분석화학실험 -...
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분석화학실험
2014학년도 1학기
담당교수: 이원용 (연구실: 과 443-C, 전화: 2123-2649, 전자우편: [email protected])
분광분석/ /분리분석/전기분석화학
General concept
Spectrophotometric experiment
I
Electrochemical experiment
t
E
Lamp Monochromator Optical cell
with sample Phototube
A
Power
supply
i
t
i
Instrumental Analysis
Spectroscopy: the science that deals with “interactions of matter
with electromagnetic radiation or other forms energy”
Spectrophotometry: a more restrictive term,
- any procedure that uses light to measure chemical concentrations.
- the quantitative measurement of the intensity of electromagnetic
radiation at one or more wavelengths with photoelectric detector.
acoustic waves, beams of particles such as ions and electrons
Chapter 18:
Fundamentals of Spectrophotometry
18-1. Properties of Light
Electromagnetic radiation ; EM wave ; radiation ; radient ray ; ray ; light
One linearly (or plane) polarized and consists of a single frequency, that is, is monochromatic.
18-2 Absorption of light
Absorption of light: increases the energy of molecule
(the molecule is promoted to an excited state)
Emission of light: decreases the energy of molecule
Ground state: lowest energy state of a molecule
M + h• υ M*
(life time: 10-6 ~10-9 S)
M* M + light (fluorescence, phosphorescence)
or M* M + heat
Excitation
Relaxation
Absorption
Light
source
Wavelength selector
(monochromator)Sample
Light
DetectorPo P
b
Single-beam spectrophotometric experiment
When light is absorbed by a sample
the radiant power of the beam of light is decreased
Radiant power (P): the energy per second per unit area of the light beam
Transmittance (T): T = P/Po (T = 0 ~ 1)
Absorbance (A), or optical density: A = log (Po/P) = -log T
(if 90% light is absorbed, 10% transmitted: T = 0.1Po/Po = 0.1, A= - log T=1)
Absorption spectrum: absorbance vs wavelength
Absorption: Beer’s Law
The part of molecule responsible for light absorption: chromophore
Absorbance is directly proportional to the concentration
Beer-Lambert law: A = εbc
ε : molar absorptivity (extinction coefficient)
characteristic of a substance that tells how much light is absorbed
at a particular wavelength
b: path length
c: concentration
Beer’s law works for monochromatic radiation passing through a dilute solution< 10 mM
Colorimetry: a procedure based on absorption of visible light
high temp
(2000-6000K)
Chapter 21:
Atomic Spectroscopy
21-1. An Overview
sample
(AES)
(AAS)
(AFS)
Most compounds Atoms in gas phasehigh temp (2000-6000K)
Mass-to-charge
(ICP-MS)
Flame in Atomic Spec.
Cuvette in Mol. Spec.
(Path length in Flame: 10 cm)
Atomic Absorption experiment
Premix burner: fuel, oxidant, and sample are premixed.
Nebulization: formation of a small droplets
Aerosol: a fine suspension of liquid (solid)particles in a gas
Nebulizer:create an aerosol from the liquid sample
“aerosol reaching the flame contains only about 5% of initial sample”
Flames
Pneumatic nebulizer
Monochromators cannot isolate lines narrower than 10-3 – 10-2 nm. To get narrow lines of the correct frequency, Use of hollow cathode lamp containing the same element that being analyzed
Filled with Ne or Ar at a pressure of 130 ~ 700 Pa
High voltage (~300V) is applied between the anode and cathode Filler gas is ionized and positive ions are accelerated toward the cathodeAccelerated positive ions strike the cathode with enough energy to sputter
metal atoms from the cathode into the gas phase Free atoms are excited by collisions with high-energy electrons: photon emission
Atomic radiation has the same frequency as that absorbed by the analyte atoms
Hollow-Cathode Lamp in AAS
Chapter 14:
Fundamentals of Electrochemistry
Electrochemistry is the branch of chemistry concerned with the interrelation of electrical and chemical effects.
The study of chemical changes caused by passage of an electric current and production of electrical energy by chemical reactions.
A galvanic cell : uses a spontaneous chemical reaction to generate electricity
To accomplish this: 1. One reagent must be oxidized2. The other must be reduced3. The two reagents must be physically separated
electrons are forced to flow through external circuit to go from one reagent to the other
14-2. Galvanic Cells (Voltaic Cells)
Anode reaction : oxidation
Cathode reaction: reduction
Cd(s) Cd2+ (aq) + 2 e-
2 AgCl(s) + 2 e- 2Ag(s) + 2Cl-(aq)
When electrons flow from the left electrode tothe right electrode : positive voltage
When electrons flow from the right electrode to the left electrode : negative voltage
Cd(s) + 2 AgCl(s) Cd2+ (aq) +2Ag(s) + 2Cl-(aq)
High input impedance potentiometer (voltmeter)
(+)
A quantitative description of the relative driving force for a half-cell reaction. A relative quantity vs standard hydrogen electrode assigned to zero volt. E0(SHE)=0
14-3. Standard (reduction) Potentials (activities of all species = 1)
H+ (aq, A = 1) + e- ½ H2 (g, A = 1)
E0(SHE)=0
SHE
Reduction :spontaneous
Oxidation:Spontaneous
Le Chatelier’s principle: increasing reactant concentrations drives the reacting to the right
The net driving force of the reaction is expressed by the Nernst equation
The Nernst equation tells us
the potential of a cell whose reagents are not all unit activity
14-4. Nernst Equation (activities of all species = 1)
Nernst Equation for a Half-Reaction
aA + ne- bB
------- (14.13)aA
bBo
AA
nFRTEE ln
ΔG = ΔGo + RT lnQ (Q; reaction quotient)
-nFE = -nFEo + RT lnQ (양변을 nF 로나누어준다)
E = Eo –(RT/nF) lnQ
R: gas constant = 8.314 J/KmolT: temperature (K)
17-1. Fundamentals of Electrolysis
Dipping Cu and Pt electrodes into a solution of Cu2+ and forcing electric current through to deposit Cu metals at the cathode and to liberate O2 at the anode
E = Eo(cathode) – Eo(anode) = 0.399 – 1.229 = - 0.890 V
the reaction of interest occur
Cathode: Cu2+ + 2e- Cu(s) E = 0.339 – (0.0592/2)log([1/[Cu2+])
Anode: H2O ½ O2 + 2H+ + 2e- E = 1.229 – (0.0592/2)log(1/pO21/2•[H+]2)
H2O + Cu2+ Cu(s) + ½ O2 + 2H+
0.2 M Cu2+ and 1.0 M H+ and liberates O2 at a pressure of 1.00 bar
E = Eo(cathode) – Eo(anode) = 0.318 – 1.229 = - 0.911 V
This voltage would be read on the potentiometer if there were negligible current not spontaneous
A power supply is needed to force the reaction occur (electrolysis)
Pote
ntia
l, V
vs S
CE
Time, s
Forward scan
Backward scan(reverse scan)
+0.8 +0.8-0.2
20 40 80600
Figure. Cyclic voltammetric excitation signal used to obtained voltammogram
Scan rate = V/s : 1.0 V/20 s = 50 mV/s
Epc : cathodic peak potential, Epa : anodic peak potential
ipa : anodic peak current, ipc : cathodic peak current
Figure. Cyclic voltammogram for 6.0 mM K3Fe(CN)6 in 1.0 M in KNO3.
Cyclic Voltammtry
irreversible
reversible
Cyclic Voltammtry(CV)
0.06 mM 2-nitropropane
1 mM O2
“Reversible” means the reaction is fast enough to maintain equilibrium
concentrations of the reactant and product at the electrode surface
∆Ep = Epa-Epc = 59 mV/n
Peak current for a reversible system the working electrode(Randles-Sevcik equation)
ip = (2.69x105) n3/2AD1/2C*v1/2
A : electrode area, D : diffusion coefficient
C* : bulk concentration (mol ·cm-3)
v : scan rate (V · s-1)
ip
v1/2
ip ∞ v1/2
from slope D can be calculated
Cyclic Voltammtry(CV)
Figure (a) Effect of variation of scan rate on cyclic voltammograms and (b) plot of ip versus v1/2.
Cyclic Voltammtry(CV): Scan Rate Effect
Chapter 23: Introduction to Analytical Separations
In real analytical problems, we must identify and quantitate one or more components from a complex mixture
Separation of mixture into each component is the first step in analysis
Sample(mixture)
Component1, 2, 3, --- Detection
<separation>
• Optical (absorbance, FL, CL)• Electrochemical (voltammetry)• Mass-to-charge
Mobile phase
Stationary phase
SampleInjection
Detector
Gas: gas chromatographyLiquid: liquid chromatography
Solid: GSC, LSCLiquid: GLC, LLC (partition chromatography)
Martin and Synge: Nobel Prize in 1952
-Chromatography operates on the same principle as extraction, but one phase is held in place while the other moves past it.
23-2. What is Chromatography?
Solute A has a greater affinity than solute B for the stationary phase: (A is more polar) Solute A is more strongly adsorbed than solute B on the solid particles Solute A spends a more time in stationary phase solute A moves down the column more slowly than solute B (longer retention time)
Column packing(stationary phase):solid particles (silica: polar)filled with solvent
Solvent (mobile phase):Non-polar organic solvent
Fluid entering the column: eluentFluid emerging from the end of column: eluate
Chromatography (LSC)
The process of passing liquid or gas through a chromatography column is called elution
Types of Chromatography
Adsorption chromatography Partition chromatography
Stat. phase: solidMobile phase: gas/liquid
Stat. phase: liquidMobile phase: gas/liquid
23-3. Chromatogram
- Retention time for each component: tr- Dead time for unretained species: tm- Adjusted retention time (tr’) = tr – tm- Capacity factor (k’) = (tr – tm)/tm = tr’/tm- Relative retention (α) for any two components (A, B) = (tr’)B / (tr’)A = (k’)B/ (k’)A
unretained species
Selectivity factor = KB / KA (partition coefficient)
Band Broadening & Efficiency of Separation
• Plate theory: theoretical plates (1941, Martin & Synge)• Rate theory: Van Deemter (1956)
• One main cause of band broadening is diffusion
Definition of diffusion coefficient (D):
Flux (mol/m2•s) = J = - D • dC/dx concentration gradient
Standard deviation of band : σ = (2Dt)1/2
Chapter 24:
Gas Chromatography
Mobile phase (carrier gas): gas (He, N2, H2)
- do not interact with analytes
- only transport the analyte through the column
Analyte: volatile liquid or gas
Stationary phase:
- solid (GSC) or non-volatile liquid (GLC)
GSC (gas-solid adsorption chromatography)
- semi-permanent retention of active or polar molecules
- severe tailing of elution peaks
GLC (gas-liquid partition chromatography)
- non-volatile liquid is coated on the inside of the column or on a fine solid support
- In 1955, the first commercial apparatus for GLC appeared on the market
Gas Chromatography
24-1. The separation process in gas chromatography
Temp of a sample injector port:
50 oC above the b.p. of least volatile component of the sample rapidly evaporates
(thermostated)
(2-50 m)
The column should be hot enough to provide sufficient vapor pressure for analyte
to be eluted in a reasonable time.
Open Tubular ColumnsThin coating: small C-term (decreased H) :
Compared with packed columns,
OTC offers higher resolution, shorter analysis time, greater sensitivity, lower sample capacity
Length: 15-100 m
Liquid Sta. Phase
Choice of liquid phase for a given problem:
“like dissolves like”
- Nonpolar columns: best for nonpolar solutes
- Polar columns for polar solutes
- As a column ages,
stationary phases bakes off
surface silanol groups (Si-OH) are exposed
peak tailing (polar analyte)
Therefore, stationary phase is covalently
attached to silica surface
Non-polar column Polar column (retention time: hydrocarbon<ketone<alcohol)
H-bonding
Dipole interaction
The Retention Index
Column oven temp = 70oC
Temperature ProgrammingTemp of column (oven) increases Solute vapor pressure increase decrease retention time
Isothermal at 150 oC
Temp programming:
50 – 250 oC at 8 oC/min
Precaution: at too high temp.
thermal decomposition of analyte
Sample Injection in GCLiquid samples are injected into GC by syringe through a rubber septum into a heated port
Gaseous samples use gas-tight syringe
<Sample size>
Packed column: sub L – 20 L, Capillary column: 10-3 L (split injection)
Spilt injection delivers only 0.2-2% of the sample to the column
Quantitative and Qualitative Analysis by GCQualitative analysis:
- retention time (GC-FID, TCD, ECD…): comparison with authentic sample
- mass (GC-MS)
Quantitative analysis:
- peak area or peak height
Thermal Conductivity Detector (TCD)
<Advantages of TCD>
- simple system
- wide linear dynamic range (~ 104)
- general response to organic and inorganic species
- non-destructive
<Limitation of TCD>
- relatively low sensitivity
Chapter 25:
High-Performance Liquid Chromatography
Mobile phase: liquid
Analyte: non-volatile liquid
Stationary phase:
- solid (GSC) or non-volatile liquid (GLC)
HPLC; uses high-pressure pump to deliver liquid mobile phase
Mobile
phase
High-pressure
pumpinjector column detector
<HPLC system>
HPLC
Elution Process
In adsorption chromatography,
solvent molecules compete with solute molecules for sites on the stationary phase
Elution occurs when solvent displaces solute from the stationary phase
Normal- vs Reversed-Phase ChromatographyNormal-phase chromatography (e.g. adsorption chromatography based on silica gel)
Stationary phase: polar (e.g. silica)
Mobile phase: non-polar (hexane, i-propylether)
Reversed-phase chromatography
Stationary phase: non-polar (hydrocarbon) or weakly polar
Mobile phase: more polar (water, methanol, acetonitrile)
C B A
<Normal Phase C> <Reversed- Phase C>
Mobile phase: low polarity
C B A
MP: medium polarity
Polarity: A>B>C
A B C
MP: high polarity
A B C
MP: medium polarity
time
- Ultraviolet detector: most common
- Refractive index (universal)
- Fluorescence
- Electrochemical
- Conductivity (ion-exchange C)
- Mass spectrometry
- Chemi-(electrochemi-)luminescence
Detectors in HPLC