W4 Potentiometry Flashcards
what is a chemical sensor
device that responds to a particular analyse 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 measure 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 has 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%)
