Chapter 22: Introduction to Electroanalytical Chemistry

analyte in solution is part of an electro-chemical cell

advantages:

1. may be specific for specific oxidation state2. relatively cheap instrumentation3. provide activities (effective conc) rather

than concentrations

electrochemistry - interchange of chemical and electrical energy.

oxidation - loss of electronsreduction - gain of electrons

review: balance redox equations

Typical electrochemical cell:

Cu2+ + Zn → Cu + Zn2+

anode - electrode where oxidation occurs cathode - electrode where reduction occurs salt bridge - allows for passage of ions

line notation : Zn / Zn2+( M) // Cu2+( M) / Cu

charge is carried in 3 ways:

1. electrons carry charge in electrodes and in external circuit

2. anions and cations carry charge in solutions and in salt bridge

3. oxidation and reduction at electrode surfaces

galvanic cell - uses a spontaneous redox reaction to generate electricity

electrolytic cell - consume electricity to drive nonspontaneous reaction

cell potential - driving force for electron flow; measured in volts; also called emf or electromotive force

standard reduction potential( E o ) - potential of a half-reaction, written as a reduction, under standard conditions (P = 1 atm, [X] = 1 M), compared to the std hydrogen electrode

2 H+ + 2 e- → H2(g) Eo = 0.0 V

These can be found in tables.

Eocell = Eo

cathode + Eoanode

If Ecell > 0, this is a spontaneous process and the cell is a galvanic cell.

ΔGo = - nFEo n = moles e- changedF = Faraday's cst = 96,485 C/mole e-

1 C = 1 amp x 1 sec

note: ΔG = - RT ln K, so this is one method of determining K

If Ecell < 0, this is a nonspontaneous process and the cell is an electrolytic cell.

dependence of cell potential on concentration is given by the Nernst equation:

E = Eo - RT/nF ln (aproducts / areactants)

At 25o C, this may be simplified to

E = Eo - 0.0591/n log (aproducts / areactants)

so one can calculate Ecell if concentrations are known.

At equilibrium, Ecell = 0.00 V and (aproducts / areactants) = K

so this becomes Eo = 0.0591/n log K,

or log K = nEo / 0.0591.

notes:

liquid junction - interface between two solutions containing different electrolytes or different concentrations of the same electrolyte; will cause a potential difference

all cell voltage measurements measure ΔV; cannot measure absolute V

cell potentials are affected by 1. pH2. precipitates and complexing agents

Types of electroanalytical methods:

Chapter 23: Potentiometry

based on measurement of potential of electro-chemical cells with negligible current drawn

Ecell = Eind + Eref + Ej

Eind = potential of indicator electrodeEref = potential of reference electrodeEj = liquid junction potential

Ideally, Ej = 0.0 V and Eref is constant

Reference electrodes should

1. be easy to assemble2. provide reproducible potential3. remain unchanged in potential with passage

of small amounts of current

Standard Hydrogen Electrode

2 H+ + 2 e- → H2(g) Eo = 0.00 V

Calomel Electrode

Hg2Cl2 + 2 e- → 2 Hg(l) + 2 Cl-

Eo depends on KCl solution conc; for saturated calomel electrode @ 25o C = 0.2444 V

Silver / Silver Chloride Electrode

AgCl(s) + e- → Ag(s) + Cl-

Indicator Electrodes

1. first kind - metal in contact with its ions

Ex: Cu / Cu2+

2. second kind - metal in contact with an anion with which its ion forms a ppt or complex

Ex: Ag / AgCl electrode

3. third kind - metal responsive to other cations

membrane electrodes - many types of ion specific membrane electrodes

glass pH electrode - potential forms across thin glass membrane separating two solutions of different pH. Potential is due to ion exchange processes on membrane surfaces

note: actually measures activity of H+ directly

must standardize pH meter

potentiometric titrations - follow progress of titration by measuring potential

may be used for titrations for which there is no good indicator

nonaqueous titrations

Chapter 24: Coulometry

method is based upon complete oxidation or reduction of an analyte

1. constant-potential coulometry2. constant-current coulometry

both of these measure amt of electricity

3. electrogravimetry - deposit material on electrode and weigh

electrolysis - a nonspontaneous reaction is driven by an application of electricity

overvoltage - amount of potential required above calculated E value to enable reaction to proceed

current-voltage relationships:

1. constant-potential

2. constant-current

Chapter 25: Voltammetry

method based upon current behavior as a function of applied voltage at a polarizable (micro)electrode

This differs from coulometry in that only a small portion of the analyte is oxidized (or reduced), compared to all in coulometry.

Polarography - one of the first techniques; usually utilizes dropping Hg electrode

voltammogram and information obtained:

note: must eliminate dissolved O2 or it will reduce at cathode. This can be done by bubbling an inert gas through the system.

Chapter 26: Introduction to Chromatographic Separations

chromatographic separations - based on physical properties of a substance; the tendency of a substance to be attracted to another substance.

chromatography - differential migration method based upon the rate of migration of a solute in a mobile phase through a stationary phase

mobile phase - provides driving force- usually gas or liquid- force is flow (gravity or pressure)

stationary phase - retarding force- paper, finely divided solid, finely divided solid coated with thin film of viscous liquid- force is adsorption onto solid or

partition between liquid coating of solid and mobile phase

discovered by Russian botanist Tswett

Types of chromatography:

Classification by mobile phase:

1. gas chromatography2. liquid chromatography

Classification by retardation mechanism:

1. adsorption (GSC)2. partition (GLC)

Classification by equipment:

1. column chromatography2. thin-layer chromatography (TLC)3. paper chromatography

chromatogram - plot of amount of analyte detected v. time (or volume)

distribution constant (coefficient):

K = cS / cM cS = conc in stationary phase cM = conc in mobile phase

retention time (tR) - time required for a solute to migrate the length of a column

dead time (tM) - time required for unretained species to migrate length of column

retention factor (k) -

k = (tR - tM) / tM

selectivity factor (α) - (for solutes A and B)

α = KB / KA = kB / kA (note: α > 1)

peak shape - should be Gaussian, but tends to broaden with longer retention times.

column efficiency - usually measured as theoretical plates, which are equilibrium steps in elution.

N = number of platesN = L / H L = column length

H = plate height (HETP)

Observe elution peak. Let tR = x and let y = peak base width.

N = 16 (y/x)2

HETP = L / N

Van Deemter Eq:

HETP = A + B/μ + C μ (μ = mobile phase velocity)

optimal separation when HETP is minimal

resolution(R) - measure of ability of a column to separate two analytes

R = 2( tRB - tRA) / (wA + wB)

notes:

If α = 1, no separation. α depends on nature of liquid phase and on T.

In general, 30o increase in T will halve K, thus doubling rate of component migration.

Lower T = better resolution.

Chapter 27: Gas Chromatography

Mobile phase is gas.

Stationary phase is finely divided solid or finely divided solid coated with nonvolatile liquid contained in a column.

packed column - mobile phase percolates through stationary phase; usually 6 - 10 ft long and 2 - 4 mm id

capillary column - stationary phase lines column wall; usually 150 - 300 μm id and up to 50 m long.

retention volume(VR):

VR = tR F (F = flow rate)

Instrumentation:

carrier gas - must be nonreactive; P constant

usually He or N2

sample injection system - sample must enter column as a vapor in a short period of time in a reproducible manner without upsetting equilibrium of the column

liquid sample - inject into heated metal block with microsyringe through septum. Temperature must be above boiling point for all components, but below decompo-sition point.

gas sample - inject with gas syringe or use vacuum line

solid sample - pyrolize or make volatile derivative

column - support material is usually diatomaceous earth with CaCO3. Particle size is given in mesh (smaller size = larger mesh)

1. must have large surface area2. uniform pore structure3. chemically inert4. thermal stability

- Thousands of specialty phases for liquid coating for GLC; use depends on sample to be separated. Usually use nonpolar phase for nonpolar samples and polar phase for polar samples for maximal retention.

- Sample must be soluble in liquid phase, or no separation will occur. - temperature range for liquid

- liquid must be chemically inert

Temperature - isothermal or ramped; must be reproducible

usually better separation at lower T

Detectors:

1. thermal conductivity detector - based on fact that a hot body will lose heat at a rate dependent upon composition of surrounding gas.

- advantages - cheap, durable, nondestructive

- disadvantages - poor sensitivity

2. flame ionization detector - effluent from column is burned in a small hydrogen flame, producing ions

- advantages - reliable, sensitive- disadvantages - sample destruction; not good for halogens

3. electron capture - effluent is passed through ionization chamber. Organics will interfere with current flow. Must add quenching gas (CH4). Very sensitive for halogenated compounds.

4. specialty detectors - mass spec, UV, IR

Advantages of GC:

1. speed2. resolution3. qualitative analysis (tR)4. quantitative analysis - peak area5. sensitivity

thermal conductivity - 100 ppmflame ionization few ppm

6. simplicity

Chapter 28: High Performance Liquid Chromatography (HPLC)

"most widely used of all analytical separation techniques" since only about 20% of compounds can be analyzed by GC because of 1)volatility or 2) thermal stability.

Early attempts to apply pressure to speed liquid flow through column were unsuccessful due to increase in HETP. This was remedied by smaller particle size.

Mechanisms of separation:

1. partition (L - L)2. adsorption (L - S)3. ion exchange4. size exclusion

Instrumentation:

Pump - considerations1. pressure delivery over broad flow range2. maximum pressure3. compatibility with different solvents4. noise level in detector

reciprocating pump (about 90% of HPLC's)

- small volume chambers with reciprocating pistons to work directly on solvent or use diaphragms and hydraulic fluid.- constant pressure due to out of phase piston relationship

displacement pump (syringe type)

- limited capacity (250 - 500 mL)- pulseless flow- high pressure

pneumatic pump

- mobile phase is in a collapsible container; use gas pressure

injection system:

microsyringe with blunt tipstopped flow systemsample size set by sample loop

column: (packed)

- heavy walled glass or stainless steel- must be smooth walled- generally straight- usually about 5 mm id for analytical

detector: (many types)

bulk property detector - measures change in solvent property, such as refractive index

- fairly insensitive- good temperature control

solute property detector

- UV-vis (usually fixed λ of 254 - 280 nm)- fluorescence- electrochemical

phases:

solvent - must be degassedgradient elution - change composition reverse phase - hydrophobic packing (usually C8 or C18 functional group and polar (usually aqueous) mobile phase

Chapter 11: Atomic Mass Spectrometry

- used for identification and quantification of almost all elements in periodic table

- advantages of technique1. sensitivity2. easy interpretation3. can measure isotope ratios

- disadvantages of technique

1. cost2. instrument stability3. interferences

- steps in analysis

1. atomize sample2. convert to stream of ions3. separate by mass/chg ratio (m/z)4. count number of ions of each type

- note: amu: 1 atom of 12C is exactly 12.00isotopic masses are exactatomic mass is avg of all isotopes

Instrumentation:

Inlet:

1. gas samples2. liquid samples4. solid samples

Ion Source:

1. ICP2. spark3. glow discharge

Mass Analyzer:

1. Quadrupole2. Time-of-flight

Detector:

1. Electron multiplier2. Faraday cup

Readout:

Mass spectrum - plot of mass (x-axis) v. relative abundance (y-axis)

Chapter 20: Molecular Mass Spectroscopy

When a molecule is ionized, it frequently fragments, and the fragmentation pattern is a fingerprint, which can be used for qualitative analysis.

Molecular ion → M+ (parent peak)

Base peak - most abundant ion

Isotope peaks - due to isotopes

Collision product peaks - may be heavier than parent peak (usually M + 1)

Instrumentation:

Inlet systems

batch inlet - vaporize sample externally

direct probe - used for solid or nonvolatile liquids

Sources:

gas phase source - sample is vaporized, then ionized

desorption sources - sample is converted directly to gaseous ions; can be used for nonvolatile or thermally unstable species

hard sources - rupture more bonds than softer sources ex: electron impact v chem. Ionization

Mass analyzers:

magnetic deflection analyzer

KE = mv2 / 2 = constant

M / Z = β2r2e / 2V β = mag. fielde = electron chgV = voltage