COLOR-REACTIONS. 35 



molecule, as a very similar reaction is obtained with urea, and is in 

 that case referable to the formation of biuret. It is possible, how- 

 ever, that its occurrence may be due to a cyanic radicle, which is 

 likewise contained in biuret, and, as we have seen in the preceding 

 chapter, hydrocyanic acid is in all probability intimately concerned 

 in the synthesis of albuminous substances. Both hydrocyanic acid 

 and hydrocyanuric acid, moreover, give this reaction, and it is to be 

 noted that the former yields the purplish color of the albumoses, 

 while the latter gives rise to the violet color of the native albumins. 

 The test is conducted as follows : A few c.c. of the solution to be 

 examined are treated with an excess of a strong solution of sodium 

 hydrate, and then drop by drop with a 2 per cent, solution of copper 

 sulphate. In the presence of albumins, with the exception of phy- 

 tovitellin, a pure violet color is obtained, while with albumoses and 

 peptones a rose color develops. If larger amounts of albumin are 

 present, the reaction is obtained without difficulty. Should traces 

 onlv be present, great care must be taken not to add too much of the 

 copper solution, as otherwise the violet color is obscured by the blue 

 of the copper solution. Where larger amounts are present, it is 

 necessary to add more of the reagent. An excess of neutral salts, 

 which is often present when this test is employed, does not interfere 

 with the reaction. If ammonium sulphate is present, however, it is 

 advisable to use a large quantity of the sodium hydrate solution, in 

 order to bring out the color. Should magnesium sulphate be con- 

 tained in the solution, a precipitate of magnesium hydroxide is 

 formed, and is allowed to settle. Sodium chloride does not interfere 

 with the reaction. 



5. Boiling with Hydrochloric Acid. The reaction apparently 

 depends upon the formation of furfural, which yields a violet 

 color when brought in contact with some other substance which 

 is formed from the albuminous molecule at the same time. What 

 this other substance is, however, we do not know. The albuminous 

 material, best after extraction with hot alcohol and subsequently with 

 ether, is boiled for several minutes with concentrated hydrochloric 

 acid, to which a drop of concentrated sulphuric acid has been added. 

 The albumin passes into solution and a deep-violet color results. 



6. The Sulphur-test. The albuminous solution is heated with 

 an excess of sodium hydrate in the presence of a small amount of 

 acetate of lead. At first the solution turns brown, and later a 

 precipitate of black sulphide of lead results. 



7. Molisch's Test.-^The reaction is referable to the presence of a 

 carbohydrate group in certain alhumins, which gives rise to the 

 formation of furfurol on treating with concentrated sulphuric acid. 

 The test is conducted as follows : A small amount of the material is 

 treated with a few drops of a 15 per cent, alcoholic solution of 

 a-naphtol, and with 1 or 2 c.c. of concentrated sulphuric acid. In 

 the presence of a carbohydrate group the liquid assumes a beautiful 

 violet color. 



