Instrumental Methods

Potentiometry is the field in electrochemistry in which potential is measured under conditions in which there is no current flow. The measured potential used to determine concentration of the electroactive species of interest present in solution.

Indicator electrode—immersed in the solution of the analyte (right-hand electrode)

Reference electrode—half-cell with accurately known electrode potential independent of analyte concentration or other ions in the solution under study (lefthand electrode)

Salt bridge—prevents the components of the analyte solution from mixing with those of the reference electrode

• 1) Known and constant potential that is insensitive to composition of solution under study
• 2) Rugged
• 3) Easy to assemble
• 4) Maintain constant potential even when there is a net current

• 1) Calomel electrodes
• 2) Silver-silver chloride electrodes

Consists of mercury in contact with a solution that is saturated in mercury (I) chloride with known concentration of potassium chloride. Saturated calomel electrodes widely used since easy to prepare.

• Disadvantages:
• 1) Much larger temperature coefficient
• 2) As temperature is changed, solubility equilibrium takes long time to be reestablished so long time to obtain new potential value

Consists of silver electrode immersed in solution of potassium chloride that is saturated with silver chloride. Piece of glass tubing with narrow opening at the bottom connected to Vycor plug for making contact with analyte solution. Can be used at high temperatures, but can also react with more sample components leading to junction pluggings.

• 1) Electrodes of the first kind
• 2) Electrodes of the second kind
• 3) Electrodes of the third kind
• 4) Inert redox electrodes

Pure metal electrode in direct equilibrium with its cation in solution.

• Not widely used because:
• 1) Not very selective
• 2) Responds not only to their own cations but also other more easily reduced cations
• 3) Often have to be used in neutral or basic solutions
• 4) Restricted to solutions that have been deaerated since many metals are easily oxidized
• 5) Also some metals do not provide reproducible potentials

Involves metal electrode being responsive to activity of anion where its ion forms a precipitate or stable complex ion

• Example:
• Electrode for measuring activity of EDTA anion in the presence of EDTA comple

Involves metal electrode responding to a different cation and referred to as electrode of third kind.

• Example:
• Mercury electrode used for determination of pCa of calcium containing solutions.

Electrodes made from platinum, gold, palladium, or other inert metals. Serve as indicator electrodes for oxidation-reduction reaction. Inert electrode serves as source or sink for electrons transferred from a redox system in the solution.

Often called ion- selective electrodes due to high selectivity. Also referred to as pIon electrodes due to output being recorded as pfunction. Types include non-crystalline membrane electrodes and crystalline membrane electrodes.

• 1) Minimal solubility—solubility in analyte solutions approaches zero, usually consists of silica glasses or polymer resins
• 2) Electrical conductivity—must have some electrical conductivity such as migration of singly charged ions within the membrane
• 3) Selective reactivity with the analyte—must be capable of selectively binding the analyte ion

Most widely used electrode. Involves measuring potential difference across a glass membrane separating the analyte solution from reference solution of fixed acidity. Concept extended for development of membrane electrodes for many other ions.

• 1) Glass indicator electrode consisting of thin pH-sensitive glass membrane sealed onto one end of heavy-walled glass or plastic tube.
• 2) Silver-silver chloride or saturated calomel reference electrode.

Glass membrane surface must be hydrated before it functions as a pH electrode. Should be hygroscopic to serve as a pH electrode. Non-hygroscopic glasses have no pH function.

Hydration process involves ion-exchange reaction between singly charged cations in the glass lattice and protons in solution.

Glass membrane must conduct electricity to serve as an indicator for cations. Conduction involves movement of hydrogen ions. Sodium ions are the charge carriers in the interior of the membrane.

• 1) Boundary potential
• 2) Potential of internal Ag-AgCl reference electrode
• 3) Small asymmetry potential

Consists of two potentials, each associated with one of the two glass surfaces. Boundary potential is the difference between these two potentials and is a measure of the hydrogen ion activity of the external solution.

When identical solutions placed in two sides of glass membrane, boundary potential should be in principle zero. However, small asymmetry potential usually observed that changes gradually with time.

• Sources:
• 1) Differences in strain on the two surfaces of the membrane created during manufacture
• 2) Mechanical abrasion on the outer surface during use.
• 3) Chemical etching of the outer surface.

Glass electrodes respond to concentration of both the hydrogen ion and alkali metal ions solution at pH of 11 or 12 or higher in basic. This results in pH values that are lower than true values. The amount of variation varies with composition of the glass membrane.

Typical glass electrodes have an error in solutions with pH less than 0.5 that is opposite in sign to that of alkaline error. Here, the pH readings are higher than their true values. The magnitude of the acid error depends on many factors.

• 1) Errors in low ionic-strength solutions
• Due to non-reproducible junction potentials because of partial clogging of fritted plug or porous fiber used to restrict flow of liquid from salt bridge to analyte solution

• 2) Error in pH of the standard buffer
• inaccuracies in preparation of the buffer, changes in buffer composition during storage.

• 3) Errors due to temperature changes
• when pH measurements made at other temperatures, pH –meters must be adjusted to compensate for change in Nernstian response of the glass electrode.

Referred to as ISFETs and are microfabricated ion-selective electrodes. ISFETs are covered with an insulating layer of silicon nitride. Changes in hydronium ion concentration of solution results in change in concentration of adsorbed protons. Change in concentration of adsorbed protons gives rise to changing electrochemical potential between the gate and the source. This in turn changes the conductivity of the channel of the ISFE.

• 1) Very rugged
• 2) Small in size
• 3) Inert toward harsh environments
• 4) Provides rapid response
• 5) Also has low electrical impedance
• 6) Does not require hydration before use and can be stored in dry state for long time

• 1) Can be easily used portable instruments.
• 2) Can be used in the field at remote locations.
• 3) Also typically used in instruments for clinical bedside applications for detection of various ions.

Electrochemical cells made of specificion electrode and reference electrode immersed in internal solution retained by thin gas-permeable membrane. Very selective and sensitive devices for determining dissolved gases or ions converted to dissolved gases.

Consists of a thin, porous membrane that is easily replaceable. This membrane separates analyte solution from internal solution containing sodium bicarbonate and sodium chloride. pH-sensitive glass electrode with flat membrane held in position so very thin film of internal solution sandwiched between it and the gas permeable membrane. pH of the film of liquid next to glass electrode provides measure of carbon dioxide content of the analyte solution on the other side of the membrane.

Microporous—made from hydrophobic polymers with porosity of 70% and pore size of less than 1 micron. Water molecules and electrolyte ions excluded from pores. Gas molecules free to move in and out of the pores.

Homogeneous—solid polymeric substances where analyte gas passes by dissolving in the membrane, diffusing, and desolvating into internal solution. Silicon rubber most common materials for constructing these films. Generally thinner than microporous membranes to hasten transfer of gas and rate of response of the system.

Selectivity of biochemical materials and reactions with electrochemical transducers can be combined to give highly selective biosensors for biological biochemical compounds. Enzymes, DNA, antigens, antibodies, bacteria, cells and whole animal/plant tissue. When analyte molecules react with these biological materials, triggers production of species that can be monitored directly or indirectly by ion- or molecular selective electrodes. Selectivity advantage counterbalanced by disadvantage of limited stability of many biochemical substances.

Most widely studied and useful biosensors. Sample brought into contact with immobilized enzyme that reacts with analyte to yield different species. Concentration of product is proportional to analyte concentration and determined by transducer.

• 1) Complex organic molecules can be determined with convenience, speed, and ease
• 2) Enzyme-catalyzed reactions occur under mild temperatures and pHs and low substrate concentration
• 3) Combining selectivity of enzymatic reaction and electrode response provides results that are free from interferences.

• 1) High cost of enzymes especially for routine or continuous measurements.
• 2) Can be overcome by using immobilized enzyme media where small amount of enzyme can be used for repetitive analysis of hundreds of samples.

• 1) Automated monitor to analyze blood samples at bedside of patients
• 2) Can determine broad range of clinically important analytes such as potassium, sodium. chloride, pH, glucose, etc.
• 3) Results sufficiently reliable and cost effective