W4 Potentiometry Flashcards
what is a chemical sensor
device that responds to a particular analyte in a selective way through a chemical reaction > can be used for qualitative or quantitative determination of analyte
what are the 2 main parts of a chemical sensor
region where selective chemistry takes place > reaction produces signal such as change in colour or change in electrical potential of surface
transducer responds to this signal and translates the magnitude of the signal into a measurement of the amount of analyte
relationship between concentration of ions present and electrical potential difference
greater concentration > greater potential difference
what are potentiometric measurements of analysis based on
based on measuring the potential of electrochemical cells without drawing appreciable current
what are ion selective electrodes
used to selectively measure concentration of specific ion > respond to presence of specific ion while ignoring other ions
selective chemistry takes place at surface of electrode > produce interfacial potential
surface must contain a component which will react selectively and reversibly with analyte > achieved by using ion-selective membranes to make up sensor surface
what are the 3 categories of membranes used in potentiometric chemical sensors
glass membranes
sparingly soluble inorganic salt membranes
polymer-immobilised ionophore membranes
what are sparingly soluble inorganic salts
ionic conductors at room temperature > can undergo ion exchange interactions at the solid/water interface to generate an interfacial potential
what is liquid junction potential
occurs at boundary between two electrolyte solutions with different ionic compositions and concentrations
ions in the two solutions diffuse at different rates across junction > create imbalance in charge distribution > can cause errors in electrochemical measurements
how does dissolved gas sensing probes work
make use of glass sensing membranes to determine concentration of dissolved gases in aqueous solutions
combination pH/reference electron located in barrel of ammonia probe
a gas permeable membrane inserted into end of barrel > barrel filled with 0.1M ammonium solution
glass membrane situated close to inside surface of gas permeable membrane and monitors pH of ammonium chloride solution
what are the 4 main materials in poly vinyl chlorine (PVC) chemical sensor
the polymer (30%)
the plasticiser: a solvent that keeps the polymer in a viscous liquid like state (65-70%)
an ionophore: complexing agent that selectively binds the ion or molecule of interest > move about in the liquid channels of the plasticiser within the PVC (1%)
a salt is sometimes added to increase conductivity and acts as counter-ion (1%)
properties of ion selective electrodes
potentiometric chemical sensors as their selective chemistry takes place at surface of electrode
species recognition achieved through chemical equilibrium reaction at sensor surface > surface must contain component that reacts selectively and reversibly with analyte
(reversibly if not electrode will only go in one direction > sensor can only react to concentrations getting higher, but not lower)
how do glass sensor in pH meters work
glass electrodes have SiO4- groups > conditioned by putting into HCl > react with H+ to form SiO-H+ groups
sensor dipped into sample > H+ from sample interacts with glass surface > SiO-H+ lose/gain H+ (depending on how many H+ in solution) > exchange of H+ sets up electrical charge potential at interface between glass and liquid
reference electrode in same solution but does not react with H+ > potential difference between glass and reference electrode depends on how many H+ ions in sample solution
what is EG, EG’, ER, ER’ and EJ
EG and EG’: potentials of glass/water interfaces
ER and ER’: potentials of the two reference electrodes
EJ: liquid junction potential
how to make ISE respond to concentration rather than activity
add ionic strength adjuster (ISA)
consists of an electrolyte that is added in same amount to every solution to adjust ionic strength to a constant value > making activity coefficient constant for every solution
