Alkali Darkening

High-yield (80%) snlfite pnlp also darkens in alkali [78], bnt an additional chromophore is involved. Althongh the absorption increases significantly in the visible, the largest increase is in the nltraviolet, with at about 380 nm. This is dne at least 

Leary and Giampaolo [70] found that unbleached softwood TMP behaved differently upon alkali treatment than peroxide-bleached TMP. Unbleached softwood TMP was bleached with peroxide (4% charge) to an ISO brightness of 78% however, alkali-darkened pulp could only be bleached to 68% with the same peroxide charge. Borohydride reduction of the pulp before the alkali treatment inhibited the darkening and almost fully restored its bleachability. The absorption maximum for alkali darkening of unbleached pulp appeared at 420 nm, but was shifted to 360 nm if the pulp was pretreated with borohydride. 

Peroxide-bleached TMP regained all the brightness lost on alkali darkening upon subsequent peroxide bleaching. Borohydride pretreatment inhibited the alkali darkening, but was not required to maintain full bleachability of the pulp, except at 120°C. The maximum in the absorption difference spectrum was at 360 nm. 

Following the earlier suggestion of Gellerstedt [101], Leary and Giampaolo [70] attribute the irreversible darkening of unbleached TMP and the absorption maximum at 420 nm to the formation of hydroxylated o-quinones. The precursors of these hydroxylated quinones are lignin groups that can be removed by peroxide or borohydride, e.g., coniferaldehyde, quinones or aryl carbonyls. 

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Stilbenes are claimed to exist in lignin-containing pnlps. One mechanism for their formation is the retroaldol condensation of (3-1 and (3-5 lignin structures in the presence of alkali (see the Alkali Darkening section). 

Svensson et al. [100] compared dithionite bleaching of spruce TMP at pH 10 and pH 5, using principal components analysis to reduce reflectance spectra to contributing subspectra. The authors concluded that at pH 5 (the common pH for commercial dithionite bleaching) quinones were the major chromophores bleached. At pH 10, quinones were formed due to alkali darkening. In spite of the higher redox potential of dithionite at pH 10, bleaching was less efficient than at pH 5, and there was no evidence that any chromophores other than quinones were removed. 

The maximum that was observed at 360 nm for alkali darkening of peroxide or borohydride treated pulps was attributed to the formation of stilbenes [70], Stilbenes, which are most likely formed by a retro-aldol condensation when ()-l or phenylcou-maran lignin structures are treated with alkali (Figure 3.3), have been isolated from peroxide-bleached mechanical pulps [110]. The action of alkali and oxygen removes such phenolic stilbenes [111]. 

W Giust, F McLellan, and P Whiting. Alkali Darkening and Its Similarity to Thermal Reversion. J. Pulp Paper Sci. 17 173-179, 1991. 

Physical properties. Majority are liquids except p toluidine and 1- and 2-naphthylamine. All are colourless when pure, but rapidly darken on exposure to air and light. All are very sparingly soluble in water, but dissolve readily in dilute mineral acids (except the naphthyl-amines, which are only moderately soluble in adds). They form colourless crystalline salts e.g., CjHjNH2,HCl) which are soluble in water these aqueous solutions usually have an add reaction owing to hydrolysis, and give the reactions of both the amine and the acid from which they are derived. Addition of alkali to the acid solution liberates the amine. 

The color of the reaction mixture darkens during the addition of the alkali but becomes light yellow toward the end. 

EtOH), is insoluble in benzene or ether, and sparingly in chloroform, but dissolves readily in alkali, the solution darkening in air. The salts are crystalline but unstable. The base forms a crystalline metbiodide and contains two methoxyl and two phenolic hydroxyl groups. On methyla-tion it yields a mixture of corydine and fsocorydine. On exhaustive methylation corytuberine yields eventually trimethylamine and 3 4 5 6-tetramethoxy-8-vinylphenanthrene, m.p. 69°. 

White granular powder or cubic crystals refractive index 2.071 darkens on exposure to hght density 5.56 g/cm Moh s hardness 2.5 melts at 455°C vaporizes at 1,547°C vapor pressure 1 and 5 torr at 912 and 1,019°C insoluble in water, alcohol and dilute acids soluble in ammonia solution and concentrated sulfuric acid, alkali cyanide, ammonium carbonate also soluble in potassium bromide and sodium thiosulfate solutions. 

Light yellow hexagonal crystals or powder darkens on exposure to hght density 5.68 g/cm melts at 558°C vaporizes at 1,506°C insoluble in water, most acids and ammonium carbonate solution moderately soluble in concentrated solutions of alkali chloride, bromide, and thiosulfate readily soluble in solutions of alkali cyanides, iodides and in hot concentrated hydriodic acid. 

To a solution of 122 g. of pure salicylaldehyde (Note 1) in 1000 cc. of normal sodium hydroxide solution at room temperature, is added 1420 g. of 3 per cent hydrogen peroxide. The mixture darkens slightly in color and the temperature rises to 45-50°. The solution is allowed to stand for fifteen to twenty hours, whereupon a few drops of acetic acid are added in order to neutralize any excess alkali, and the solution evaporated to complete dryness on the water bath under reduced pressure. 

Alcohol Antimicrobial preservative, disinfectant, solvent In acidic conditions may react with oxidizing materials. May react with residual amounts of aldehyde in alkali conditions darkening solution. Incompatible with aluminum containers... 

TNT is almost insoluble in water, sparingly soluble in alcohol and will dissolve in benzene, toluene and acetone. It will darken in sunlight and is unstable in alkalis and amines. Some of the properties of TNT are presented in Table 2.12. 

The Selenide [(NHCgHg.CO CSe. —This occurs when malon-anilide (5 mols.) and selenium tetrachloride (4 mols.) react in absolute ether at 27° to 30° C. The product darkens at 217° C. and melts at 222° to 223° C. Malon-m-chloroanilide also occurs during the preparation. Reduction of the selenide by alkali hydrosulphite affords the original anilide and hydrogen selenide, whilst treatment with bromine gives dibromomalonanilide and selenium bromide. 

The flash point of furfural is 143°F by Tag Closed Cup. Because of its chemical reactivity, furfural should be kept away from strong acids, alkalies or strong oxidizing chemicals. When furfural is stored for long periods in contact with air, there is a gradual darkening of color, increase in acidity, and formation of a soluble polymer. 

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