II.] 



COLOUR TESTS. 



39 



4. The reagent is prepared by dissolving one part by weight of mercury 

 in twice its weight of concentrated nitric acid (Sp. gr. i'42). The mixture is 

 slightly warmed towards the end. It is then treated with twice its bulk of 

 distilled water, allowed to settle over-night, and filtered. It contains mercurous 

 and mercuric nitrates, excess of nitric acid, and a small amount of nitrous acid. 



5. The reaction should never be attempted with a strongly alkaline fluid, 

 since the alkali will precipitate the yellow or black oxides of mercury. 



6. If an excess of the reagent be employed the red colour is often dis- 

 charged on boiling. 



7. The reaction is given with all aromatic substances that contain a 

 hydroxyl group attached to the benzene ring. Thus it is given by phenol, 

 salicylic acid, and naphthol, but is not given by benzoic acid. 



The aromatic substance derived from protein that is responsible for the 



"OH (i) 



.CH 2 .CH.NH 2 .COOH (4)' 



8. 



reaction is tyrosine. C 6 H 4 = 



(See Ex. 92.) 



23. The glyoxylic reaction. (Hopkins and Cole.) Treat 2 

 or 3 cc. of the fluid with the same bulk of " reduced oxalic acid " 

 ("glyoxylic reagent "). Mix and add an equal volume of con- 

 centrated sulphuric acid, pouring it down the side of the tube. 

 A purple ring forms at the junction of the fluids. Mix the fluids 

 by shaking the tubes gently from side* to side. The purple colour 

 spreads through the whole fluid. 



NOTES. i. The "glyoxylic reagent " is prepared by one of the follow- 

 ing methods : 



A. Treat half a litre of saturated solution of oxalic acid with 40 

 grammes of 2 per cent, sodium amalgam in a tall cylinder. When all the 

 hydrogen has been evolved the solution is filtered and diluted with twice 

 its volume of distilled water. The solution now contains oxalic acid, 

 sodium binoxalate, and glyoxylic acid (COOH.CHO). It should be kept 

 in a closed bottle containing a little chloroform. 



B. In a flask place 10 grammes of powdered magnesium and just 

 cover with distilled water. Slowly add 250 cc. of saturated oxalic acid, 

 cooling under the tap at intervals. Filter off the insoluble magnesium 

 oxalate, acidify with acetic acid, dilute to one litre with distilled water, and 

 bottle as above. 



2. The reaction does not succeed in the presence of nitrates, chlorates, 

 nitrites, or excess of chlorides. 



3. The colour is not well seen if the protein is mixed with certain carbo- 

 hydrates (e.g. cane-sugar), owing to the char produced by the strong sulphuric 

 acid. 



4. It is important to use pure sulphuric acid for this test. It sometimes 

 fails owing to the presence of impurities in the acid employed. At the same 

 time it must be admitted that a very minute trace of ferric chloride does 

 sometimes increase the intensity of the colour. 



