Visible spectrum spectrophotometr

Although three early papers briefly discussed reactions between methylcobalamin and mercury compounds (30-32), the most systematic investigation has come from Wood and co-workers (33). They proposed the mechanism shown in Fig. 1, with values for the various rate constants presented in Table I. Species 2 and 3, in which the benzimidazole nitrogen no longer bonds to the cobalt atom, are termed base-off compounds, whereas 1 is base-on methylcobalamin and 4 is aquocob(III)alamin, the usual product of aqueous transmethylation by 1. Each one of these species has a unique ultraviolet-visible spectrum, which allows quantitative studies by spectrophotometric techniques to be made (28, 32, 33). The mercuric acetate-1 exchange is so rapid that it must be studied using stopped-flow kinetic techniques (33). 

Reaction of Cytochrome cIinn with Bis(ferrozine)copper(II) Knowledge of the redox properties of cytochrome c was an encouragement to initiate a kinetics investigation of the reduction of an unusual copper(II) complex species by cyt c11. Ferrozine (5,6-bis(4-sulphonatophenyl)-3-(2-pyridyl)-1,2.4-triazine)286 (see Scheme 7.1), a ligand that had come to prominence as a sensitive spectrophotometric probe for the presence of aqua-Fe(II),19c,287 forms a bis complex with Cu(II) that is square pyramidal, with a water molecule in a fifth axial position, whereas the bis-ferrozine complex of Cu(I) is tetrahedral.286 These geometries are based primarily upon analysis of the UV/visible spectrum. Both complexes are anionic, as for the tris-oxalato complex of cobalt in reaction with cytochrome c (Section 7.3.3.4), the expectation is that the two partners will bind sufficiently strongly in the precursor complex to allow separation of the precursor formation constant from the electron transfer rate constant, from the empirical kinetic data. 

It is important to appreciate that particular values for the wavelength exponent, n, and the attenuation index, a, at some one wave length will characterize the entire transmittancy curve in the visible spectrum of the sugar solution. When a sugar liquor is treated with a solid adsorbent, the changes in the spectrophotometric curves in the visible spectrum are adequately defined by changes in the values of n and a (see Table I). This procedure promises to be an effective tool, both in research and in applications.31... 

The formation of these complexes profoundly affects the visible spectrum of TNP-amino acids. This complication is resolved in the spectrophotometric procedure of Plapp et al. (1971) by following TNP-amino acid formation at 367 nm, the isobestic point for e-trinitrophenyl-a-acetyllysine and its sulfite complex. The procedure described by Fields (1971, 1972) exploits the higher extinction coefficient of the sulfite complex at 420 mii. 

As phosphate does not absorb in either the usable UV region or within the visible spectrum, options for spectrophotometric detection are hmited to either indirect methods or some form of precolumn... 

This section provides correlation charts and operational information for the design and interpretation of ultraviolet-visible spectrophotometric (UV-Vis) measurements. While UV-Vis is perhaps not as information-rich as infrared or nuclear magnetic resonance, it nonetheless has value in structure determination and sample identification. Moreover, it is extremely valuable in quantitative work. Typical UV-Vis instruments cover not only the UV and visible spectrum, but the near-infrared as well. Although there is overlap among the ranges, the approximate breakdown is ... 

In recent years, a new generation of colorimeters based on spectrophotometers have been developed. The spectrophotometric colorimeter does not mimic the human eye. Instead, it makes a spectrophotometric measurement at sixteen 20-nm intervals over the entire range of the visible spectrum. The percentage reflectance value obtainedby the spectrophotometer is converted into tristimulus values through the use of a microprocessor. One other useful feature added to these spectrophotometric colorimeters is the ability to select various types of CIE illuminants. Even though the actual light source remains the same, the microprocessor computes the colors that would be seen if the samples were viewed under various illuminants. 

The commercial nonionic surfactants do not absorb radiation in the visible spectrum. The simplest form of spectrophotometric analysis of nonionics is the direct measurement of the UV absorbance of the sample. The ethoxylated alkylphenols are the only compounds which can be readily determined by this method, with a maximal absorbance at about 223 nm and another peak at 276 nm (69). Ethoxylated amides may be determined in model systems by their absorption of light at 202 nm (70), but many other compounds found in typical samples also have absorbance in this region. Because of the sensitivity of direct UV analysis to interference, it can only be used in well-defined situations. Interferences often encountered in nonionic surfactants are oxidation inhibitors like butylated hydroxytoluene. 

Fig. 4. H-dependence of the visible absorption spectrum of bovine Co(II) carbonic anhydrase. The broken curve represents the basic spectral form, from ref. (58) (near infrared) and at pH 11.6 (visible). The solid curves were obtained in imidazole-sulfate buffers, ionic strength 0.1, pH 7.80, 7.00, 6.55, 6.10, respectively. The spectrum of lowest intensity was obtained by extrapolation and represents the acidic spectral form. Insert Spectrophotometric titration at 640 nm. The curve was calculated for the titration of a single group with pKa = 6.6...

Generally, ROH or B is an indicator, so that RO-HB+ absorbs strongly in the visible region of the spectrum, and its concentration may be determined spectrophotometrically. The kinetics of reactions of this type have been observed by the temperature-jump method with microwave or laser heating. 

The scope of the present treatment does not include details of the various instrumental methods for the detection of EDTA titration end points. Nevertheless, we may mention spectrophotometric detection methods, which are of two types. The first is based on instrumental observation of the color changes of metal ion indicators. The second is based on the absorption of radiation in the visible or ultraviolet regions of the spectrum by the metal-EDTA complex. For example, MgY shows appreciable absorbance at a wavelength of 222 nm, whereas the reagent HjY ... 

Although product analysis seems essential for the clarification of complex ET [wo-cesses involving biological molecules, only few attempts have so far been made. Ohde et al. [15,35] conducted bulk electrolysis to determine spectrophotometrically some redox products of interfacial ET reactions. Recently, Sawada et al. [39] have developed a microflow coulometric cell with a hydrophobic membrane-stabilized O/W interface. This microflow cell can accomplish complete electrolysis, and thus determination of the number of electrons for complex ET reactions at O/W interfaces. Also, its use for an on-line spectrophotometric detection of electrolysis products was made [43]. Figure 8.5 shows the spectrum change of the electrolyzed solution for the ET between Fc in NB and Fe(CN)6 " in W. When relatively small potentials were applied to the microflow cell, Fc" " could be detected in the electrolyzed solution. The characteristic absorbance peak at 620 nm showed an undoubted existence of Fc+ in the W phase as the electrolysis product. This result would also support the IT mechanism. In situ UV-visible spectroscopy [44-46] also deserves attention for its usefulness in product analysis and clarification of reaction mechanisms. 

If the band of wavelengths selected for spectrophotometric measurements corresponds to a region of the absorption spectrum in which the molar absorptivity of the analyte is essentially constant, departures from Beer s law will be minimal. Many molecular bands in the UV/visible region fit this description. For these. Beer s law is obeyed, as demonstrated by Band A in Figure 24-17. Some absorption bands in the UV/visible region and many in the infrared region are very narrow, however, and departures from Beer s law are common, as illustrated for Band B in... 

In the spectrophotometric method, color is assessed by means of a standard absorption spectrophotometer. In this case, a description of the hue, intensity and brightness of a non-turbid colored solution is possible in the visible region (400-700 mn) of the electromagnetic spectrum. Since pH will have a significant impact on the color of certain dyes, the pH of the dye solution employed must be measured and reported. Details regarding the method are provided in section 2120 C of the Standard Methods book, and can be summarized as follows ... 

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