell = Eind - Eref + Ej
Electrode response is related to analyte _____ rather than analyte concentration
Activity
is used to control the
ionic strength and the pH of samples
and standards, in ion-selective electrode measurements.
total ionic strength adjustment
buffer (TISAB)
involves determining the potential of the electrode system before and after a measured volume of a standard has
been added to a known volume of the analyte solution. Multiple additions can also be made.
The standard addition method
Errors Affecting pH Measurements
Alkaline error
The acid error
Dehydration
Errors in low ionic strength solutions
Variation in junction potential
Error in the pH of the standard buffer
This happen when the ordinary glass electrode becomes somewhat sensitive to alkali metal ions and gives low readings at pH values greater than 9
Alkaline error.
This happen when the Values registered by the glass electrode tend to be somewhat high when the pH is less than about 0.5.
The acid error
This causes erratic electrode performance.
Dehydration
It has been found that significant errors
(as much as 1 or 2 pH units) 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
Errors in low ionic strength solutions
A fundamental source of uncertainty for which a
correction cannot be applied, resulting from differences in the composition of the standard and the unknown solution.
Variation in junction potential
Any inaccuracies in the preparation of the
buffer used for calibration or any changes in its composition during storage cause
an error in subsequent pH measurements.
Error in the pH of the standard buffer
we measure the potential of a suitable indicator
electrode as a function of titrant volume
potentiometric titration,
The four membrane potentials that develop in a cell
Reference electrode potential
Junction potential
Boundary potential
