Decolorization

Thus far we have considered situations that tacitly assume the presence of only a single contaminant, whereas actual situations usually involve several impurities. Moreover the separate impurities can have different adsorbabilities. This may cause a behavior in which the decolorization proceeds smoothly until a certain amount of color is removed, beyond which further additions of carbon provide little or no additional decolorization. In such a situation, carbon may be considered for the easily adsorbable color, and other means sought for the less adsorbable portion of color. 

Industrial applications often seek to accomplish more than one objective for example, the treatment of corn sugars and syrups may aim to remove color, bitter flavor, iron, hydroxymethylfurfural, and foam-formers. Inasmuch as the operating environment favorable for improving one property (e.g., color) may not be best for another (e.g., foaming), an experimental study of a new venture should include a simultaneous examination of all properties to be corrected. To do this, the filtrate obtained from each variation in experimental conditions is examined to learn the effect a particular set of operating conditions may have on each of the properties to be improved. This brings us to a discussion of useful functions of activated carbon in industrial applications. 

More data are available on decolorization than on other liquid-phase functions of activated carbon, and this is understandable in view of the comparative ease of measuring color. Long before the introduction of modern instrumentation, there were satisfactory visual methods of measuring color which continued in use until relatively recently. Photoelectric colorimeters which are now in general use avoid the personal equation and expedite evaluations. Spectrophotometers offer further advantages, especially when it is desirable to measure the separate adsorption of several different colored bodies. 

Data on decolorizing are presented in varied forms and in diverse units of measurement, depending on the terminology in vogue for the substance or product being treated, and on the instrumentation in use. Consequently, sets of data taken from different sources are not always readily correlated if left in their original form. Inasmuch 

An expensive carbon may provide additional benefits in some operations, but a price tag must not be taken as an index of value for all purposes. A carbon may be expensive because of a higher cost required to impart special adsorptive powers or to ensure freedom from soluble ash. Except where such properties are needed the purchase of such a carbon can be an extravagance, in fact, in some applications a suitable low-cost carbon can be more effective on a pound-for-pound comparison. 

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SchifT s reagent A solution of rosaniline in water decolorized with sulphurous acid. Aliphatic aldehydes and aldose sugars give a magenta colour with this reagent with aromatic aldehydes and aliphatic ketones the colour develops more slowly aromatic ketones do not react. 

Castile soap is manufactured from olive oil, transparent soap from decolorized fats and liquid green soap from KOH and vegetable oils. Soaps are sometimes superfatted in that they contain some free fatty acid. 

Selenium is used in Xerography for reproducing and copying documents, letters, etc. It is used by the glass industry to decolorize glass and to make ruby-colored glasses and enamels. It is also used as a photographic toner, and as an additive to stainless steel. 

White gold is an alloy of gold decolorized by the addition of palladium. Like gold, palladium can be beaten into leaf as thin as 1 /250,000 in. The metal is used in dentistry, watchmaking, and... 

As ceric sulfate is used extensively as a volumetric oxidizing agent in quantitative analysis. Cerium compounds are used in the manufacture of glass, both as a component and as a decolorizer. 

The toluene solution from the previous step was treated with an ethanol solution of NaOEt (0.1 mol in 100 ml) at O C. When about a quarter of the solution had been added a thick precipitate formed and ether (100 ml) was added to maintain a fluid slurry. The remainder of the NaOEt was added and the slurry was stirred overnight. The solid was collected and w ashed with ether. It was then mixed with ether (200 ml) and 2NHC1 (75 ml) and shaken in a separatory funnel until the solid dissolved. The ether layer was washed with 2NHHC1 (2 X 50ml) and water and dried over MgS04. The solution was decolorized with Magnesol and evaporated to give the a-nitro ester as a red oil. 

Fig. 2. Pore size distribution of typical samples of activated carbon (small pore gas carbon and large pore decolorizing carbon) and carbon molecular sieve (CMS). A / Arrepresents the increment of specific micropore volume for an increment of pore radius.

If lighter colors than these are desired it is necessary to decolorize all the melanin in the hair in a preliminary step, and then add color back to the desired depth in a second treatment. This is known as a double-process treatment. The decolorization step consists of treating the hair with an alkaline mixture of persulfate salts and peroxide. The persulfate is added to the peroxide as a dry powder immediately before applying to the hair. Although the persulfate salts alone do not have any bleaching effect, the persulfate—peroxide mixture can remove all the melanin in the hair. Dark brown or darker hair can be lightened to a light blonde shade in about an hour. 

The amount of oxygen evolved is not related to the degree of bleaching (40). Oxidative decoloring is caused by hydrogen peroxide or by the HO ions present in alkaline solution. Hydrogen peroxide is also an effective solvent for melanin (41). 

Optimum resin decoloration and overall catalyst life were achieved using a catalyst with a specific fresh surface area of 181 ni2/g (64). 

Research has shown that ascorbic acid can be produced from hulls of immature walnuts by extracting the hull with 0.2% sulfur dioxide solutions, and purifyiag the extract by adsorption on and elution from anion-exchange resias (see Ion exchange). Eluates from the anion-exchange step are concentrated, purified by organic solvent fractionations, decolorized, and crystallized (35). 

Both technical- and reagent-grade phosphoms pentoxide is typically >99% P O q. Phosphoms pentoxide sublimes near 360°C at atmospheric pressure. Lower oxides, which may account for <0.3% (as P40 ) in technical-grade material, are present at <0.02% in reagent-grade phosphoms pentoxide. Lower oxides are detected by decolorization of a dilute potassium permanganate solution (Table 11). 

GaUic acid is heated with about half its weight of water in a copper autoclave until the pressure reaches 1.2 MPa (12 atm) and the temperature is 175°C. Steam and carbon dioxide are released but sufficient water is retained to maintain the pyrogaHol as a Hquid. The cooled solution is decolorized with animal charcoal and is then evaporated until the volatile pyrogaHol distills into iron receivers. The sohdified material is purified by repeated distillation, sublimation, or vacuum distillation at 200°C in the presence of diaLkyl phthalates (8). 

C), the yield of more than 90% purity L-glutamic acid crystals is very high. The glutamic acid crystals appear as both the metastable a- and stable P-forms. The a-form consists of prismatic crystals which are easy to filter, whereas the P-form needle crystals are difficult to filter. Control of crystallisation conditions of a-crystals are requited (13). The cmde L-glutamic acid crystals are suspended ia water and neutralized with caustic soda or sodium hydroxide. The solution is decolorized with activated carbon to produce a transparent solution and MSG is crystallized under reduced pressure. 

DecoloriZation. Filtration, often a refinery botdeneck, especially with poor-quaHty raw sugar, is foUowed by decolorization with bone char (traditional), granular activated carbon (now most common), ion-exchange resias, or any combination of these. Comparative merits and regeneration of these decolorizing systems are a frequent topic ia the Hterature (r6—r8,rll). 

The Reich test is used to estimate sulfur dioxide content of a gas by measuring the volume of gas required to decolorize a standard iodine solution (274). Equipment has been developed commercially for continuous monitoring of stack gas by measuring the near-ultraviolet absorption bands of sulfur dioxide (275—277). The deterrnination of sulfur dioxide in food is conducted by distilling the sulfur dioxide from the acidulated sample into a solution of hydrogen peroxide, foUowed by acidimetric titration of the sulfuric acid thus produced (278). Analytical methods for sulfur dioxide have been reviewed (279). 

Dithionite is a stronger reducing agent than sulfite. Many metal ions, eg, Cu", Ag", Pb ", Sb ", and Bi ", are reduced to the metal, whereas TiO " is reduced to (346). Dithionite readily reduces iodine, peroxides, ferric salts, and oxygen. Some of the decolorizing appHcations of dithionite, eg, in clay bleaching, are based on the reduction of ferric iron. 

Antimony trioxide and sodium antimonate are added to specialty glasses as decolorizing and fining agents, and are used as opacifiers in porcelain enamels. Antimony oxides are used as white pigments in paints, whereas antimony trisulfide and pentasulfide yield black, vermilHon, yeUow, and orange... 

In the glass (qv) and ceramic industry (see Ceramics), barite can be used both as a flux, to promote melting at a lower temperature or to increase the production rate, and as an additive to increase the refractive index of glass. The viscosity of barite-containing glass often needs to be raised. Alumina in the form of feldspar is sometimes used. To offset any color produced by iron from the barite addition, more decolorizer may be needed. When properly used, barytes help reduce seed, increase toughness and brilliancy, and reduce annealing time. Barite is also a raw material for the manufacture of other barium chemicals. 

Bleaching and decolorization can occur by destroying one or more of the double bonds in the conjugated chain, by cleaving the conjugated chain, or by oxidation of one of the other moieties in the conjugated chain. The result of any one of the three reactions is an increase in the energy gap between the... 

Analysis. Butenes are best characterized by their property of decolorizing both a solution of bromine in carbon tetrachloride and a cold, dilute, neutral permanganate solution (the Baeyer test). A solution of bromine in carbon tetrachloride is red the dihaUde, like the butenes, are colorless. Decoloration of the bromine solution is rapid. In the Baeyer test, a purple color is replaced by brown manganese oxide (a precipitate) and a colorless diol. These tests apply to all alkenes. 

Activated carbon products used for decolorizing food products in Hquid form must meet the requirements of the Tood Chemical Codex as prepared by the Pood Nutrition Board of the National Research Council (63). 

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