분석화학실험 -...

분석화학실험

2014학년도 1학기

담당교수: 이원용 (연구실: 과 443-C, 전화: 2123-2649, 전자우편: wylee@yonsei.ac.kr)

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