Oxidative bleaching Subject

Oxidation has an important effect on the color of fats and oils. Although oxidation bleaches the carotenoid pigments, it tends to develop the color of other types of coloring material and, in certain cases, even develops colored compounds of qui-noid nature from the fatty acids or acylglycerols of the oils. Cottonseed and, in certain cases, soybean oil are subject to marked darkening upon oxidation. The effect of oxidation or aeration on the color stability of oils is shown in Figure 7 (7). 

As described in Section 4-1. one important class of chemical reactions involves transfers of protons between chemical species. An equally important class of chemical reactions involves transfers of electrons between chemical species. These are oxidation-reduction reactions. Commonplace examples of oxidation-reduction reactions include the msting of iron, the digestion of food, and the burning of gasoline. Paper manufacture, the subject of our Box, employs oxidation-reduction chemishy to bleach wood pulp. All metals used in the chemical industry and manufacturing are extracted and purified through oxidation-reduction chemistry, and many biochemical pathways involve the transfer of electrons from one substance to another. 

ACR139). The corresponding thieno[3,4-c]pyrroles (12) and (13) are also bleached by air and light, but the oxidation products have not been identified. However, the products from the reaction of thienopyrrole (13) and peracetic acid were identified as diketone (151) and its mono AT-phenylimine (154) showing that the pyrrole ring is subject to preferential oxidative attack (Scheme 45) (75ACR139). 

Uloses are derivatives of carbohydrates, which contain a further keto group. Aldosuloses are obtained from aldoses and diuloses from ketoses. They are important intermediates in the synthesis of carbohydrates. Uloses can be prepared by oxidation of derivatives in which all the hydroxyls except that one subjected to oxidation are blocked. Uloses are formed as intermediates during pulp bleaching (see Section 8.1.3). 

The traditional elastomeric threads have been subject to certain inherent limitations, however. The presence of unreacted double bonds makes them sensitive to oxidation, especially with exposure to the ultraviolet radiation of direct sunlight. They also have low resistance to laundry and household bleaches and dry-cleaning fluids. 

The carboxyl contents of different lignins are shown in Table 7.5.1. In the case of lignins isolated or modified without recourse to strong oxidation procedures, the carboxyl content is generally less than 10%. However, when lignin is subjected to the action of strong oxidants, as is the case, for example, in the bleaching of chemical pulps, the carboxyl content may substantially exceed this value. 

When potassium ferricyanide is warmed with a solution of bleaching powder to 70° C. a considerable evolution of gas takes place, and a reddish deposit of ferric oxide and calcium carbonate is formed. The filtered solution is concentrated and the potassium nitroprusside extracted with alcohol, and converted into the insoluble copper salt by addition of cupric chloride. This latter is decomposed with sodium hydroxide, yielding the sodium salt, which may be further purified by dissolving in a little water, addition of alcohol, and subsequent evaporation after filtering off any insoluble material.2 The constitution to be assigned to sodium nitroprusside in particular, and hence to nitro-prussides in general, has been a subject of debate, Browning s3 formula is —... 

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