Transmittance absorbance, relation with

Review with your teacher how a colorimeter works. How are absorbance (A) and transmittance (%T) related ... 

With non-absorbing films, A = 0, and for reflectance and transmittance the relation R + T = 1 is valid. It is therefore sufficient to know either R or T. Reflectance R is considered in the following treatment. 

After extraction, in the filtered extract, phosphorus is estimated colorimetrically by adding ammonium molybdate and thereafter reducing the molybdenum-phosphate complex in acidic medium with a reducing agent for which stannous chloride is used. The intensity of the blue colour molybdenum blue is directly related to the quantity of orthophosphate ion and thus provides a measure for the concentration of P in test solution. The absorbance or transmittance is measured spectrophotometrically at 660 mp,. wavelength. 

It was shown that the transition moment of the )3-carotene molecule is isotropically distributed in the plane of the SC film. As regards the normal direction ofthe film, the observed angular dependence ofboth reflectance and transmittance was in excellent agreement with the theoretical curves calculated assuming the isotropic absorbing medium. This means that the refractive index ofthe SC film is isotropic in the normal ofthe film. Since the refractive index can be directly related to the polarizability of a molecule using Clausius-Mosotti s law and a transition moment linearly depends on the polarizability, the present investigation reveals that the transition moment of the /3-carotene molecule is isotropically distributed both in the plane and in the normal of the SC film. The transition moment lies parallel to the molecular axis of /3-carotene, hence we can conclude that the /3-carotene molecules are randomly oriented in the SC film. 

This equation states that the absorbance is directly proportional to solute concentration for a given solute/solvent, (i.e., e, the molar absorptivity) and for a fixed path length b. It must be remembered that the absorbance and the molar absorptivity are dependent on the wavelength. It becomes important in practice for an analyst to know how the molar absorptivity varies with wavelength, X. The percent transmittance, %T = lOOP, a common term used with stand-alone speetrophotometers, can be related to the absorbance by manipulating the above definition for A according to... 

Like UV/vis spectra, IR spectra are now usually presented in terms of absorbance, rather than transmittance, which used to be the norm. This allows application of the Beer-Lambert Law (Section 2.9), which states that the absorbance is directly proportional to the concentration of the absorbing species. In Raman spectra, the intensity of a peak due to a given species is directly related to its concentration. So monitoring of the absorbance (IR) or scattering (Raman) at a single frequency or, better, several frequencies or the whole spectral range, allows the concentration of a compound to be compared with that in a standard sample. However, the spectrum of a compound is different in different phases, and varies to some extent with concentration, because of interactions with neighboring molecules, so reference spectra need to be obtained under conditions similar to those used for the unknown sample. 

The amount of light absorbed is a function of the so-called absorption coefficient k ) and of the optical path length in the atomiser cell (ft) k depends on the frequency of the selected analytical line and on the concentration of the analyte absorbing atoms. The general absorbance law (Lambert-Bouguer-Beer law) relates transmittance (and so measured intensities I and 1q) with k and b through the following equation ... 

Since the bands that are measured in GC/FT-IR experiments are almost invariably very weak, their integrated absorbance varies linearly with the quantity of each analyte injected. On the other hand, when chromatograms are constructed using the Gram-Schmidt algorithm, the signal that is measured is proportional to the absorp-tance, 1 — T v), of each band. Since transmittance and absorbance are related as... 

Analysis for the purpose of accurately determining the quantity of a chemical species existing in a sample is called quantitative analysis. Quantitative infrared spectroscopic analysis mainly deals with the intensity of an infrared absorption band. In this chapter, basic aspects of quantitative spectroscopic infrared analysis for a target substance (the analyte) in solution samples are described. The subjects to be described include the characteristics of a Fourier transform infrared (FT-IR) spectrometer, the relation between percentage transmittance and absorbance, Lambert-Beer s law on the relationship between the intensity of an infrared band and the concentration of a sample, the use of a working curve in quantitative analysis, and the origins of deviations from Lambert-Beer s law. 

Emission from a body occurs from thermally excited atoms and molecules within the body. The basic principle of thermal emission is described by Kirchhoff s law which states that the ratio between the energy of radiation emitted by a body in a thermal equilibrium and its absorptance is a function of only the temperature of the body and the wavenumber (or wavelength) of the radiation it does not depend on the material constituting the body. The absorptance mentioned above may be defined as follows. When a body is irradiated, the radiation is partly reflected, partly absorbed, and the remainder passes through the body, if scattering by the body is ignored. If the proportions of the reflection, absorption and transmission are expressed, respectively, by reflectance (r), absorptance (a), and transmittance (t), the following relation holds r -i- a -t- t = 1. It is clear that each of the three quantities is a dimensionless constant with a value between 0 and 1. (As each of them is a function of wavenumber v, they are expressed as r(v), a(v), and t(v) when necessary.) Usually, transmittance is denoted by T, but it is not used in this chapter to avoid confusion with temperature T. 

---