Colour reaction adsorbed

The interference of impurities extracted from the adsorbent is appreciably harder to avoid. The usual silica gels and aluminas contain inorganic impurities, in particular iron and chloride colour reactions influenced by iron show divergences when an eluent is used which dissolves iron [212]. Such divergences can be eliminated by using special preliminary purification procedures. The evaluation of bile acid chromatograms is an example illustrating this [797] the silica gel G was... 

The reagent serves in many cases as eluent also and measurement is carried out at the suitable wave length after centrifuging. If strongly polar, acid or basic reagents are used, binder and impurities are easily dissolved out of the adsorbent this can affect the colour reaction [212] (cf. p. 150). 

Difficulties of this sort have been reported, e.g., in the extraction of phenols which had been converted to azo dyes on the adsorbent layer [623]. The reaction of sulphonamides with a diazonium reagent has been taken to completion by adding excess reagent again after having extracted the coloured product [69]. The determination of monosaccharides with benzidine-glacial acetic acid has been carried out similarly [38]. The method has been used also for the quantitative analysis of amino acids, after colour reaction with ninhydrin [42, 95] corticosteroids, after reaction yielding formazans [64] and of cannabinols after conversion into azo compounds [370]. 

A semi-quantitative determination can be made by comparison with a series of standards on the same layer, before or after carrying out a colour reaction. A flawless quantitative determination after elution has not so far been achieved as a result of the extreme instabflity of the vitamin A compounds, especially on active adsorbents. Success may perhaps be gained by conversion to the more stable anhydro-form and following then with TLC. The radioactive measurement of isotopes has been carried out frequently [68, 98]. 

Some of the frequently used colour reactions of classical fat analysis, like the Halphen-test, can be carried out on the adsorbent layer after thin-layer fractionation this enables us to discover which substances are responsible for these colour reactions [144]. The sterol fraction is especially easily identified with various spray reagents (see Chapter L) and functions thus as a characterising feature ( marker ). 

The crude product of an organic reaction may contain a coloured impurity. Upon recrystallisation, this impurity dissolves in the boiling solvent and is partly adsorbed by the crystals as they separate upon... 

Upon heating in air the TGA-DTA measurement showed that the spent catalyst sample lost 5% weight and two exothermal peaks (at 300°C and 400°C) were observed. Similar measurements with spent alumina (without CuO) showed only one peak in the DTA diagram (405°C). A fresh and spent Cu0/A1203 sample were characterized with EDAX. The analysis showed that chlorine was present on the spent catalyst, whereas it was totally absent in the fresh sample. The origin of chlorine was from a chlorine containing impurity in the hydroxy ketone I which irreversibly adsorbed on the catalyst. Adsorbed chlorine is known to increase the acidity of the alumina support and thereby may enhance cracking or polymerisation processes which finally lead to catalyst deactivation. The dark-yellow colour of the initially white supports after use in the reaction indicated that residues were retained on the catalyst. The yellow colour disappeared after calcination in air at 500°C,... 

The acid or base strength is defined by comparing the interaction between the sites and reference (probe) molecules. For example, the acid strength is the ability of the site to convert an adsorbed reference neutral base into its conjugate acid. This process may be monitored by following the colour changes of the indicators or by such techniques as IR or NMR spectroscopies. Several criteria must be considered in the selection of these probes their pK value must be adapted to the acidity or basicity of the surface under study the size of the probe molecules should ensure that all sites are readily accessible finally, the probe should be selected so that its interaction with the surface is easily studied by the available experimental techniques. Catalytic test reactions are sometimes used... 

It is interesting to note that the colour formation caused by chlorhexi-dine/black tea interaction occurs after only short exposure times and this is speculated to be at a rate much faster than nonenzymatic browning reactions at body temperature. Thus, the overall evidence indicates that the most likely mechanism of staining by chlorhexidine involves the interaction of dietary chromogens with adsorbed chlorhexidine. 

The acceptors and donors need to be pre-adsorbed at the surface to allow the slower chemistry to compete effectively with the ultrafast electron-hole recombine-ation at the surface (Serpone et al., 1995b). hi a real Ti02 dispersion, the donor is typically OH and/or FI2O, and the acceptor is the ubiquitous O2 molecule, which is reduced to produce OFI(ads) radicals and the hydroperoxy radical, FI02 (ads) (reactions 5.48 and 5.49, and 5.51 and 5.52). The surface Ti species imparts a blue colour to the Ti02 particles in the presence of hole scavengers in oxygen-free dispersions. 

If the substances being separated are coloured, the location of the spots of the adsorbed substances that have ascended the paper by capillary action is easily achieved. In the case of colourless substances, the spots have to be located by either conversion to a coloured derivative by reaction with a suitable detecting agent or by some suitable detecting device. 

From previous results (3) obtained for the reaction using Ni (aq) and the related coloured ligand pyridine-2-azo-p-dimethylaniline (PADA), it was concluded that in the presence of excess micellar SDS, both Ni (aq) and PADA were strongly adsorbed at the micelle surface where reaction occurs. The rate of reaction was markedly pH-dependent due to the protonation of the ligand, and a shift in... 

Optical enzyme sensors are designed preferably as extrinsic sensors, i.e. as op-todes. There are, however, some examples of intrinsic sensors. As an example, an optical fibre has been described which was manufactured on the basis of a polystyrene fibre coated with adsorbed enzyme and indicator molecules. The colour changes brought about by the enzymatic reaction was detected using the evanescent field. 

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