672 MICROCHEMICAL TESTS 



Gardiner {Proc. Camb. Phil. Soc, iv, 1883, p. 387) adds a 

 solution of ammonium molybdate in a strong solution of ammo- 

 nium chloride ; many tannins give a copious yellow precipitate, 

 while digallic acid gives a red coloration, destroyed by oxallic 

 acid. 



With gallic acid, KCN gives a pink coloration and Nessler's 

 reagent a grey-green precipitate. 



Vinson {Bot. Gaz., xlix., 1910, p. 222) fixes and stains tannins 

 in situ by precipitation with vapour of amyl or ethyl nitrate. 

 Whole organs are exposed to the vapour. A 20 per cent, alcoholic 

 solution made by diluting the 90 per cent, commercial nitrous 

 ethyl is recommended. Amyl nitrite is disagreeable to use. Sweet 

 spirits of nitre (containing about 4 per cent, ethyl nitrite) require 

 a longer exposure. 



1351. Proteins. Iodine gives a yellow to brown coloration, and 

 osmic acid a brown coloration. A brick-red colour is obtained 

 when they are warmed in a few drops of Millon's reagent. This 

 reagent is a mixture of mercuric and mercurous nitrates and 

 nitrous acid. Plugge (Arch. Pharmacie, ccxxviii, 1890, p. 44) 

 dissolves 1 part by weight of mercury in 2 parts of nitric acid of 

 s.g. 1-42 and then dilutes it with twice its volume of water. 



Xanthoproteic Reaction. Warm with a few drops of strong 

 nitric acid. A yellow colour, changed to orange by moistening 

 with strong ammonia results. This is non-specific. 



Biuret Reaction. Add a solution of copper hydrate in KOH 

 solution ; or steep twenty to sixty minutes in 0-2 per cent, 

 aqueous KOH, wash and place in 10 per cent, copper sulphate 

 for thirty to sixty minutes, wash in water and mount in 2 per cent, 

 aqueous KOH. A mauve to violet colour indicates proteins. 

 Protein stored in granular form is preserved by ordinary fixatives. 

 Lyons blue gives an intense stain. Aleurone grains are well fixed 

 by alcoholic mercuric chloride or picric acid. Stain the ground 

 substance with alcoholic eosin ; the globoid and crystalloid will 

 show by contrast. 



1352. Alkaloids. A chocolate brown precipitate is given by 

 iodine-potassium iodide ; colourless amorphous precipitates by 

 tannic acid, phosphotungstic acid and mercuric iodide in potas- 

 sium iodide ; and crystalline precipitates by auric and platinic 

 chlorides. 



In examining plant tissues for alkaloids, Errera {Ann. Soc. 

 beige Micr. Mem., xiii) recommends testing fresh sections with 

 alkaloid reagents and also sections that have been soaked with 

 5 per cent, alcoholic tartaric acid solution, which is a solvent of 

 alkaloids. In the latter case, no precipitates should be obtained. 

 The final identification of the various alkaloids depends chiefly 

 upon the colour reactions, for which see various monographs. 

 See also Niethammer, Biochem. Zeits., ccxiii, 1929, p. 138. 



