Uv-visible spectrophotometric methods

Another type of method is based on the spectrophotometric properties of Cr (VI), sometimes used as standard solution [38]. Moreover, the spectrophotometric determination of Cr (VI) is well known in the field of water examination, as it can be used for COD alternative measurement [39] (see Chapter 4). Two other UV-visible spectrophotometric methods have been proposed. The first one [40], designed for natural water, uses the peak height measurement at 372 nm, for a basified sample (pH > 9). The peak height is calculated from the absorbance values at three wavelengths (310, 372 and 480 nm), taking into account a very simple third-degree-polynomial interference signal. 

The second UV-visible spectrophotometric method [41] was proposed for natural and urban wastewaters with a more general mathematical compensation of interferences. Both UV-visible spectrophotometric methods are rapid and simple with a detection limit of around 5 p,g/L for a 50-mm optical pathlength. Unfortunately, they cannot be applied for industrial wastewater survey, as the presence of specific compounds cannot be modelled by the mathematical tools used for interferences removal. 

Figure 27. Comparison between the UV-visible spectrophotometric method and the corresponding reference methods for (a) copper, (b) mercury and (c) iron determination, with or without standard additions of metals in sample.

UV-visible spectrophotometric methods can also be used, such as are detailed in the AOAC Official Methods of Analysis. They are less accurate and less selective than LC methods as they measure total carotenes however, they offer a rapid, relatively simple analytical procedure. 

Iron oxide Iron oxide causes coloration in glass depending on its oxidation state. Iron(II) causes a bluish-green coloration, iron(III) a yellowish coloration. For determination of total iron expressed as Fc203, a UV-visible spectrophotometric method based on 1,10-phenanthroline is used. Hydroxyl-amine is used to ensure all the iron is present as iron(n). 

In the early days of vitamin Ki(20) discovery, colorimetric and UV-visible spectrophotometric methods were applied for identification and quantitative purposes. Also, the formation of fluorescent derivatives has been described. Other physicochemical methods include titration with cerium(IV) sulfate, polarogra-phy, microcoulometry, and phosphorimetry. These methods, however, have now been largely replaced by modern chromatographic techniques such as LC and to a lesser extent GC and TLC. 

For the quantitative analysis of the solute distributed in one or both phases, the most commonly used analytical methods include UV-visible spectrophotometric analysis for compounds with chromophore groups and gas-liquid chromatography (GLC). Colorimetric methods have also been used for specific compounds. 

The rapid evolution of microcomputers has led to the derivative transformation of spectral data, which offer a powerful tool for both qualitative and quantitative analysis of mixtures of organic compounds. The method has found increasing application in UV-visible spectrophotometric analysis of organics for background correction and for resolution enhancement. The ability to eliminate matrix interferences such as irrelevant absorption and light scattering has been of particular value. 

Experimental determination of the rate law is the most important and perhaps the most fnndamental aspect of the use of reaction kinetics in diagnosing the reaction mechanisms. There are several methods used for the determination of the experimental rate law, and these methods are critically and well described in several books on reaction kinetics. The most convenient and commonly used method involves the UV-visible spectrophotometric technique, and an attempt is made to describe this particular method in some detail in this chapter. 

For many years, flow injection analysis has been used for automated spectrophotometric determination of anionics. Although UV absorbance at 224 nm has been reported for FLA of LAS (68), FIA is normally based upon the more robust methylene blue visible spectrophotometric method (69-71). Other cationic dyes may be used in place of methylene blue (72,73). The methylene blue method requires a two-phase system, a feature which is a continuing target of optimization experiments. Most commercial systems rely on gravity... 

Although considered a basic technique, ultraviolet-visible (UV-vis) is perhaps the most widely used spectrophotometric technique for the quantitative analysis of pure chemical substances such as APIs in pharmaceutical analysis. For pharmaceutical dosage forms that do not present significant matrix interference, quantitative UV-vis measurements may also be made directly.114,115 It is estimated that UV-vis-based methods account for 10% of pharmacopoeia assays of drug substances and formulated products.116... 

A common technique for measuring the values has been to employ species that produce anions with useful ultraviolet (UV) or visible (vis) absorbances and then determine the concentrations of these species spectropho tome trie ally. Alternatively, NMR measurements could be employed, but generally they require higher concentrations than the spectrophotometric methods. A hidden assumption in Eq. 5 is that the carbanion is fully dissociated in solution to give a free anion. Of course, most simple salts do fully dissociate in aqueous solution, but this is not necessarily true in the less polar solvents that are typical employed with carbanion salts. For example, dissociation is commonly observed for potassium salts of carbanions in DMSO because the solvent has an exceptionally large dielectric constant (s = 46.7) and solvates cations very well, whereas dissociation occurs to a small extent in common solvents such as DME and THE (dielectric constants of 7.2 and 7.6, respectively). In these situations, the counterion, M+, plays a role in the measurements because it is the relative stability of the ion pairs that determines the position of the equilibrium constant (Eq. 6). 

Because the Meisenheimer adducts are usually characterized by intense UV-visible absorption bands, the equilibrium constants for their formation (K) can be determined spectrophotometrically by standard methods, whether complete conversion to the adduct can be attained39 or not,60 depending on the equilibrium constant value (K 5 10). 

Visible and UV spectral data have been reported for the parent unsubstituted heterocycle (62JCS493). The apparent pKa values of several derivatives of this system were determined using spectrophotometric methods. The parent heterocycle has a pKt of 2.51. Values of 1.14, -0.25, -0.53 and -0.02 were obtained for the pKa of the 2-methyl, 2,3-dimethyl, 2,3,6-trimethyl and 2,3,6,7-tetramethyl derivatives of this ring system, respectively (63JCS4304). ESR data for the parent heterocycle have also been reported (71JA5850). 

The benzhydryl chlorides and BC13 react with formation of ion pairs (ionization constant, Ki) which dissociate to give the free ions (dissociation constant, KD). Because paired and free diarylcarbenium ions show only slightly different UV-visible spectra, [41], spectrophotometric measurements allow the determination of the total carbocation concentration. On the other hand, only free ions are detected by conductometric analysis, and a combination of both methods allows the determination of Ki and Kd using the theory of binary ionogenic equilibria [42,43]. 

In principle, any measurable property of a reacting system that is proportional to the extent of reaction may be used to monitor the progress of the reaction. The most common techniques are spectrophotometric (UV-visible, fluorescence, IR, polarimetry and NMR) or electrochemical (pH, ion-selective electrodes, conductivity and polarography). Either a "batch" method can be used, in which samples are withdrawn from the reaction mixture and analyzed, or the reaction may be monitored in situ. By far the most widely used technique involves UV-visible spectrophotometry. 

Spectrophotometric Methods Measurement of UV/visible absorption can also be used to detennine the end points of titrations (see Section 26A-4). In these cases, an instrument responds to the color change in the titration rather than relying on a visual detennination of the end point. 

The conventional spectrophotometric techniques (UV-visible, IR, NMR) are of limited use in structural determination of melanins. Consequently, an array of degradation techniques that yield easily identifiable, low molecular weight fragments has been developed. Many of these methods were developed in the 1950s and 1960s and are documented by Nicolaus (7). The degradation methods are classified as reductive, oxidative, pyrolytic, and photochemical, and recent findings are described below. 

Nowadays, spectrophotometry is regarded as an instrumental technique, based on the measurement of the absorption of electromagnetic radiation in the ultraviolet (UV, 200-380 nm), visible (VIS, 380-780 nm), and near infrared region. Inorganic analysis uses UV-VIS spectrophotometry. The UV region is used mostly in the analysis of organic compounds. Irrespective of their usefulness in quantitative analysis, spectrophotometric methods have also been utilized in fundamental studies. They are applied, for example, in the determination of the composition of chemical compounds, dissociation constants of acids and bases, or stability constants of complex compounds. 

Two Wavelengths Method The spectral overlapping of the components of a mixture is one of the most important limitations of the spectrophotometric methods when one component has to be determined in the UV-visible range. When two analytes have to be determined in the same solution, two different wavelengths have to be chosen in such a way that one analyte does not interfere with the other. The general relation between the two concentrations to be determined and the two measured absorbance values is ... 

As previously mentioned, phosphorus compounds are commonly classified into orthophosphates (PO ), acid-hydrolysable (condensed) phosphates and organic phosphates. It must be noticed that acid-hydrolysable phosphates (as pyrophosphates) are negligible in sewage [30], The general procedure illustrated in Fig. 11 includes two main steps an indirect UV-visible measurement (PO -) and a photo-oxidation step followed by a UV-visible measurement (Pgl)- First, orthophosphates are determined by spectrophotometric measurement of a phosphomolybdate complex (formed with addition of ammonium molybdate 40 gL-1) using the spectrum deconvolution method. 

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