70 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



E, a small cylinder for the introduction of amyl alcohol to solutions 

 containing proteoses, or proteins, which froth considerably when treated 

 with nitrous acid. An occasional drop during the evolution of the 

 nitrogen generally suffices. 



The connecting tubes are of 6-7 mm. external diameter and i mm. 

 bore ; the tube to A is of 2 mm. bore. 



The gas burette F is of 150 c.c. capacity ; its upper and narrower 

 portion holding 40 c.c. is graduated in tenths ; its lower and wider 

 portion in 10 c.c. divisions. This large burette will hold all the gas 

 which is liberated whilst the apparatus is freed from air. The gas, 

 which is to be finally measured, should only fill a portion of the narrow 

 accurately graduated part. It is filled with I per cent, sulphuric acid. 

 G connects the gas burette with the Hem pel pipette H, which contains 

 a solution of 50 grams of potassium permanganate and 25 grams of 

 caustic potash per litre. 



The nitric oxide reduces the permanganate, but the manganese 

 dioxide formed is in such a fine state of division that it does not interfere 

 with the manipulation. Several determinations can be made with the 

 same solution. Deposition of manganese dioxide in the capillary tube 

 G is prevented by allowing water from the gas burette to remain in this 

 tube, instead of permanganate. Any carbon dioxide evolved is 

 absorbed by the alkali. 



Excess of sodium nitrite solution (30 gm. per 100 c.c.) is used 

 and it is decomposed by glacial acetic acid ; less nitric oxide is evolved 

 by its use than by the use of mineral acid. The amino substance may be 

 dissolved in semi-normal acid, 50 per cent, acetic acid, or normal alkali. 



Only a small correction for the reagents is necessary ; commercial 

 nitrite gave O'2 c.c. of nitrogen for five minutes' reaction, 0-3 c.c. for 

 thirty minutes and 0*5 c.c. for two hours. 



The estimation is performed in the following way : 



I. The amino solution, containing less than 20 milligrams of amino 

 nitrogen, is placed in B and 5 c.c. of water in A ; 28 c.c. of the nitrite 

 solution is placed in D and then 7 c.c. of glacial acetic acid. Whilst 

 the nitric oxide is being evolved, the vessel D is closed with the stopper, 

 the stop-cock c in the exit tube C being open. The small volume of air 

 in D is driven out by letting water enter from A till the bottle is filled 

 and liquid rises in C. 



The air dissolved in the nitrous acid solution is removed by closing 

 c, leaving a open and shaking D. It is best to hold A, B, C in the 

 left hand during this operation. Rapid evolution of nitric oxide occurs 



