Download Kimia Analitik 2 PENGANTAR SPEKTROSKOPI bagian 2

Kimia Analitik 2
PENGANTAR SPEKTROSKOPI
bagian 2
Sonny Widiarto
Jurusan Kimia FMIPA
Universitas Lampung
Spektroskopi Absorpsi Molekul
• Absorpsi radiasi UV, vis dan IR sangat luas
digunakan untuk identifikasi maupun penentuan
spesi anorganik, organik maupun biokimia
• Spektroskopi absorpsi molekul UV-vis digunakan
dalam analisis kuantitatif dan paling banyak
dipakai pada laboratorium kimia maupun klinik
dibanding teknik lain
• Spektroskopi absorpsi IR merupakan alat yang
sangat efektif untuk identifikasi dan penentuan
struktur senyawa organik maupun anorganik
Absorbing Species
• solvent effect  1,2,3,4-tetrazine
Absorption by Organic Compounds
• Absorption of radiation by organic molecules in
the wavelength region between180 and 780 nm
results from interactions between photons and
electrons that either participate directly in bond
formation (and are thus associated with more
than one atom) or that are localized about such
atoms as oxygen, sulfur, nitrogen, and the
halogens.
• The wavelength of absorption of an organic molecule
depends on how tightly its electrons are bound
• single bonds are firmly held  their excitation requires
energies corresponding to wavelengths in the vacuum
ultraviolet region below 180 nm
• Electrons in double and triple bonds of organic
molecules are not as strongly held and are therefore
more easily excited by electromagnetic radiation
• Unsaturated organic functional groups that absorb in
the ultraviolet or visible regions are known as
chromophores
Typical spectra for organic
compounds
•
Kromofor
A chromophore is the part of a molecule responsible for its color. The color arises when
a molecule absorbs certain wavelengths of visible light and transmits or reflects others.
The chromophore is a region in the molecule where the energy difference between two
different molecular orbitals falls within the range of the visible spectrum. Visible light
that hits the chromophore can thus be absorbed by exciting an electron from its ground
state into an excited state.
Saturated organic compounds containing such heteroatoms as oxygen,
nitrogen, sulfur, or halogens have nonbonding electrons that can be excited
by radiation in the 170- to 250-nm range
Absorption by Inorganic Species
• In general, the ions and
complexes of elements in
the first two transition
series absorb broad bands
of visible radiation in at
least one of their oxidation
states. As a result, these
compounds are colored
• Absorption occurs when
electrons make transitions
between filled and unfilled
d-orbitals with energies that
depend on the ligands
bonded to the metal ions.
• The energy differences
between these d-orbitals
(and thus the position of the
corresponding absorption
maxima) depend on the
position of the element in
the periodic table, its
oxidation state, and the
nature of the ligand bonded
to it.
Qualitative Applications
Solvents
• Ultraviolet spectra for qualitative
analysis are usually measured
using dilute solutions of the
analyte.
• For volatile compounds, however,
gas-phase spectra are often more
useful than liquid-phase or
solution spectra
• Gas-phase spectra can often be
obtained by allowing a drop or
two of the pure liquid to
evaporate and equilibrate with
the atmosphere in a stoppered
cuvette.
• must be transparent in the
regionof the spectrum where the
solute absorbs
• The analyte must be sufficiently
soluble
• consider possible interactions of
the solvent with the absorbing
species
Prosedur
• A first step in any photometric or spectrophotometric
analysis is the development of conditions that yield a
reproducible relationship (preferably linear) between
absorbance and analyte concentration.
• Wavelength Selection.
• Variables That Influence Absorption the nature of the
solvent, the pH of the solution, the temperature, high
electrolyte concentrations, and the presence of
interfering substances
• The Relationship between Absorbance and
Concentration
Metode Standard Adisi
Single-Point
Berikut ini adalah contoh metode standard adisi single-point
Multiple-point
Studi Ion Kompleks
• Spektrofotometri dapat digunakan untuk
penentuan komposisi ion kompleks dalam
larutan dan untuk penentuan konstanta
pembentukan kompleks
• Terdapat 3 teknik dalam mempelajari ion
kompleks: (1) metode variasi kontinyu (2)
metode perbandingan mol (3) metode
perbandingan slope
a). Cara Variasi kontinyu
* Kation M + ligan L ==== kompleks ML
* Buat konsentrasi M dan L tepat sama
* Buat campuran M dan L pada variasi volume,
tetapi volume total tetap sama
* Ukur serapannya, buat kurva hubungan A
terhadap fraksi volume salah satu (M atau L) Ekstrapolasi:
Vm/(Vm+VL)= 0,34
A
VL /(Vm+VL)= 0,66
Perbandingan M:L =
0,34 : 0,66 = 1 : 2
0
0,2
0,4
0,6
0,8
1,0
Vm/(Vm+VL)
1,0 0,8
0,6
0,4
0,2
0,0
VL/(Vm+VL)
Rumus kompleks: ML2
b). Cara angka banding mol:
* pada pencampuran [M] konstan ,[L] berubah
* Diukur pada  di mana salah satu menyerap kuat
* Buat kurva A terhadap perbandingan mol ligan (L)
dan mol kation
A
0
1
2
3
4
5
Mol L/Mol M
c). Cara angka banding lereng:
* Khusus untuk kompleks lemah
(Kstab kecil)
* mengukur serapan larutan kompleks dengan
kelebihan yang besar dari L atau M
* kurva A terhadap [L] total dan A terhadap
[M] total
Reaksi: mM + nL == MmLn
Pada [M] >>>> maka
A
MmLn ~CL/n
An =  b MmLn ~  b CL/n
[M] atau [L]
Lereng: Sn = An/CL=  b/n
Dengan cara yang sama untuk [L] >>
Am =  b
MmLn
=  b CM/n
Lereng Sm = Am/Cm =  b/m
Angka banding rasio Sm/Sn= ( b/m)/( b/n)
= n/m
Metode variasi kontinyu
Spektrofotometer IR
• Dispersif dan FTIR
Dispersif