ELEMENTARY CHEMICAL PHYSIOLOGY 209 



The excess of sodium acetate in the above mixture reacts with the phenyl- 

 .hydrazine hydrochloride so as to form the acetate. 



When it is desired to obtain osazones from dilute sugar solutions, a more 

 certain way to proceed is as follows : Mix two drops of phenyl-hydrazine 

 (fluid) with ten drops glacial acetic acid and add to this 5 c.c. of the sugar 

 solution, shake, and place the test tube for one hour in the boiling water bath. 

 After cooling examine under the microscope for the crystals. With stronger 

 sugar solutions this method yields crystals after a few minutes' heating. 



The advantage of the phenyl-hydrazine hydrochloride is that it does not 

 readily decompose on keeping, whereas the free base does. 



The osazone crystals are valuable for distinguishing between the different 

 sugars. Besides microscopical examination, a determination of the melting 

 point is often of value. 1 For this purpose the crystals of osazone are collected 

 on a filter paper, washed with water acidulated with acetic acid, recrystalli sed 

 from water, alcohol or acetic acid, and dried by placing in a desiccator over 

 H 2 SO 4 . They are then placed in a narrow glass tube closed at one end and tied 

 on to the bulb of a thermometer by a fine platinum wire. The thermometer is 

 suspended in a long-necked flask (about 50 c.c. capacity) in which is concentrated 

 H 2 SO 4 (almost saturated with K 2 SO 4 to prevent fuming) and the tempera- 

 ture gradually raised by heating the flask over wire gauze. The bulb of the 

 thermometer should dip into the sulphuric acid. The exact temperature at 

 which the crystals begin to melt and the temperature of complete fusion are 

 noted. For accurate work, a correction is necessary because the mercury 

 thread is cooler than the bulb of the thermometer. 2 



The following are the melting points of some of the most important osazones: 

 Dextrosazone, 3 . . . 204-205 C. 



Lactosazone . . . 200 C. (Begins to melt at this temp.). 



Maltosazone . . ' / . 206 C. 



If the crystals are pure, melting occurs at once, but if they are impure there 

 may be a considerable difference in temperature between the points of com- 

 mencing and complete fusion. 



Ketoses. -As mentioned above, some carbohydrates belong to 

 the group of substances called ketones. 



A ketone is the oxidation product of a secondary alcohol and it contains 

 the group CO which is situated somewhere in the chain between other 

 groups and not at the end of it as in the case of the CHO group of the 

 aldehydes. The simplest ketone is acetone CH 3 CO CH 3 which may be 

 obtained by oxidation of secondary propyl alcohol, 



CH 3 - CHOH - CH 3 + O = CH 3 - CO - CH 3 + H 2 O. 

 Secondary propyl alcohol. Acetone. 



Ketones form compounds with phenyl hydrazine, but only some of them 

 reduce metallic oxides in alkaline solution. Those ketones which belong to the 

 carbohydrates manifest this reducing power. The only well-known ketose is 

 laevulose. 



There are several reactions characteristic of ketoses, of these the 

 following is important. 



1 Too much reliance must not be placed on a determination of the melting 

 points of osazones in identifying unknown sugars, for they vary with the rate 

 of heating and with the method of purification of the osazone. 



2 To make the above correction, a second thermometer must be suspended 

 in the flask with its bulb opposite the middle of the column of mercury of the 

 main thermometer, the formula for correction is then L(T t) (0-000154) 

 where L = the height of the mercury column of the main thermometer above 

 the sulphuric acid measured in degrees ; T = the reading of the main ther- 

 mometer ; t, the reading of the air thermometer. This correction must be 

 added to the reading T of the main thermometer. 



3 Lsevulose forms the same osazone as dextrose. 



