Absorbance of light

Concentration is not the only property that may be used to construct a titration curve. Other parameters, such as temperature or the absorbance of light, may be used if they show a significant change in value at the equivalence point. Many titration reactions, for example, are exothermic. As the titrant and analyte react, the temperature of the system steadily increases. Once the titration is complete, further additions of titrant do not produce as exothermic a response, and the change in temperature levels off. A typical titration curve of temperature versus volume of titrant is shown in Figure 9.3. The titration curve contains two linear segments, the intersection of which marks the equivalence point. 

Fig. 10-8. Single particle scattering to mass ratio for particles of four different compositions. Carbon particles are also very efficient absorbers of light. Source U.S. Environmental Protection Agency, "Protecting Visibility," EPA-450/5-79-008, Office of Air Quality Planning Standards, Research Triangle Park, NC, 1979.

Singlet oxygen, O2, can readily be generated by irradiating normal triplet oxygen, 2 in the presence of a sensitizer, S, which is usually a fluorescein-type dye, a polycyclic hydrocarbon or other strong absorber of light. A spin-allowed transition then occurs ... 

The ultraviolet-visible spectrophotometer is the most widely used detector for HPLC. The basis of UV-VIS detection is the difference in the absorbance of light by the analyte and the solvent. A number of functional groups absorb... 

The measurement of absorbance of light by a dyestuff-anionic surfactant complex, which has been extracted into an organic solvent is a key feature of many methods, and Sodergren has successfully used segmented flow colorimetry for an automated version of this procedure (2 ). An alternative is the two phase titration technique, pioneered by Herring (3) which uses dimidium... 

In this example, the Hg atom is the primary absorber of light. If the primary absorber is regenerated, it can participate in subsequent cycles, and is called a photosensitizer. In other cases, the photoactive species yields the active species directly. Thus, chlorine molecules can absorb light and dissociate into chlorine atomic radicals ... 

Matrix interferences can be observed in a number of different forms. Components in the extract may interfere with excitation of electrons. Absorbance of light by unexpected metals or organic compounds, generally or specifically, would also cause interference. Another source of interference is the complex-ation of the analyte with extract components such that the metal of interest is protected from the heat source. Other interferences such as changes in the viscosity of the extract are also possible, although they are less common. Whenever soil samples are analyzed by atomic spectroscopic methods, it is essential to make sure that no interfering species are present in the soils. If they are, then steps must be taken to correct for these interferences. 

Electrocyclic reactions can be brought about by heat, by ultraviolet irradiation and sometimes by use of metal catalysts. The thermal reaction is generally not reversible and as written above cyclobutenes have been converted to 1, 3 dienes by heating between 100° and 200°C. But the photochemical conversion can be carried out in either direction. Generally 1, 3 dienes can be converted to cyclobutenes rather than the reverse because the dienes because of n electrons are strong absorbers of light at the used wavelengths. 

But the reverse conversion does not proceed smoothly. This is because the diene is a stronger absorber of light at wave lengths used in the reaction. So it is the diene which is excited. The most instructive examples are those where the diene forms part of a ring so that a fused cyclobutene is formed. 

Of more apparent significance in the aquatic environment are redox processes induced or enhanced on absorbance of light by chromophores at metal oxide surfaces in which the metal of the oxide lattice constitutes the cationic partner. Light induced electron transfer within such a chromophore often results in disruption of the oxide lattice. The photoredox-induced dissolution of iron and manganese oxides by such a mechanism has been proposed as a possible means of supply of essential trace-metal nutrients to plants and aquatic organisms (29-31). ... 

A rigid, planar, conjugated porphyrin ring, which functions as an efficient absorber of light. 

The absorbance of light passing through a medium is proportional to the concentration of the medium. 

The more concentrated the solution, the greater the absorbance of light. 

Tryptophan and tyrosine, and to a much lesser extent phenylalanine, absorb ultraviolet light (Fig. 3-6 Box 3-1). This accounts for the characteristic strong absorbance of light by most proteins at a wavelength of 280 nm, a property exploited by researchers in the characterization of proteins. 

A calibration curve shows the response of an analytical method to known quantities of analyte.8 Table 4-7 gives real data from a protein analysis that produces a colored product. A spectrophotometer measures the absorbance of light, which is proportional to the quantity of protein analyzed. Solutions containing known concentrations of analyte are called standard solutions. Solutions containing all the reagents and solvents used in the analysis, but no deliberately added analyte, are called blank solutions. Blanks measure the response of the analytical procedure to impurities or interfering species in the reagents. 

G. G3 Calibration curve. (You can do this exercise with your calculator, but it is more easily done by the spreadsheet in Figure 4-13). In the Bradford protein determination, the color of a dye changes from brown to blue when it binds to protein. Absorbance of light is measured. 

In a spectrophotometric titration, absorbance of light is monitored as titrant is added. For many reactions, there is an abrupt change in absorbance when the equivalence point is reached. The Fajans titration is based on the adsorption of a charged indicator onto the charged surface of the precipitate after the equivalence point. The Volhard titration, used to measure Ag+, is based on the reaction of Fe3+ with SCN- after the precipitation of AgSCN is complete. 

The minimum requirements for a spectrophotometer (a device to measure absorbance of light) were shown in Figure 18-4. Light from a continuous source is passed through a monochromator, which selects a narrow band of wavelengths from the incident beam. This... 

From published papers and proposed methods, this type of detection is very popular because of its ease of use, broad application area, and low purchase price. The detector measures the change in absorbance of light in the 190-350-nm region (UV) or the 350-700-nm region (visible). 

A / is the absorbance of the sample when the light is polarized parallel to a reference axis, and Aj is the absorbance of light which is polarized perpendicular to this axis. The strength of the absorption depends on the orientation of the electric field vector of the light and the transition moment of the chromophore - parallel orientation results in maximum absorption whereas perpendicular orientation leads to zero absorption. By dividing the LD value by the absorbance of the unoriented sample under isotropic conditions (Aiso), the reduced linear dichroism (LDr), i.e. the wavelength-dependent LD, is obtained (Eq. 7) [36]. 

As the name implies, photochemical reactions result from the absorbance of light... 

Carbon blacks are strong absorbers of light over the entire spectrum and are the... 

Figure 5.3 Diagram illustrating possible promotion of electrons on absorbance of light.

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