Kimia Analitik 2

PENGANTAR SPEKTROSKOPIbagian 2

Sonny WidiartoJurusan Kimia FMIPAUniversitas 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-tetrazineAbsorption 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 ApplicationsSolvents• 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-PointBerikut 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)

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)

A

Ekstrapolasi:Vm/(Vm+VL)= 0,34VL /(Vm+VL)= 0,66Perbandingan M:L =0,34 : 0,66 = 1 : 2Rumus 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

0 1 2 3 4 5 Mol L/Mol M

A

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

A

[M] atau [L]

Reaksi: mM + nL == MmLn

Pada [M] >>>> makaMmLn ~CL/nAn = b MmLn ~ b CL/nLereng: Sn = An/CL= b/n

Dengan cara yang sama untuk [L] >> Am = b MmLn = b CM/n Lereng Sm = Am/Cm = b/mAngka banding rasio Sm/Sn= ( b/m)/( b/n)

= n/m

Metode variasi kontinyu

Spektrofotometer IR

• Dispersif dan FTIR

Dispersif