Organic dyes adsorption

Organic dyes are fixed to the support by dispersion in a binder or by direct adsorption. Many organic dyes are used after precipitation with metal salts or metal hydroxides (mordants) as alum K fSCLfr. Al2(S04)3.24H20 or potassium tartrate. 

The end points of precipitation titrations can be variously detected. An indicator exhibiting a pronounced colour change with the first excess of the titrant may be used. The Mohr method, involving the formation of red silver chromate with the appearance of an excess of silver ions, is an important example of this procedure, whilst the Volhard method, which uses the ferric thiocyanate colour as an indication of the presence of excess thiocyanate ions, is another. A series of indicators known as adsorption indicators have also been utilized. These consist of organic dyes such as fluorescein which are used in silver nitrate titrations. When the equivalence point is passed the excess silver ions are adsorbed on the precipitate to give a positively charged surface which attracts and adsorbs fluoresceinate ions. This adsorption is accompanied by the appearance of a red colour on the precipitate surface. Finally, the electroanalytical methods described in Chapter 6 may be used to scan the solution for metal ions. Table 5.12 includes some examples of substances determined by silver titrations and Table 5.13 some miscellaneous precipitation methods. Other examples have already been mentioned under complexometric titrations. 

Now, to return to the orange stain, formed on the surface of a pan by adsorption of capsaicin from a solution (the curry). Such organic dyes are usually unsaturated (see the structure I above), and often comprise an aromatic moiety. The capsaicin, therefore, has a high electron density on its surface. During the formation of the adsorption bond, it is common for this electron cloud to interact with atoms of metal on the surface of the pan. Electron density flows from the dye molecule via the surface atoms to the conduction band of the bulk metal. The arrows on Figure 10.4 represent the direction of flow as electron density moves from the charge centroid of the dye, through the surface atoms on the substrate, and thence into the bulk of the conductive substrate. 

The surface morphologies of PAMAM dendrimers have been studied extensively by Turro and co-workers [16-23]. As shown in Scheme 4, one approach was to study the adsorption of organic dye molecules and metal complexes on the dendrimer surface by UY-Vis and fluorescence spectroscopy another approach took advantages of electron transfer processes between two adsorbed species on a single dendrimer surface or between the adsorbed species on a dendrimer surface and other species in aqueous solution. 

The link between colloids and surfaces follows naturally from the fact that particulate matter has a high surface area to mass ratio. The surface area of a 1cm diameter sphere (4jtr ) is 3.14 cm, whereas the surface area of the same amount of material but in the form of 0.1 pm diameter spheres (i.e. the size of the particles in latex paint) is 314 000 cm. The enormous difference in surface area is one of the reasons why the properties of the surface become very important for colloidal solutions. One everyday example is that organic dye molecules or pollutants can be effectively removed from water by adsorption onto particulate activated charcoal because of its high surface area. This process is widely used for water purification and in the oral treatment of poison victims. 

The area determinations by dye adsorption from solution discussed here are applicable to aqueous dispersions. Although saturation coverage of silver halides by Pseudocyanine remained unchanged in 40% methanol by volume, it is known that in organic solvents where ion-pairs may be adsorbed, the molecular cross section of the cyanine can vary with the dye s anion—cf. Reference 23 for discussion and literature citations. Recent determinations of Agl areas by adsorption of Pseudocyanine were reported to have been unrealistic and salt-dependent (van den Hul, H. J., Lyklema, J., J. Phys. Chem. 90, 3010 (1968)). A likely reason for this result is the circumstance that these investigators carried out their measurements in alcohol dispersions of the substrate where the cited solvent-dependent limitations would apply. 

The reduction of organic dye methyl orange (MO) over CdS colloids with the particles size d = 2R - 5 nm has appeared to be a convenient reaction for detail studying the kinetics of photocatalytic processes. This dye is readily reducible with no dimers formation. The MO adsorption spectrum in the pH range of 10-12, practically does not change. This allows simplifying the interpretation of the experiments on redox transformations of MO and considering the reaction of photostimulated reduction of MO as a model one. 

The photon absorption efficiency of the oxides can be greatly enhanced by the adsorption of organic dye molecules on the surface of the oxides. Such sensitized systems are promising in giving efficient electron-transfer systems. 

The practical and fundamental interests in dye adsorption stem from three considerations (I) their ubiquity in industrial waste waters [499-511,395] (2) their use as surrogates for natural organic matter [512], and (3) their use for the determination of porous structure of adsorbents [451,513,514,346,515-520,512]. 

An adsorption indicator is typically an organic dye, such as fluorescein and its derivatives. Most adsorption indicators are weak acids. Their use is thus confined to basic, neutral, or slightly acidic solutions in which the indicator exists predominantly as the anion. Some cationic adsorption indicators are suitable for titrations in strongly acidic solutions. In this case, adsorption of the dye and coloration of the precipitate occur if the precipitate particles possess a negative charge. 

Other important organic electrolytes are the dye molecules. The adsorption of dyes is of interest largely because they are pollutants frequently found in textile wastewaters and because some of them were proposed as molecular probes to characterize the pore texture of carbon adsorbents. However, this last apphcation should be viewed with caution [1] because dye adsorption is profoundly affected by the carbon surface chemistry and solution pH. Thus, Graham [40] found a good linear relationship between a decreased uptake of the anionic metanil yellow and an increased carbon surface acidity. This author concluded that acidic groups on the carbon surface tend to reduce the capacity for anionic adsorbates in general. The adsorption of dyes was subsequendy investigated by other authors [1]. For instance, Nandi and Walker [41] studied the adsorption of acid and basic dyes on different carbon materials and found that the area covered by a dye molecule depended on the nature of the solid surface. 

The coloured adsorption compounds formed by Mg(OH)2 with organic dyes are essential parts of the methods for determining magnesium (e.g., the Titan Yellow method). Other methods are based on soluble coloured magnesium complexes formed with some organic reagents (e.g., Eriochrome Black T) in ammoniacal media. 

Besides Titan Yellow, several other organic dyes have been recommended which react with Mg(OH)2 to form adsorption compounds in alkaline media, e.g., Magneson II, Phenazo [11], and polymethine dyes [19-21]. 

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