03: electrochemistry and potentiometry

electrochemistry

branch of analytical chemistry that uses electrical measurements of chemical systems for analytical purposes

ionic strength

measure of the net effect of dissolved electrolyte

activity (a)

represents the effective concentration of each species

activity coefficient (γ)

relates activity to concentration

redox reaction

change in the oxidation states of the reactants

reduction

gain of electrons (oxidation state decreases)

oxidation

loss of electrons (oxidation state increases)

anode

electrode where oxidation occurs

cathode

electrode where reduction occurs

salt bridge

• commonly a tube with a porous frit at each end and filled with electrolyte

• prevents direct mixing of the two electrolyte solutions

• ions conduct current across to complete the circuit

potential difference (voltage)

reflects the driving force of the associated redox reaction toward equilibrium

cell potential

combination of standard potentials

positive cell potential

galvanic cell (spontaneous reaction; can perform electrical work)

negative cell potential

electrolytic cell (non-spontaneous; electrical work must be done to drive the chemical reaction)

standard potential

• measure of the driving force for a reaction from a state of unit activity for reactant(s) and product(s) to their equilibrium concentrations when coupled with the SHE half-cell

• how far the reaction is from equilibrium

standard hydrogen electrode (SHE)

reference point to which other half-cells are compared

thermodynamic electrochemical cell potential

the difference between the cathode potential and anode potential (both written as reductions)

loading error

• when the measurement circuit draws non-trivial current and causes a voltage drop

• minimized with high resistance measurement circuits

inert electrodes

• respond to redox couples without participating directly in the reaction

• serve as an interface for transferring electrons from one half-cell to the other

electrodes of the first kind

metal electrode in direct equilibrium with its metal cation in solution

electrodes of the second kind

respond to ions that form an insoluble salt with the metal

equivalence point

amount of oxidant/reductant added is equal to the amount of the analyte

endpoint

observable change that (approximately) signals the equivalence point

auxiliary oxidizing/reducing agents

used for pre-oxidation or pre-reduction of samples

indicators

reagents that undergo half-cell reactions with an associated color change

liquid junction potential (E_j)

potential difference that develops across an ion permeable boundary between different electrolyte solutions

membrane potential (E_m)

separation of charge at the double-layer

boundary potential (E_b)

difference between two membrane potentials (each side of the interface)

alkaline error

at high pH, monovalent metals can associate with the glass membrane and generate a change in potential

separate solution method

determines selectivity coefficients by measuring the analyte and interferent ion(s) separately

fixed interference method

determines selectivity coefficients by preparing a calibration curve with different activities of analyte and a fixed activity of interference

total ionic strength adjustment buffer (TISAB)

• buffer solution that increases the total ionic strength to a high level

• overwhelms the contribution from the sample, such that the sample and calibration standards can be measured at approximately the same ionic strength, even though the ionic strength of the sample is unknown

ionophore

hydrophobic organic molecule that serves as an ion exchanger