Darkene

C10HJ4N2. When pure nicotine is a colourless liquid, b.p. 247 C, but darkens on exposure to air and light. Crude nicotine contains smalt amounts of other alkaloids, but ( —)-nicotine is the principal component. 

CgHgNa. While crystals m.p. 147 C, b.p. 267"C, darken rapidly in air. Prepared by reducing p-nitroaniline or aminoazobenzene. Oxidizing agents convert it to quinone derivatives, hence it cannot be diazotized with nitric acid. 

C10H10N4O2S. White powder, which darkens on exposure to light m.p. 255-256 C. Prepared by condensing p-acet-amidobenzenesulphonyl chloride with 2-aminopyrimidine and subsequent hydrolysis. Soluble sulphadiazine is the sodium salt. Sulphadiazine is the least toxic of the more potent sulphonamides. ... 

CgHg04. A yellow powder, darkening at 240°C and decomposing at 260°C. It occurs in asafoetida as an ester with its anhydride, umbelliferone. 

Another method which should be cited apart from the others is to pyrolyze the sample in a hydrogen atmosphere. The sulfur is converted to H2S which darkens lead-acetate-impregnated paper. The speed of darkening, measured by an optical device, provides the concentration measurement. This method attains sensitivity thresholds of 0.02 ppm. 

Detection of this particle accumulation has so far been done visually. To make the particles more easily visible, they have been chemically treated in order to make them light up or flouresce when struck by an ultraviolet light. The operator sits in a darkened room in which the test pieces are illuminated by ultraviolet light. Cracks show up very clearly and in principle this method of inspection is acceptable. Despite the effiency of this method it is well known that a large number of defective pieces pass this test. Why is it so ... 

The product, commonly called calomel, is a white solid, insoluble in water in its reactions (as expected) it shows a tendency to produce mercury(II) and mercury. Thus under the action of light, the substance darkens because mercury is formed addition of aqueous ammonia produces the substance HjN—Hg—Hg—Cl, but this also darkens on standing, giving HjN—Hg—Cl and a black deposit of mercury. 

II). Pure Compounds decomposing slightly before melting. Lactose. Melts slowly between about 205 and 215 , with preliminary darkening and subsequent decomposition. 

Ethyl iodide is a heavy liquid, of b.p. 72° and of d, 1 94 insoluble in water, When freshly distilled it is colourless, but on prolonged exposure to light it darkens in colour owing to the liberation of free iodine. Its chemical properties are almost identical with those of ethyl bromide given on pp. 102 and 103. 

Phenylhydrazine is, however, frequently supplied in the form of its hydro chloride or sulphate, since these salts on exposure to light darken less rapidly than the free base. If these salts are used, however, osazone formation is unsatisfactory, partly because the mineral acid formed by hydrolysis of... 

To a cold aqueous solution of picric acid, add about an equal volume of dilute potassium cyanide solution. An orange coloration develops and rapidly darkens to a deep red. 

Phenylhydrazine on exposure to light slowly darkens and eventually becomes deep red in colour salts of the base share this property but to a lesser degree, the sulphate and acetate (of the common salts) being most stable to light. Phenylhydrazine is largely used in organic chemistry to characterise aldehydes and ketones as their phenyl-hydrazones (pp. 342, 345), and carbohydrates as their osazones (pp. 136-140). It is readily reduced thus in the process of osazone formation some of the phenylhydrazine is reduced to aniline and ammonia. On the... 

During this period hydrogen chloride continues to be liberally evolved, and the product darkens considerably in colour. Now pour the product cautiously into 500 ml. of dilute hydrochloric acid and 100 g. of chipped ice in a separating-funnel, and shake the mixture thoroughly this operation removes the dark colour, and the toluene solution becomes yellow. Run off the lower acid layer, and extract the toluene three times with water. Finally dry the toluene solution over calcium chloride. 

Cane sugar.—Melts, darkens, then chars, and finally burns, with a marked odour of burnt sugar. Typical of the changes given by mono- and di-saccharides. 

Place a small quantity (t.e., about o-i g.) of the following substances (finely powdered) in a clean dry test-tube and add about I ml. of cone. H2SO4. Note any reaction in the cold, and then warm gently. Note any effervescence and any pronounced blackening (as distinct from mere darkening in colour). 

Uric acid C5H4N4O3 Purines Dissolves on gentle warming and is reprecipitated unchanged on dilution. The solution in cone. H2SO4 darkens on heating. 

I. Action of sulphuric add. To 0 5 ml. of the alcohol, add 0 5 ml. of cone. H2SO4 and shake the mixture. Heat is evolved and a white gelatinous polymer gradually separates. The reaction is hastened by warming and the product darkens. 

Volatility in steam. Add about 0 1 g. of benzoquinone to 3 ml. of water in a test tube and boil gently. The benzoquinone dissolves to give a yellow solution, which rapidly darkens in colour. Note the irritating and characteristic odour of benzoquinone which has volatilised in the steam. Also given by />-toluquinone and 1,4 naphthoquinone but not by the other quinones mentioned above. 

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. 

Removal of sulphur as sodium sulphide. Dissolve about 0 i g. of c> stine in a few ml. of 10% NaOH solution. Add a few drops of lead acetate solution and boil for 1 minute. The solution darkens owing to the formation of lead sulphide. 

Note. A large variety of organic compounds darken to some extent (without actual blackening) when heated with cone. HjSO,. 

Green coloration. Catechol (colour rapidly darkens). [Aniline (pale green), o-toluidine (pale green initially), mono-methylaniline, and diphenylamine, each in dil. HCl.]... 

Cone. H,SO, CO — co+co. darkens on strong heating black- ens. effer- vescence chars, effer- vescence yellow. effer- vescence — darkens slightly — —... 

Primary alcohols, lower than hexyl, dissolve there may be some darkening, but the solution remains clear. 

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