Summary: (Last lecture) Absorption spectroscopy definition electromagnetic spectroscopy matter absorption spectroscopy fundamental terms (transmittance, absorbance absorptivity, molar absorptivity)

Absorption spectroscopy Molar absorptivity    A =  bc  = the molar absorptivity, L mol-1 cm-1 (the characteristic of a substance that tells how much light is absorbed at particular wavelength) b = the pathlength of cell, cm C = the concentration of absorbing species, M

Quantitative aspects of absorption measurements Absorption spectroscopy Beer ’ s Law A =  bc (The heart of spectrophotometry) *Application of Beer ’ s Law to mixture

A solution containing more than one kind of absorbing substances: Absorption spectroscopy A total = A 1 + A 2 + … + A n =   bc 1 +   bc 2 + … +  n bc n Conditions: no interaction among the various species

Absorption spectroscopy Limitations to the Applicability of Beer ’ s Law A  c monochromatic radiation dilute solutions (  0.01 M) only Why?

At high concentration (> 0.01M): in concentrated solution, solutes molecules influence one another as a result of their proximity. When solute molecules get close to one another, their properties (including molar absorptivity) change somewhat. Absorption spectroscopy Limitations to the Applicability of Beer ’ s Law

Absorption spectroscopy The solute becomes the solvent. Properties of a molecule are not exactly the same in different solvent. At high concentration (> 0.01M): Limitations to the Applicability of Beer ’ s Law

Absorption spectroscopy 1. Chemical Deviations 2. Instrumental Deviations DEVIATIONS Polychromatic Radiation Stray Light Deviations of Beer ’ s Law

Chemical deviations arise when an analyte dissociates, associates, or reacts with a solvent to produce having a different absorption spectrum from the analyte. Ex: acid/base indicators HIn = H + + In - colour 1 colour 2

Ex: The molar absorptivity of the weak acid HIn (Ka=1.42 x ) and its conjugate base In - at 430 and 570 nm were determined by measurements of strongly acidic and strongly basic solutions of the indicator (where essentially all of the indicator was in HIn and In - forms respectively). The results were HIn 6.30 x x 10 3 In 2.06 x x 10 2  430  570

Derive absorbance data for unbuffered solutions having total indicator concentrations ranging from 2 x to 16 x M Soln. Calculate the [HIn] and [In - ] in a solution in which the [indicator] is 2.00 x M Here HIn = H + + In - (1)

From the equation for the dissociation process; Substitution of these relationships into (1) for K a :

From Beer’s Law:

At 570 nm:

Note: The direction of curvature is opposite at the two wavelengths.

Instrumental deviations polychromatic radiation Consider a beam consisting of just two wavelengths and  at,

at , When an absorbance measurement is made with radiation composed of both wavelengths, the measurement A, A m :

when

In experiment, deviations from Beer’s Law resulting from the use of a polychromatic radiation is not appreciable.

Instrumental deviations stray light Causes: scattering and reflections from various internal surface Characteristic: differs greatly in wavelength from that of the principal radiation may not have passed through the sample

P s is the power of nonabsorbed stray radiation Instrumental deviations stray light

At high concentration and at longer path lengths, stray radiation can also cause deviations from the linear relationship between ABS and path length. note M.R. Share, Anal. Chem. 1984, 56, 339A

Summary: The instrumental deviations result in absorbance that are smaller than theoretical. OR The instrumental deviations always lead to negative absorbance error. Instrumental deviations: stray light

Analysis of Mixtures of Absorbing Substances :  : two components behave independently of one another.

Example 1 The molar absorptivities of compounds X and Y were measured with pure samples of each. (nm) X Y A mixture of compounds X and Y in a cm cell has an absorbance of at 272 and at 327 nm. Find the concentrations of X and Y in the mixture.  (M -1 cm -1 )

Example 2 The figure shows the spectra of 1.00x10 -4 M MnO 4 -, 1.00x10 -4 M Cr 2 O 7 2-, and unknown mixture mixture of both. Absorbances at several wavelengths are given in the table. Find the concentration of each species in the mixture Wavelength MnO 4 - Cr 2 O 7 2- Mixture (nm) standard standard

Quiz 2: สารละลายของสารอินทรีย์ตัวหนึ่งเตรียมขึ้นจากสารละลาย mg ในเอธานอล 10 mL พบว่าหากใช้เซลที่มี ความหนา 1.0 cm จะให้ค่าการดูดกลืน 1.25 ที่ 305 nm จงคำนวณ molar absorptivity กำหนดให้น้ำหนักโมเลกุล ของสารเท่ากับ 500

Summary: key terms Beer’s Law the relationship between a sample’s absorbance and the concentration of the absorbing species Stray Light any light reaching the detector that does not follow the optic path from the source to the detector

Transmittance the ratio of the radiant power passing through a sample to that from the radiation’s source Absorbance The attenuation of photons as they pass through a sample (A) Absorbance spectrum a graph of a sample’s absorbance of electromagnetic radiation versus wavelength (frequency or wavenumber)

photon a particle of light carrying an amount of energy equal to hv Instruments for absorption measurements Next topic:

Instrument components: UV-VIS signal processor optical source h  sample h  detector selector

Instrument components: UV-VIS Sources: A sources must: generate a beam of radiation with sufficient power for easy detection and measurement provide output power that is both stable and intense Types of spectroscopic sources: 1. continuous sources 2. lines sources

Instrument components: UV-VIS continuous sources lines sources hollow cathode lamp Hg vapor lamp laser H 2 and D 2 lamp Tungsten filament lamps Xe arc lamp

Instrument components: UV-VIS Tungsten filament lamp: Vis/near IR source nm

Instrument components: UV-VIS Quartz Tungsten Halogen (QTH) lamp nm high temperature (3500 K) Evaporation: W(s) W(g) W(g) + I2(g) WI2(g) Redeposition: WI2(g) + W(s) W(s) + I2(g)

Instrument components: UV-VIS H 2 and D 2 lamp provide continuous spectrum in the UV region ( nm) by electrical excitation of deuterium or hydrogen at low pressure mechanism H 2 + E electrical  H 2 *  H(KE1) + H(KE2) + hv ‘ bond dissociation energy ’

Instrument components: UV-VIS sample containers

Instrument components: UV-VIS sample containers Note: a liquid sample is usually contained in a cell called a cuvet that has a flat material fused silica glass  only Vis quartz

Instrument components: UV-VIS wavelength selectors Types 1. Filters 1.1 interference filters 1.2 absorption filters 2. Monochromators

Instrument components: UV-VIS Filters “a wavelength selector that uses either absorption, or constructive and destructive interference to control range of selected wavelengths” the simplest method for isolating a narrow band of radiation

Instrument components: UV-VIS Absorption filters work by selectively absorbing radiation from a narrow region Interference filters use constructive and destructive interference to isolate a narrow range of wavelengths

Absorption filters use coloured glass provide effective bandwidths, range nm the width of the band of radiation passing through a wavelength selector measured at half the band’s height Instrument components: UV-VIS

Relationship between Absorption and Observed Colour wavelength region removed by absorption (nm) colour observed complementary colour of the residual light, as seen by eye violet yellow-green blue yellow green-blue orange blue-green red green purple yellow-green violet yellow blue orange green-blue red blue-green Instrument components: UV-VIS