/2 PHYSIOLOGICAL CHEMISTRY. 



Ex. To a few cubic centimeters of white of egg solution add five drops 

 of a 10 per cent solution of ct-naphthol in alcohol. Then carefully add 

 three or four cubic centimeters of strong sulphuric acid, which sinks below 

 the lighter solution. Note the color at the zone of contact and through- 

 out the liquid on shaking. Alkalies change the color to yellow. Thymol 

 solution is sometimes used instead of a-naphthol. This gives a deep red 

 color. These furfuraldehyde reactions are extremely delicate, and appear 

 in a great variety of tests. Their general character and importance should 

 therefore be recognized. 



The Xanthoproteic Reaction. This is a delicate test, 

 depending on the formation of yellow nitro derivatives of the 

 phenol groups in the protein complex. Similar reactions are 

 given by many simpler organic substances where nitric acid 

 is mixed with them and heat applied. The color produced by 

 nitric acid in contact with the skin is due to the same general 

 reaction. 



Ex. In illustration, add some strong nitric acid to white of egg solu- 

 tion. On application of heat the yellow color appears. By neutralizing 

 with ammonia the color changes to orange yellow. 



Make a similar test by warming some phenol solution with nitric acid. 

 In this case a nitro phenol is formed. Pure phenol and strong nitric acid, 

 it will be recalled, yield trinitrophenol or picric acid, CeHa' OH -(NO 2 )3. 

 Add ammonia to neutralize, as before. 



The Lead Hydroxide Test. The protein bodies contain 

 sulphur which may be removed by action of an alkaline lead 

 solution, or alkaline bismuth solution or mixture. The sec- 

 ond reaction has some importance as it is the source of a fallacy 

 in the so-called bismuth test for sugar in urine. In presence 

 of albumin, sulphide of bismuth is formed in place of the re- 

 duction product indicative of sugar. As all protein bodies con- 

 tain sulphur the test is a general one. It may be made as 

 follows : 



Ex. Produce first a soluble alkaline compound of lead by adding to a 

 few cubic centimeters of lead acetate solution enough strong alkali, sodium 

 or potassium hydroxide, to form a precipitate and redissolve it. Then add 

 the protein substance, white of egg for example, and boil. A brown or 

 black color appears and sometimes even a precipitate of black lead sul- 

 phide. Only a part of the sulphur, however, may be separated in this 

 simple manner. Another portion seems to be much more firmly combined 

 in the protein molecule. 



