potentiometry Flashcards
based on the measurement of the potential of electrochemical cells without drawing appreciable current
potentiometry
no current or only a negligible current flows while measuring a solution’s potential, the solution’s composition remains unchanged
quantitative technique
made using a potentiometer to determine the difference in potential between a working or, indicator, electrode and a counter electrode
potentiometric measurements
a half cell with an accurately known electrode potential Eref that is independent of the concentration of the analyte or any ions in the solution
reference electrode
has a potential Eind that varies in a known way with variation in the concentration of an analyte
indicator electrode
has a potential that is accurately known, constant, and completely insensitive to the composition of the analyte solution
ideal reference electrode
seldom used as a reference electrode for day to day potentiometric measurements because it is somewhat inconvenient to use and maintain and is also a fire hazard
standard hydrogen electrode (SHE)
what is an ideal indicator electrode
responds rapidly to changes in the concentration of an analyte ion and is selective to a group of ions
types of indicator electrode
- metallic
- membrane electrodes
- ion-sensitive field effect transistor (ISFET)
types of metallic indicator electrode
- electrode of the first kind
- electrodes of the second kind
- Inert electrode
potential difference is created
membrane electrodes
characteristics of membrane
- low solubility
- some electrical conductivity
- selectivity
two general types of membrane electrodes
crystalline and non crystalline membranes
non crystalline membranes
- glass - silicate glasses for H+ , Na+
- liquid - liquid ion exchanger for Ca2+
- immobilized liquid - liquid/PVC matrix for Ca2+ and NO3-
glass membrane structure
- SiO4⁴- framework with charge balancing cations
- SiO2 72%, Na2O 22%, CaO 6%
by altering the glass composition
direct potentiometric measurement of single charge cation could be done
glass electrodes for these analytes are available
Na+, K+, NH4+, Rb+, Cs+, Li+
based on potential that develops across two immiscible liquids with different affinities for analyte
liquid membrane electrodes
porous membrane used to separate liquids
liquid membrane electrodes
the potential develops across the interface between the solution containing the analyte and the ion exchanger
liquid membrane electrodes
the reservoir forces exchanger into the membrane; the exchanger forms complexes with the species of interest
calcium electrode
crystalline membranes
- single crystal - LaF3 for F
- mixed crystal - AgS for S2- and Ag+
usually ionic compound
crystalline membrane electrodes
single crystal, crushed powder, melted and formed
crystalline membrane electrodes
sometimes doped (Li+) to increase conductivity
crystalline membrane electrodes
operation similar to glass membrane
crystalline membrane electrodes
other potentiometric sensors
- gas sensors
- biosensors
gas membrane allows small gas molecules to pass and dissolve into internal solution
gas sensors
an analytical device for the detection of an analyte that combines a biological component with a physicochemical detector component
biosensor
make use of biocomponents capable of recognizing (bio) chemical species and transforming them into a product through a chemical reaction
catalytic
working range for most ion-selective electrodes is from a maximum concentration of _______ to a minimum concentration of _________
maximum: 0.1 - 1 M
minimum: 10-⁵ - 10-10 M
errors in potentiometry may be due to
- interfering ions
- finite current drawn through the cell while measuring the potential
- difference in the analyte’s activity coefficient in the sample and standard solutions
- liquid junction potentials
precision of potentiometric measurement is limited to
variation in temperature and the sensitivity of the potentiometer
selectivity of potentiometry
most ion-selective electrodes respond to more than one analyte
potentiometry provides what in terms of completing methods
rapid and relatively low cost means for analyzing samples
