Ch 15 - Electrodes and Potentiometry

Problem with reference electrodes:

porous plugs become clogged leading to sluggish and unstable chemical response

silver - silver chloride reference electrode

(Ag|AgCl)

Ion Selective Field Effect Transistors

(pH sensor) ISFET -path from source to drain involve reverse bias so little current flows -+ charged chemically sensitive layer attracts e-'s to surface of base between source and drain, allowing current to flow -more + gate = more current between source and drain

solid-state ISE

-based on the field effect transistor, another type of pH electrode ex) fluoride electrode

junction potential

an electric potential that exists at the junction between two different electrolyte solutions or substances. it arises in solutions as a result of unequal rates of diffusion of different ions -found at each end of the salt bridge

The selectivity coefficient, 𝐾PotA, X,�A, XPot, of an ion‑selective electrode (ISE) indicates

the relative response of the ISE to the ion of interest (A) and an interfering ion (X).

glass electrode

(most common ISE) an electrode for measuring pH from the potential difference that develops on the thin glass membrane when it is dipped into an aqueous solution containing H+ ions -Na+ ions diffuse OUT of the hydrated gel layer (they can move through glass) while H+ anions bond to replace the cations that left -pH is fixed for the internal solution, pH of external solution changes

In an alkaline solution with a high Na+Na+ concentration, a glass pH electrode tends to indicate a pH that is lower than the actual pH. Why?

The Na+ ions bind to the same exchange sites as the H+ ions on the surface of the glass electrode. The electrode responds to Na+ as if H+ is present, leading to an apparent pH that is lower than the actual pH.

How can liquid junction potential be minimized?

When mobilities of cation and anion are similar ex) saturated KCl in salt bridge (because K+ and Cl- have similar mobilities)

transistors

a semiconductor device used to amplify or switch electronic signals and electrical power -amplifies things

metal electrodes

develop an electric potential in response to a redox reaction at the metal surface ex) Pt, gold, carbon

liquid membrane ISE

hydrophobic membrane with a hydrophobic ion exchanger (ionophore) selective for analyte ion -species L which binds analyte ion C cannot leave membrane -extraction of some analyte into or out of membrane generates potential difference at each interface, dE responds only the changes in analyte in EXTERNAL solution

Why is SHE difficult to use?

it requires H2 gas and a fresh catalytic Pt surface that is easily poisoned in solution

liquid junction potential

kinetic potential difference originating with different mobilities of ions in two contacting electrolyte solutions -May limit accuracy of potential measurements because we do not know the contribution of junction to the voltage.

reference electrode

one that maintains a constant potential against which the potential of another half-cell may be measured -half cell + salt bridge -mostly on the left side ex) Ag|AgCl, SHE, SCE

Indicator electrode (working electrode)

responds to analyte activity 1. Metal electrodes 2. ion selective electrodes ex) Pt

ion selective electrodes (ISE)

selective binding of one type of ion to a membrane generates electric potential -do NOT involve redox processes ex) glass electrode, glass combination pH electrode, solid-state ISE, liquid membrane ISE

The smaller the value of 𝐾PotA, X,

the more selective is the ISE

ion exchange equilibrium

the reaction in which H+ replaces cations in the glass

potentiometry

the use of electrodes to measure voltages that provide chemical information -electrodes respond selectively to specific analytes in solution or gas phase

fluoride electrode

used to monitor the fluoridation of municipal water supplies HOW: (LaF3 + EuF2) F- ions migrates through creating vacancies, F- ions fill in and leave vacancies -> this is how they diffuse from one side to the other -this creates a tiny electric current

Calomel Electrode (SCE)

Advantage: [Cl-] does not change if liquid evaporates -uses mercury

Explain how a chemical‑sensing FET generates a response to an analyte.

1. A voltage is applied between the source and the drain, but no current flows. 2. A positive potential is applied to the gate, which attracts electrons from the base. 3. A current flows between the source and the drain as a conductive channel forms. 4. The sample is introduced to the gate. 5. Analyte absorbs onto the chemically sensitive gate, resulting in a change in potential of the gate. 6. Current flowing between the source and the drain changes. 7. Potential is applied to the external circuit to return the current to its original value.

Steps of an Ion Selective Electrode:

1. Electrode is placed in aqueous solution. 2. Analyte ions in the external sample solution with equilibrate with ion exchange sites in the membrane. 3. Analyte ions diffuse out of the membrane. 4. A charge imbalance (potential difference) develops across the interface. 5. Potential diff is measures using internal and external reference electrodes. 6. Concentration of analyte ion is determined in external solution by potential difference.

Types of Ion selective electrodes

1. Glass membranes - for H+ and monovalent cations 2. Solid-state electrodes - based on inorganic crystals of conductive polymers 3. Liquid-based electrodes - use hydrophobic polymer membrane saturated with a hydrophobic liquid ion exchanger 4. Compound electrodes - analyze-selective electrode enclosed by a membrane that separates analyte from other species or that generates analyte in a chemical reaction

Errors in pH measurement

1. standards - pH measurement cannot be more accurate than our standards (which are +/- 0.01 pH unit) 2. junction potential - (at porous plug) may be different between analyte and standard solution 3. Alkaline error/sodium error - when [H+] is very low and [Na+] is very high, electrode responds to Na+ = apparent pH LOWER than true pH 4. acid error - saturation of ion sites at HIGH [H+] causes pH to be HIGHER than true pH 5. equilibration time - 30s-many minutes 6. hydration of glass - dry electrodes require many hours of soaking 7. temperature and cleaning

Mechanism of ISE's

Have a very slightly conductive membrane separating two regions of solution. At each interface an equilibrium is established between some ion in solution and that ion bound to sites in the membrane. Different concentrations of the ion on the two surfaces of the membrane result in a potential different across the membrane (junction potential). -ISE (pH electrode) uses two reference electrodes (2 Ag\AgCl's) for which dE is constant. Membrane is in between the two reference electrodes -Potential on interior is CONSTANT since the compartment is sealed **Net dE responds to change at EXTERIOR membrane only