Color reversion

Other Borohydrides. Potassium borohydride was formerly used in color reversal development of photographic film and was preferred over sodium borohydride because of its much lower hygroscopicity. Because other borohydrides are made from sodium borohydride, they are correspondingly more expensive. Generally their reducing properties are not sufficiently different to warrant the added cost. Zinc borohydride [17611-70-0] Zn(BH 2> however, has found many appHcations in stereoselective reductions. It is less basic than NaBH, but is not commercially available owing to poor thermal stabihty. It is usually prepared on site in an ether solvent. Zinc borohydride was initially appHed to stereoselective ketone reductions, especially in prostaglandin syntheses (36), and later to aldehydes, acid haHdes, and esters (37). 

Mary compounds car charge their color reversibly when... 

When exposed to daylight, the sulfide and selenide halides HgsY2X2 are blackened within a few minutes. This black color reversibly disappears when the sample is heated to 90 to 120°C, or stored in the dark for several days 204, 375-377). The nature of this phototropic behavior has now been widely investigated by analytical, spectroscopic, structural, magnetic, EPR, and radiotracer investigations 205, 233, 375-377, 379, 380, 382). During irradiation of the compounds, electrons belonging to or I ions are excited to upper states. The result-... 

The members of Wolfbeis team constructed an optical sensor for ammonia-based on ion pairing76. They immobilized pH-sensitive dye (bromophenol blue) as an ion pair with cetyltrimethylammonium bromide (CTABr) in a silicone polymer matrix. Bromophenol blue, while contact the ammonia (both in water as well as in gaseous form) changes its color reversibly from yellow to blue. The immobilized dye shows long wave absorption with a good photostability. 

When the nitrile IV is heated it develops color reversibly at 100° and above. Bases, however, cause the formation of a product analogous... 

Reciprocity failure of the film has two practical implications. The first is seen with the use of color-reversal film. Each of the three color-sensitive layers of the film has a different reciprocity failure factor resulting in incorrect color reproduction during long exposures. To correct for this problem, color-compensating filters can be placed in the light path to the camera. 

A less accurate way to measure pH is to use indicators. An indiccitor is a compound, such as phenolphthalein or methyl orange, that changes color reversibly at different pH values. For example, phenolphthalein is a colorless substance in any solution with a pH value smaller than 8.3. It turns red-violet in solutions with a pH value greater than 8.3. 

Figure 3.6 Film structure of the FUJICHROME 64T Type II Professional color reversal film (from Fuji Photo Film Co., LTD. 2005a). Reproduced by permission of FUJIFILM.

Table 9.13 summarizes the effect of the kind of reductant on the photochromic behavior of thionine dye. All reductants afforded transparent films upon irradiation and complete color recovery in the dark. The greatest photosensitivity was obtained with TEA as the reductant. The reversibility decreased in the order, tri->di->mono-ethanolamine. The difference cannot be explained by an electron-donating character such as the basicity of the reductant (P A a value of their ammonium salt tri-mono-ethanolamine). A potential explanation is that TEA(bp, 335.4°C/760mmHg) is so hygroscopic that the moisture absorbed in TEA and TEA itself acts as a plasticizer, providing tough films that afford higher color reversibility, as mentioned earlier. 

Brightness reversion or yellowing has been attributed in the past to nearly every constituent of pulp and paper. A considerable amount of work has been published in recent years on the influence of various pulp constituents on brightness reversion. Yet, the mechanism of the color reversion is of a complex nature, and the various pulp constituents can either contribute directly or react with each other. 

The effect on color reversion by traces of heavy metal ions has been reported (129,155,156,157,158). The presence of ferric and cupric ions... 

Figure 4-16a shows the logo of a sporting goods store in Boston, Massachusetts. Geometrical correspondence is gone, yet we have no difficulty in recognizing the antimirror symmetry relationship. The antireflection plane relates a half-snowflake and a half-sun, symbolizing winter and summer, respectively. There are two coke machines in the picture of Figure 4-16b. There is no geometrical correspondence, but there is color reversal, and reversal of yet another, more important, property, the sugar content. This makes the two machines an example of antisymmetry with some abstraction.

This mechanism appears to be most commonly reported for transition-metal-catalyzed autoxidation of organic substrates. However, at least a few other possibilities exist (23). The effect of oxygen absorption and subsequent oxidation and cleavage of cellulosic chains on the permanence as well as the appearance of paper can be very significant. The absorption of even the smallest amounts of oxygen is known to produce a substantial loss in mechanical properties of polymeric materials. In the case of paper, oxygen absorption leads to a decrease in the degree of polymerization, color reversion, and a loss in mechanical properties. 

Burgess, H. D. The Color Reversion of Paper After Bleaching , Preprints The Conservation of Library and Archive Materials and the Graphic Arts Cambridge, 1980 Conference, 171, (1980). 

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