Chapter 19: Potentiometry Flashcards
are based on measuring the potential of
electrochemical cells without drawing appreciable current
potentiometric methods
The equipment for potentiometric methods is simple and inexpensive and includes a
reference electrode
indicator electrode
potential-measuring device
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
has a potential that varies in a known way
with variations in the concentration
of an analyte.
indicator electrode
TRUE or FALSE
Potassium chloride is a nearly ideal electrolyte for the salt bridge because the mobilities of the K1 ion and the Cl2 ion are nearly equal.
TRUE
TRUE or FALSE
The ideal reference electrode 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.
TRUE
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
mercury (I) chloride is also known as
calomel
is available as translucent flakes, is a
heteropolysaccharide that is extracted
from certain East Indian seaweed.
Solutions of _____ in hot water set to a
gel when they are cooled.
agar
what kind of reference electrode is the most widely marketed reference electrode system that consists of a silver electrode immersed in a solution of potassium chloride that has been saturated with silver chloride
silver/silver chloride reference electrodes
TRUE or FALSE
Silver/silver chloride electrodes have the advantage that they can be used at temperatures greater than 60C, while calomel electrodes cannot
TRUE
When two electrolyte solutions of different compositions are in contact with one
another, there is a potential difference across the interface. It is the result of an unequal distribution of cations and anions across the boundary due to differences in the rates at which these species diffuse
liquid junction potentials
TRUE or FALSE
The magnitude of the liquid junction potential can be minimized by placing a salt bridge between the two solutions. The salt bridge is most effective if the mobilities of the negative and positive ions in the bridge are nearly equal and if their concentrations are large
TRUE
responds rapidly and reproducibly to changes in the concentration of an analyte ion (or group of analyte ions).
ideal indicator electrode
types of indicator electrode
metallic
membrane
ion-sensitive field effect transistors
classification of metallic indicator electrodes
electrodes of the first kind,
electrodes of the second kind, and
inert redox electrodes.
is a pure metal electrode that is in direct equilibrium with its cation in the solution. single reaction is involved
electrodes of the first kind
TRUE or FALSE
Electrode systems of the first kind are not widely used for potentiometric determinations for several reasons. For one, metallic indicator electrodes are not very selective and respond not only to their own cations but also to other more easily reduced
cations.
TRUE
Metals not only serve as indicator electrodes for their 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 type
TRUE or FALSE
the most convenient method for determining pH involves measurement of the potential that appears across a thin glass membrane that separates two solutions with different hydrogen ion concentrations.
TRUE
are sometimes called p-ion electrodes because the data obtained from them are usually presented as p-functions, such as pH, pCa, or pNO3
membrane electrodes/ membrane indicator electrodes
contains two reference electrodes: the external calomel electrode and
the internal silver/silver chloride electrode.
glass electrode system
TRUE or FALSE
The potentials of the two reference electrodes depend on the
electrochemical characteristics of their respective redox couples, but the potential across the glass membrane depends on the physicochemical characteristics of the glass and its response to ionic concentrations on both sides of the membrane
TRUE
Glasses that absorb water are said to
be
hygroscopic
TRUE or FALSE
The two surfaces of a glass membrane must be hydrated before it will function as a pH electrode. Nonhygroscopic glasses show no pH function. Even hygroscopic glasses lose their pH sensitivity after dehydration by storage over a desiccant
TRUE
varies with the pH of the analyte solution
boundary potential
components of potential glass electrode
boundary potential
potential of the internal Ag/ AgCl reference electrode
small asymmetry potential
It consists of a conducting membrane that selectively binds calcium ions, an internal solution containing a fixed concentration of calcium chloride, and a silver electrode that is coated with silver chloride to form an internal reference electrode
Liquid membrane electrodes
is a measure of a response of an ion-selective electrode to other ions.
selectivity coefficient
TRUE or FALSE
The sensitivity of the liquid-membrane electrode for calcium ions is reported to be 50 times greater than for magnesium ion and 1000 times greater than for sodium or potassium ions.
TRUE
has been widely used for membranes,
consists of approximately 22% Na2O, 6% CaO, and 72% SiO2. Membranes made from this glass exhibit excellent specificity to hydrogen ions u to a pH of about 9
Corning 015 glass
used for membranes consists of an
infinite three-dimensional network of groups in which each silicon atom is bonded
to four oxygen atoms and each oxygen atom is shared by two silicon atoms.
silicate glass
The cell consists of a glass indicator
electrode and a saturated calomel reference electrode immersed in the solution
of unknown pH.
glass electrode for measuring pH
The indicator electrode consists of a thin pH-sensitive glass membrane sealed onto one end of a heavy-walled glass or plastic tube
glass electrode for measuring pH
TRUE or FALSE
The membrane of a typical glass electrode (with a thickness of 0.03 to 0.1 mm) has an electrical resistance of 50 to 500 MV.
TRUE
The resulting difference in potential between the two surfaces of the glass is the ______, which is related to the activities of hydrogen ions in each of the solutions by the Nernst-like equation
boundary potential
TRUE or FALSE
Nonhygroscopic glasses show no pH function. Even hygroscopic
glasses lose their pH sensitivity after dehydration by storage over a desiccant
TRUE
have been developed for the direct potentiometric measurement of numerous polyvalent cations as well as certain anions
liquid membrane electrodes
The potential of ____________ develops across the interface between the solution containing the analyte and a liquid ion exchanger that selectively bonds with the analyte ion.
liquid-membrane electrodes
which electrodes have both their ion exchanger dissolved in an immiscible organic liquid that is forced by gravity into the pores of a hydrophobic porous disk. This disk
then serves as the membrane that separates the internal solution from the analyte solution.
liquid-membrane and glass electrode
can be used to make measurements of ion activities within a living organism.
ion-selective microelectrodes
The sensitivity of the liquid-membrane electrode for __________ is reported to be
50 times greater than for magnesium ion and 1000 times greater than for__________.
calcium ions
sodium or potassium ions
The performance of the electrode is independent of pH in the range between 5.5 and 11.
liquid membrane electrodes
TRUE or FALSE
At lower pH levels, hydrogen ions undoubtedly replace some of the calcium ions on the exchanger; the electrode then becomes sensitive to pH as well as to pCa.
TRUE
is the parameter measured by the membrane electrode
activity
TRUE or FALSE
Membranes prepared from cast pellets of silver halides have been used successfully
in electrodes for the selective determination of chloride, bromide, and iodide ions
TRUE
developed for solid membranes that
are selective toward anions in the same way that some glasses respond to cations
crystalline membrane electrodes
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. One of the problems
in using this type of device in electronic circuits has been its pronounced sensitivity
to ionic surface impurities, and a great deal of money and effort has been expended
by the electronic industry in minimizing or eliminating this sensitivity in order to
produce stable transistors.
filed effect transistor or
metal oxide field effect transistor
offer a number of significant advantages over membrane electrodes including ruggedness, small size, inertness toward harsh environments, rapid response, and low electrical impedance
ion-sensitive field effect transistors
is very similar in construction
and function to an n-channel enhancement mode MOSFET.
ISFET
is a galvanic cell whose potential is related to the concentration of a gas in a solution.
gas-sensing probe
is also called as gas sensin electorde, which is a misnomer
gas-sensing probe
consists of a tube containing a reference electrode, a specific ion electrode, and an electrolyte solution. A thin, replaceable, gas-permeable membrane attached to one end of the tube serves as a barrier between the internal and analyte solutions.
gas sensing probes
are used widely for determining dissolved gases in water and other solvents
gas sensing probes
is fabricated from a hydrophobic polymer, which is highly porous (average pore size is less than 1 micrometer) and allows passage of gases and is water repellent polymer prevents water and solulte ions from entering the pores
microporous membrane
the ratio of
the volume of red blood cells
to the total volume of a blood
sample expressed as a percent.
hematocrit
provide a rapid and convenient method for
determining the activity of a variety of cations and anions. 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 potentiometric measurements
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.
standard addition method
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 electrodes
is a remarkably versatile tool for the
measurement of pH under many conditions. It can be used without interference
in solutions containing strong oxidants, strong reductants, proteins, and gases; the
pH of viscous or even semisolid fluids can be determined
glass/ calomel electrode system
what error indicates that the ordinary glass electrode becomes somewhat sensitive to alkali metal ions and gives low readings at pH values greater than 9.
alkaline error
Values registered by the glass electrode tend to be somewhat high when the pH is less than about 0.5.
acid error
may cause erratic electrode performance.
dehydration
measure the potential of a suitable indicator electrode
as a function of titrant volume
potentiometric titration
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
potentiometric titrations
cell potential in potentiometric analysis is represented by the equation
Ecell= Eind- Eref + Ej
consists of a thin pH-sensitive glass membrane
sealed onto one end of a heavy-walled glass or plastic tube. A small volume of dilute hydrochloric acid saturated with silver chloride is contained in the tube
indicator electrode
TRUE or FALSE
tso surfaces of a glass membrane must be hydrated before it will function as a pH electrode
TRUE
TRUE or FALSE
Nonhygroscopic glasses show no pH function
TRUE
TRUE or FALSE
Even hygroscopic glasses lose their pH sensitivity after dehydration by storage over a desiccant
TRUE
can be used to make measurements of ion activities within a living organism.
ion-selective microelectrodes
valuable tool for physiological investigation because this ion plays important roles in such processes as nerve conduction, bone formation, muscle contraction, cardiac expansion and contraction, renal tubular function, and perhaps hypertension.
calcium ion iquid-membrane electrode
is also of great value for
physiologists because the transport of neural signals appears to involve movement
of this ion across nerve membranes.
potassium ion liquid membrane electrode
also called as field effect transistor
metal oxide field effect transistor (MOSFET)
is used to control the ionic strength and the pH of amples and standards, in ion-selective electrode measurements
total ionic strength adjustment buffer
