94 PHYSIOLOGICAL CHEMISTRY. 



two solutions especially if the mixture is slightly agitated. This 

 color probably results through the union of the protein and the 

 formaldehyde. If the sulphuric acid is added to the protein before 

 the formaldehyde is added the typical end-reaction is not 

 obtained. So far as is known this is a specific test for pro- 

 teins. The reaction cannot be applied satisfactorily with concen- 

 trated formaldehyde. 



Rosenheim claims the reaction is due to the presence of oxidizing 

 material in the sulphuric acid and that when pure sulphuric acid 

 is used no reaction is obtained. He advises the use of a slight 

 amount of an oxidizing agent, e. g., ferric chloride or potassium 

 nitrite (0.005 gram per 100 c.c. of sulphuric acid) in order to 

 facilitate the reaction. Rosenheim further states that proteins 

 respond to the formaldehyde reaction because of the presence of the 

 tryptophane group, a statement which Acree does not accept as 

 proven. 



9. Bardach's Reaction. 1 This is the most recent test which 

 has been described for the detection of protein material. The 

 test depends upon the property possessed by protein substances of 

 preventing the formation of typical iodoform crystals through the 

 interaction of an alkaline acetone solution with iodopotassium 

 iodide. Instead of the typical hexagonal plates or stellar forma- 

 tions of iodoform there are produced, under the conditions of the 

 test, fine yellow needles which are apparently some iodine compound 

 other than iodoform. The technique of the test is as follows : 

 Place about 5 c.c. of the protein solution 2 under examination in 

 a test-tube, add 2-3 drops of a 0.5 per cent solution of acetone and 

 sufficient Lugol's solution 3 to supply a moderate excess of iodine 

 and produce a red-brown coloration. (The amount of Lugol's solu- 

 tion necessary will depend upon the content of protein, sugar and 

 other iodine-reacting substances in the solution under examination 

 and may vary from one drop to several cubic centimeters.) Add 

 an excess (ordinarily about 3 c.c.) of concentrated ammonium 

 hydroxide and thoroughly mix the solution. Place the tube in the 

 test-tube rack, examine the contents at intervals of five minutes, 

 and when it is evident that crystals have formed, place a drop of 



1 Bardach: Zeitschrift fur Physiologische Chemie, 1908, LIV, p. 355; also 

 Seaman and Gies: Proceedings of the Society for Experimental Biology and 

 Medicine, 1908, V, p. 125. 



2 The solution should not contain more than 5 per cent of protein material. 



8 Dissolve 4 grams of iodine and 6 grams of potassium iodide in 100 c.c. of dis- 

 tilled water. 



