Colour tests

NOTE. It is not expected that a student xvill attempt to memorise all the colour tests given in Part III. He might for example be expected to know the Phthalein Reaction for phenol itself (p. 339). Details of the divergence from the standard result are recorded primarily for reference, so that a student will not be diverted from his line of investigation if he observes that a suspected phenol does not respond exactly to the standard colour changes. 

The brush method here the substances to be separated give a colour test with a reagent. The developed column is extruded from the tube and streaked lengthwise with a brush dipped in the test reagent the colours... 

The colour test is not specific for a-amino acids other primary amino compounds and also ammonia give a blue colouration with ninhydrin. 

Methylation with diazomethane may be carried out as follows (FUME CUPBOARD )-. Dissolve 2-3 g. of the compound (say, a phenol or a carboxylic acid) in a little anhydrous ether or absolute methanol, cool in ice, and add the ethereal solution of diazomethane in small portions until gas evolution ceases and the solution acquires a pale yellow colour. Test the coloured solution for the presence of excess of diazomethane by removing a few drops into a test-tube and introducing a glass rod moistened with glacial acetic acid immediate evolution of gas should... 

All carbohydrates (mono-, di- and poly saccharides) give the Molisch colour test for details, see Section III,139,(i). ... 

Small amounts of pyridine have been purified by vapour-phase chromatography, using a 180-cm column of polyethyleneglycol-4(X) (Shell 5%) on Embacel (May and Baker) at 100°, with argon as carrier gas. The Karl Fischer titration can be used for determining water content. A colour test for pyrrole as a contaminant is described by Biddiscombe et al. [J Ghent Soc 1957 1 954]. 

If the identity of the analyte is genuinely unknown, a further problem is encountered. In contrast to GC, there are no HPLC systems, combinations of mobile and stationary phases, that are routinely used for general analyses. Therefore, there are no large collections of k values that can be used. For this reason, retention characteristics are often used for identification after the number of possible compounds to be considered has been greatly reduced in some way, e.g. the class of compound involved has been determined by colour tests or UV spectroscopy. 

Colour Tests The production of visible and distinct colouration may be achieved by ascertaining the requisite quantities of reagents and reactants, time period and above all the stability of the colour produced. 

Phase-transfer catalysed colour test for aldehydes... 

The Reissert reaction exemplifies the B pathway (equation 73), and is of course a notable method for aldehyde and alkaloid synthesis. The Fujiwara reaction (81JOC3175), a colour test, provides an example of Type D behavior (Scheme 50). 

The liberation of hydrocyanic acid due to complete hydrolysis of cyano-genic glycosides can be determined by a simple colour test using sodium picrate paper (yellow), which turns red (sodium isopurpurate) in contact with HCN. 

Methods used at this stage range from colour tests to chromatography coupled to mass spectrometry (the detection of specific frag-... 

A sensitive colour test for sulphite ions consists in adding, drop by drop, a 0-01 per cent, solution of Fast Blue R crystals, shaking after each addition, until the violet coloration disappears and a yellow solution is produced the test is sensitive to one part of sulphurous acid in about 175,000. Thiosulphates and polythionates do not interfere, but sulphides and hydroxides must be absent.1... 

If the quantity of methyl alcohol present in the sample is so small (less than 0-25%) that the coloration obtained is less than that given by standard solution No. 1,100 c.c. of the spirit are distilled with a small dephlegmating column, the first 50 c.c. (exact) of the distillate being used for the colour test. 

To refer the amount of methyl alcohol to 100 c.c. of anhydrous alcohol it is best, to avoid calculation, to make up such quantity of the distilled spirit as contains exactly 9 cm. of alcohol with water to 100 c.c. and to carry out the colour test with 1 c.c. of this solution. 

EXAMPLE If the distilled spirit in question has the alcoholic strength 4171%, 21-58 c.c. of it (corresponding with 9 c.c. Of anhydrous alcohol) are made up to xco c.c. and x c.c. of the liquid thus obtained used for the colour test. If the coloration obtained is comparable with that given by standard solution No. 2 (containing 0-05% of methyl alcohol), it follows that the... 

Oil Colour Test.—1 gram is mixed on a glass plate with a few drops of good, boiled linseed oil and then spread out on the glass and allowed to dry the colour is compared with that given by a standard ultramarine. 

Appearance or smell Flame colours Tests for aqueous cations Tests for aqueous anions Tests for gases... 

The ninhydrin test. Heat a solution of the compound with a few drops of a 0.25 per cent aqueous solution of ninhydrin (Expt 5.99). a-Amino acids give a blue-violet coloration. This highly sensitive test is also given by some /5-amino acids and by some peptides and proteins, particularly on warming. The colour test is of great value in the characterisation of the a-amino acids separated by t.l.c., and the Rf values are useful aids to identification. 

The following colour tests have been described for the identification of camphor ... 

Details of a colour test for papaverine and its tetraethoxy-analogue (ethaverine) have recently become available.12... 

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