Chapter 14: Electrodes and Potentiometry

Potentiometry

obtaining chemical information through voltag measurement

indicator electrode (working)

responds to analyte activity

• connects analyte half-cell to a second half-cell by a salt bridge

reference electrode

maintains a fixed potential 

Ag-AgCl & calomel reference electrodes are more convenient bc…

a standard hydrogen electrode (SHE) is difficult to use as it requires H2 gas and a freshly prepared catalyic Pt surface that is easily poisoned in many solutions

Ag-AgCl electrode

reference electrode that maintains a stable, known potential

• made of Ag metal coated with AgCl

• immersed in a solution containing Cl- ions, usually KCl of known concentration

calomel electrode

reference electrode that provides a stable, known potential, but it’s based on mercury (Hg) and mercury (l) chloride (Hg2Cl2)

insert metals

plantium & gold

• gold are even more inert - it does not participate in many chemical reactions

the junction potential puts a fundamental limitation on the 

accuracy of direct potentiometric measurements bc the contribution of the junction to the measured voltage is unknown

junction potential (liquid junctional potential)

a small voltage difference that develops where two different electrolyte solutions meet - such as at a salt bridge or between your refernce electrode and the test solution

two main families of electrodes

ion-selective electrodes and metal (redox)

metal (redox) electrodes

made from a metal that’s in direct contact with a solution containing its own ions

• electron activity

• Cu/Cu2+, Zn/Zn2+

ion-selective electrodes

non-metal electrodes designed to respond selectivity to a specific ion in solution (not to electrons directly)

• work using a membrane that selectively binds or transports a certain ion

liquid-based ion selective electrode

why liquid-based? because the membrane is a hydrophobic organic polymer impreganted with a viscous organic solution containing an ion exhcanger and, sometimes, a ligand that selectively binds the analyte cation, C+. 

glass electrode

measures how many hydrogen ions are in a solution by detecting the tiny voltage difference created when H+ ions move across a thin glass membrane that separates the sample from an internal solution

• voltage changes by about 59 mV per pH unit, then will pH meters will give a numerical value

why glass is special

• glass composition = allows Na+- H+ exchange at hydrated surface

• outermost layer becomes hydrated, H+ to bind/release quickly

• potential across thin layer is reproducible and Nernstian

ion-exchange equilibrium 

reaction in which H+ replaces cations in the glass

• H+ - main ion that binds significantly to hydrated gel layer

selective of ion-selective electrodes

ion-selective electrodes respond to the activity of free analyte, not complexed by any other species