Electrochemistry

Activity (electrochemistry)

The proper term for the concentration of an electrolyte measured in an electrochemical cell, used in the Nernst equation

Activity coefficient

The activity of an electrolyte divided by molar concentration. It is a measurement of the interaction of the selected electrolytes with other species in the solution

Potentiometry

An electrochemical technique that measures electric potential (E) between two electrodes under equilibrium condtiions

What are the electrodes of a potentiometric electrode

Reference electrode and indicator electode

Common reference electrode materials

Saturated calomel and silver/silver chloride

Ag/AgCl electrode features

Inside impermeable case (glass or plastic) - consists of silver wire coated in AgCl and immersed in chloride solution

Reference electrode junction

A junction (frit) to allow electrical, ionic conductivity between the sample solution and internal filling solution while preventing large scale corrective mixing

Is there an electric potential at the reference electrode junction?

Yes, a small one. Is substracted when determining electric potential of the cell

Main difference between indicator and reference electrodes

Indicator electrode has ion selective membrane to allow for potential difference

How does a potential difference occur?

When there is a difference in ion activity between each side of the ion selective membrane

How to calculate Ecell

Ecell = Eind - Eref - Ejxn

What is the Nernst equation

Describes the logarithmic relationship between E cell and the activity of an electrolyte. It is temperature dependent

4 classes of ion selective membranes

Glass, Polymer, Solid State, Gas Sensing

Use of glass ion selective membranes

Used to measure H+ and Na+. Composition determines selectivity

Composition of glass ion selective membranes

SiO2, Na2O, CaO, or Al2O3

Polymer membrane composition

Ion exchanger or ionophore dissolved in viscous, water insoluble solvent. Solvent is often PVC

What are ionophores

Molecules that reversibly bind ions

Crystalline ion selective membranes

Single type of crystal or pressed ion of interest salt pellets that have electrical conductivity (ex. Chloride electrode composed of AgCl)

How is electric potential created in crystalline ISE

Movement of ions in sample solution into vacancies in the membrane crystal lattice

Gas permeable ion selective membrane

Gas permeable electrode with thin outer membrane that is permeable to the gas of interest and an internal electrode.

Seen in CO2 and Urea

How does CO2 ISE work

O2 diffuses through the membrane and interacts with a weak bicarbonate solution to form carbonic acid; then, the dissociated H+ interacts with the internal electrode.

What are some factors that cause limitations/interferences in ISEs

Temperature

Ionic strength

pH

Biofouling

Cross-reacting ions

Electrolyte exclusion effect

What is the electrolyte exclusion effect

Indirect ISEs are diluted and rely on a calculation to determine the actual amount of analytes in the sample. Sodium concentration will be falsely low with increased proportions of solids such as hyperlipidemia or hyperproteinemia. This is caused by an increase in fractional water content

How does temperature interfere with ISEs

Nernst equation is temperature dependent = Ecell is altered

How does ionic strength interfere with ISEs

The total ionic strength of a sample affects the activity coefficient of the ion

How does pH interfere with ISEs

Some ions require conversion of the analyte to one form (calcium)

What is biofouling

Protein buildup on ISE membrane

Effect of cross-reacting ions on ISEs

Ions not usually found in sample reacts with membrane, causing interference