146 MISC. PUBLICATION 5 4 0. U. S. DEPT. OF AGRICULTURE 



Heat conduction : Metal dishes, preferably aluminum, are used to insure good 

 heat transfer to the sample. 



Time: The time has arbitrarily been set at 6 hours. Longer heating time 

 will result in a greater loss of weight and in some cases as much as 50 hours 

 may be necessary to obtain complete removal of water. However, at present the 

 6-hour period is used by Federal inspectors and therefore this period must be 

 used in order to obtain comparable results. In research work on dehydrated 

 materials it has been found desirable to heat for a longer time and for this purpose 

 16 hours is customarily used. 



Other details : Weighing must be done on an analytical balance capable of 

 determining the weight within 0.10 mg. Drying dishes must be handled with 

 metal tongs to avoid inaccuracies in weights caused by oil or perspiration from 

 the fingers. Any commercial type of vacuum oven can be used. Where difficulty 

 is encountered in obtaining a vacuum oven, a satisfactory substitute in the form 

 of a large pyrex vacuum desiccator placed in an ordinary air oven can be used. 

 Still another way is to use a battery of Abderhalden-type driers made of glass, 

 which are not subject to priority limitations. A temperature of 70° C- in the 

 Abderhalden driers can be maintained by using in the boiling flask a mixture 

 of carbon tetrachloride and chloroform (volume ratio 3.4 to 1). These liquids 

 have the advantage of being noninflammable, but inhalation of the vapors should 

 be avoided. 



AIR OVEN METHOD 



For air-oven analysis, samples are prepared in the same manner as 

 for the vacuum-oven method and then heated in an ordinary air oven 

 (unevacuated). The method has the advantage of simplified equip- 

 ment, since no vacuum oven or pump is required. At temperatures 

 near or above the boiling point of water, shorter time is required for 

 a determination than by the standard vacuum-oven method. It is 

 subject to larger errors, however, and must be checked against the 

 vacuum-oven method for each type of material tested. 



Procedure: The weighed samples are heated in the air oven for a period of 

 time predetermined by comparison with vacuum-oven determination on the 

 same material. Time of heating will depend on fineness of grinding and on 

 the temperature. Dehydrated carrots ground in a coffee grinder (very fine 

 grind) require about 2 hours at 100° C, 2% hours at 90°, and 7 hours at 80°. 

 Dehydrated potatoes ground similarly require about 1% hours at 110°, 3 

 hours at 100°, and 6 hours at 90°. As in the vacuum-oven method the loss 

 in weight is accepted as the water content of the sample. To obtain reproducible 

 results, the time, temperature, and fineness of grind decided upon for each 

 vegetable must be consistently adhered to. 



DISTILLATION METHOD 



By the distillation method the water is distilled out of the material 

 at a temperature above the boiling point of water with the aid of an 

 organic liquid immiscible with water, and the volume is read in a 

 volumetric buret. The method is described in "Official and tentative 

 methods of analysis," (4, p. 593) . It can be used successfully only when 

 calibrated against the vacuum-oven method because at the high tem- 

 peratures used in the distillation, the vegetable material usually chars 

 and the results are likely to be high and erratic. The method is more 

 applicable to wet (20 to 90 percent water) than to dry vegetables. 



Apparatus: Pyrex distilling flask (250 ml.) connected by means of distilling 

 tube receiver to a 20-inch straight-tube Liebig condenser. The receiver is 

 readily made by attaching the proper side tube to the calibrated section of a 

 5-ml. Mohr pipette. 



Procedure: Approximately 75 ml. of toluene is placed in the distilling flask 

 and to this is added the carefully weighed ground sample in an amount estimated 

 to yield from 2 to 5 ml. of water. After the apparatus is connected, fill the 

 receiving tube with toluene by pouring through top of condenser. Heat the flask 

 at such a rate that about 2 drops of distillate per second is condensed. After 

 most of the water has passed over, this rate is increased to 4 drops per second. 

 When all the water appears to have been distilled, wash down the condenser with 



