Analytical Exam 2

Potentiometry

Use of electrodes to measure voltage and obtain chemical information about analyte activity.

Indicator (working) electrode

Electrode whose potential responds to analyte activity or concentration.

Reference electrode

Electrode that maintains a known, stable potential while the indicator electrode responds to the sample.

Standard hydrogen electrode (SHE)

Primary reference standard assigned E° = 0.000 V; practical measurements often use convenient Ag/AgCl or calomel references.

Ag/AgCl reference electrode

Half-cell AgCl(s) + e- ⇌ Ag(s) + Cl-; E° = +0.222 V and about +0.197 V in saturated KCl.

Calomel reference electrode

Half-cell 1/2 Hg2Cl2(s) + e- ⇌ Hg(l) + Cl-; E° = +0.268 V and about +0.241 V in saturated KCl.

Saturated calomel electrode (SCE)

Calomel electrode containing saturated KCl; its potential is about +0.241 V under the stated conditions.

Cell potential, Ecell

Difference between the right- and left-electrode potentials under the chosen cell convention; use reduction potentials consistently and account for the reference electrode.

Nernst equation

Relates electrode potential to activities: E = E° - (RT/nF) ln Q; at 25°C, E = E° - (0.05916 V/n) log Q.

Metal indicator electrode

Conducting metal surface develops a potential through a redox equilibrium involving the analyte; examples include Pt, Au, and carbon surfaces.

Platinum indicator electrode

Common relatively inert redox indicator electrode; best response requires a large, clean surface.

Ion-selective electrode (ISE)

Electrode that responds selectively to an ion by membrane binding/partitioning rather than an analyte redox reaction.

Ionophore

Ligand in an ISE membrane that selectively binds and carries a target ion through the membrane.

Liquid-based ISE membrane

Hydrophobic polymer containing viscous organic phase, ionophore, and hydrophobic counterion; target ion can cross while other ions are largely excluded.

ISE inner filling solution

Contains a fixed target-ion concentration and counterion; its constant activity helps make the inner membrane potential stable.

Membrane potential

Potential generated by selective ion partitioning/diffusion across an ISE membrane; varies with target-ion activity in the sample.

Potassium ISE ligand

Valinomycin, selected for high affinity for K+ relative to many other ions.

ISE response equation

For an ideally selective monovalent cation at 25°C, potential changes by about 59.16 mV per tenfold activity change; slope magnitude is 0.05916/n V per decade for charge n.

Junction potential

Voltage difference at the boundary between dissimilar electrolyte solutions, caused by ions diffusing at different rates.

Observed potentiometric voltage

Observed voltage includes cell potential plus liquid-junction contribution: Eobserved = Ecell + Ejunction (with sign set by cell convention).

Reducing junction potential

Use a salt bridge electrolyte whose ions have similar mobilities, commonly concentrated KCl, to minimize charge separation.

Glass pH electrode

ISE with a thin hydrated glass bulb whose membrane potential responds mainly to external hydrogen-ion activity.

Hydrated glass gel layer

Thin water-rich layer on each glass surface where H+ can exchange with metal cations in the glass; inner and outer layers are separated by dry glass.

Glass membrane ion exchange

H+ from solution replaces mobile metal cations (such as Na+) in the hydrated glass layer; this creates the pH-dependent membrane potential.

pH electrode reference system

Two Ag/AgCl reference electrodes and fixed internal chloride/hydrogen-ion conditions measure the potential across the glass membrane.

pH calibration

Measure voltage in at least two standard buffers of known pH to establish the electrode slope/intercept and electromotive efficiency.

Electromotive efficiency, b

Practical glass-electrode slope relative to the ideal Nernst slope; typically greater than 0.98 and determined by calibration.

pH measurement errors

Important sources include standards, junction potential and drift, sodium error, acid error, equilibration time, glass hydration, temperature, and cleaning.

Sodium error

At high pH, glass electrode may respond to Na+ as well as H+, causing the indicated pH to be lower than the true pH.

Acid error

At very low pH, glass electrode response becomes non-ideal and often indicates a pH higher than the true pH.

ISE selectivity coefficient

Quantifies an electrode's relative response to an interfering ion compared with the target ion; smaller interference coefficient means better selectivity.

ISE classes

Glass membrane, solid-state, liquid-based, and compound electrodes.

Solid-state ISE

Uses an inorganic crystal membrane through which the target ion migrates selectively.

Fluoride ISE

Uses Eu2+-doped LaF3 crystal; dopant creates anion vacancies that allow F- migration; used to monitor water fluoridation.

Fluoride electrode filling solution

Typically 0.1 M NaF and 0.1 M NaCl in the described electrode.

SECTION: CHAPTER 18 - SPECTROPHOTOMETRY

Spectroscopy

The theoretical study of interactions between matter and radiated energy.

Spectrometry

The practical measurement/application of spectroscopy using instruments such as spectrometers.

Spectrophotometry

Quantitative measurement of how much a substance absorbs light as a beam passes through a sample.

Spectrophotometer

Instrument that measures transmission or reflection as a function of wavelength to produce a spectrum.

Wavelength, λ

Distance between successive wave maxima or minima; usually measured in m or nm.

Frequency, ν

Number of oscillations per second; measured in Hz or s-1.

Speed of light relationship

c = νλ; speed of light c is approximately 3.00 x 10^8 m/s.

Photon

Particle of electromagnetic radiation with energy E = hν = hc/λ.

Planck constant, h

6.626 x 10^-34 J s.

Irradiance, P

Energy per second per unit area carried by a light beam; absorption reduces transmitted irradiance.

Monochromatic light

Light consisting of one wavelength (ideally); required for Beer’s law in its simple form.

Transmittance, T

Fraction of incident light transmitted: T = P/P0, where P0 is incident and P is transmitted irradiance.

Percent transmittance

%T = 100(P/P0).

Absorbance, A

Logarithmic measure of light absorbed: A = -log10(T) = log10(P0/P).

Beer-Lambert law

A = εbc; absorbance equals molar absorptivity times pathlength times analyte concentration.

Molar absorptivity, ε

Proportionality constant in Beer’s law, typically M^-1 cm^-1; depends on analyte and wavelength.

Pathlength, b

Distance light travels through the sample, usually in cm; common cuvet pathlength is 1.000 cm.

Chromophore

Part of a molecule responsible for its light absorption.

Beer’s law assumptions and limits

Best applies to dilute solutions (about ≤0.01 M) and monochromatic light; it can fail with chemical changes, nonlinearity, or other deviations.

Absorption of light

Analyte absorbs a photon and moves to a higher-energy molecular state; transmitted irradiance decreases.

Spectrophotometer components

Continuous light source, monochromator to select a narrow wavelength band, sample, and detector for emergent irradiance.

Cuvet

Transparent sample cell used in UV-visible spectrophotometry; sample and reference cuvets should be matched and positioned consistently.

Quartz/fused-silica cuvet

Transmits UV and visible light and is suitable for UV-visible measurements.

Glass cuvet

Suitable for visible light but absorbs UV, so it is unsuitable for many UV measurements.

IR sample cells

Typically made of NaCl or NaBr, which transmit infrared radiation.

Mull

Solid sample ground and dispersed in mineral oil (Nujol), then pressed between KBr plates for IR measurement.

Single-beam spectrophotometer

Measures blank/reference irradiance P0, then replaces the blank cuvet with the sample and measures P.

Double-beam spectrophotometer

Splits or alternates light between sample and reference cuvets to compare their signals.

Baseline spectrum

Reference spectrum recorded before the sample; subtracted from the sample spectrum to correct background.

Wavelength of maximum absorbance, λmax

Chosen for analysis because sensitivity is high and absorbance changes relatively little with small wavelength shifts.

Useful absorbance range

Approximately 0.4 to 0.9; too much absorption leaves little measurable light, while too little makes sample and blank hard to distinguish.

Noise and detection

Lower instrumental noise generally allows detection of lower analyte concentrations.

Good cuvet practice

Use matched cuvets, place them consistently, and wipe fingerprints and dust from optical faces.

Analyte suitability for spectrophotometry

Analyte must absorb at a measurable wavelength and its absorption should be distinguishable from other sample components.

Molecular absorption and energy levels

Absorption promotes a molecule to an excited state; emission returns it to a lower-energy state and releases a photon.

Electronic transition

Transition between electronic energy states; requires more energy than typical vibrational or rotational transitions.

Vibrational transition

Change in molecular vibrational motion; can be induced by infrared radiation.

Rotational transition

Change in molecular rotational motion; can be induced by microwave radiation.

Vibrational relaxation

Nonradiative loss of excess vibrational energy within an electronic state, usually as heat.

Internal conversion (IC)

Nonradiative transition between electronic states of the same spin multiplicity, such as S1 to S0.

Intersystem crossing (ISC)

Nonradiative transition between states of different spin multiplicity, such as S1 to T1.

Singlet state, S

A state in which electron spins are paired; common fluorescent emission occurs from S1 to S0.

Triplet state, T

A state with parallel electron spins; phosphorescence commonly occurs from T1 to S0.

Fluorescence

Photon emission from an excited singlet state to a lower singlet state; typical lifetime about 10^-10 to 10^-8 s.

Phosphorescence

Photon emission from an excited triplet state to a singlet ground state; typical lifetime about 10^-4 to 10^2 s.

Luminescence

Light emission from an electronically excited molecule, including fluorescence and phosphorescence.

Nonradiative transition

Relaxation that releases energy without emitting a photon, commonly as heat.

Stokes shift

Emission occurs at lower energy and longer wavelength than absorption because some energy is lost before emission.

Emission spectrum

Plot of emitted-light intensity versus emission wavelength, λem.

Excitation spectrum

Plot of emission intensity versus selected excitation wavelength, λex.

Emission measurement geometry

Select an excitation wavelength and commonly observe emitted light at 90 degrees to the incident beam to reduce transmitted and scattered excitation light.

Mirror-image relation

Emission spectrum is often roughly the mirror image of the absorption spectrum, with vibronic structure shifted to longer wavelengths.

0-0 transition, λ0

Highest-energy emission transition between lowest vibrational levels of excited and ground electronic states; further emission peaks occur at longer wavelengths.

Emission intensity and concentration

At low concentration, emission intensity is proportional to absorbed irradiance and approximately proportional to analyte concentration.

SECTION: SENSORS AND ATOMIC SPECTROSCOPY

Luminescence-quenching sensor

Uses a change in excited-state emission caused by a quencher to measure quencher concentration.

Excited-state molecule, M*

Molecule after absorbing a photon; it can emit light, lose energy as heat, or transfer energy to a quencher.

Excitation rate

Rate of excited-state formation by absorption: proportional to ka[M] for incident light of fixed intensity.

Emission rate

Excited-state loss by photon emission: ke[M*].

Nonradiative decay rate

Excited-state loss as heat: kd[M*].

Quenching rate

Excited-state loss by interaction with quencher Q: kq[M*][Q].

Quantum yield, Φ

Fraction of absorbed photons that produce the specified outcome, such as emitted photons.

Unquenched emission quantum yield, Φ0

Φ0 = ke/(ke + kd) when no quencher is present.

Emission quantum yield with quencher, Φ

Φ = ke/(ke + kd + kq[Q]) for the stated dynamic quenching model.