Fading

For many molecules the reduced dipole moment p = (p /( a )) is greater than 1 and the tenns in the successive tenns in the X expansion oscillate widely. Stell, Rasaiah and Narang [M] suggested taming this by replacing the tnincated expansion by the Fade approximant... 

The position of the spots should be marked in pencil, for the colours fade after some time. 

Dissolve a few drops of nitromethane in 10% sodium hydroxide solution. Add a few crystals of sodium nitrite and shake. Now add dilute sulphuric acid drop by drop. A brownish-red coloration develops, but fades again when an excess of acid is added. The sulphuric acid has thus liberated nitrous acid, which has in turn reacted with the nitromethane to give a nitrolic acid, the sodium salt of which is CH3NO2 + ONOH = CH(N02) N0H + HgO reddish-brown in colour, probably owing to mesomeric ions of the type ... 

To 2 ml. of a freshly prepared dilute aqueous solution of sodium nitroprusside, add 2 drops of ethyl malonate and shake then add 2-3 drops of 10% aqueous sodium hydroxide solution and shake again. A red coloration at once appears, but fades in a few minutes to pale brown,... 

Now add more dilute sulphuric acid drop by drop the colour almost completely fades, as salt formation occurs on both nitrogen atoms with suppression of the resonance hybrid formation. 

Sulphur. THE LASSAIGNE SODIUM TEST. The sodium fusion will have converted any sulphur present in the original compounds to sodium sulphide. Dissolve a few crystals of sodium nitroprusside, Na8[Fe(CN)5NO],zH20, in water, and add the solution to the third portion of the filtrate obtained from the sodium fusion. A brilliant purple coloration (resembling permanganate) indicates sulphur the coloration slowly fades on standing. Note, (i) Sodium nitroprusside is unstable in aqueous solution and therefore the solution should be freshly prepared on each occasion, (ii) This is a very delicate test for sulphides, and it is essential therefore that all apparatus, particularly test-tubes, should be quite clean. 

Hydrolysis of methyl oxalate. The exceptionally rapid hydrolysis of rnethyl oxalate can be followed thus Dissolve 0 2 g. of finely powdered methyl oxalate in 10 ml. of water, and add i drop of phenolphthalein. Then add very dil. NaOH solution (1%) drop by drop until the solution just turns pink it will be noticed that the colour rapidly fades, but is restored on the Further addition of 1-2 drops of NaOH solution. The colour fades again and the addition can be repeated until hydrolysis is complete. Oxalic acid (with which methyl oxalate may be confused) gives a precise end-point when treated with NaOH solution in this way. 

Repeat using using 1- or 2-naphthol in place of resorcinol the aqueous layer becomes deep blue, fading to green. 

KMnO, + dil. Na,CO, de- colorisa-tion in the cold — slight fading — sduction 0 heating — deco lorisa-tion in the cold — de- colorisa-tion in the cold... 

The following simple test distinguishes fructose from all other carbohydrates. Upon heating a little fructose with dilute cobalt chloride solution, cooling and treating with a little ammonia solution, a violet to purple colour is developed, - the colour gradually fades and must be observed immediately after the addition of the ammonia solution. Green cobalt hydroxide is formed with all other carbohydrates. 

This nitropropene should be used within a week, or stored in the cold, as the color fades to a slight orange over a couple of weeks in room temperature, which is a sign of decomposition. 

Violet Orange at pH 7.2, beautiful violet at 7.4, color fades on standing... 

Silicate. Once important, siUcate-bonded wheels have faded almost to obhvion increasing wheel speeds have made this inherently weak bond obsolete. Some wheels are produced, however, for old grinding equipment still in existence. 

Examination. Microscopic examination (123) can identify the fibers present ia the pulp (124). Inks, watercolor pigments and media, etc, are analyzed similarly to the pigments and media for paintings. However, sample removal tends to be far more disfigutiag and hence constitutes an even more restrictive factor. Watermarks are studied with the aid of beta-radiography. Examination ia iafrared illumination can assist ia the reading of documents of which the ink has faded. 

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