Distillation Trap for Determining Moisture in Relatively D r y Materials W. N. LINDSAY Central Research Department, Food Machinery Corporation, San Jose, Calif.
A
loosely on the top of the trap, and positions the tip of the condenser near the graduated arm, B. It is important that the condenser be immersed in the condensed water and solvent. B was made of a 1-ml. Pyrex pipet with 0.01-ml. divisions. The portion of the return line inside tube A was made of 6-mm. tubing. The height of the open end of this tube \vas adjusted to cause the condensed m t e r layer to remain in the 25-mm. diameter tube above the constricted section. JVhen the trap is tilted to bring B into a vertical position, the wat'er layer falls into this graduated tube, where the water volume can be read directly to 0.01 mi. and estimated to 0.001 ml. As the zero graduation mark will not necessarily coincide with a meniscus, the volume is determined by taking the difference betreen the meniscus readings. The trap
COhlMOS method for rapid determination of moisture is distillation of the material with a solvent, immiscible with water and collection of the water in a suitable trap where its volume may be measured. Several types of distillation traps are described in the literat,ure ( I , 8-10, I d ) , the earliest being that of Bidwell and Sterling. These traps have been designed to use solvents with densities both heavier and lighter than water. Liquids keavier than water have several advantages, other than their usual nonflammability. Since most samples float on the heavy solvent, scorching on the bottom of the flask and excessive bumping are eliminated. The vapor and condensate are a t all times in the path of continuous flow-,with no stagnant pockets in the condensed solvent or water where individual droplets form and remain isolated.
\vas designed for use with perchloroethylene
(d
'4")
= 1.63 - ; other
solvents may be used if their density is near that of perchloroethylene. O P E R A T I O N OF T R A P
1 80
I 77
&152
I84
. Figure 1.
Diagram of Trap
Dimensions in mm.
The usual trap is graduated in 0.1 nil., and the volume of water collected may be estimated to 0.01 or 0.02 nil. For products of low moisture content, a large sample must be taken t o obtain the usual 5 or 10 ml. of water. Many products are too bulky for an adequate sample w i g h t , and in many instances only small quantities are available for analysis. For these reasons a trap amoullts of lnoisture has been dedesigned particularly for sired. Attempfs to modify existing heavy solvent traps by decreasing the diameter of the usual graduated tube to secure smaller unit volumes Tvere not satisfactory because the solvent layer, and the TT.atern-as did not fall droprvise through the swept out of the trap. As a result, a new type of tilt trap (Figure 1) using a heavy solvent was designed to allow precise measuremerit of small volumes of water.
The sample is weighed into a 250-ml. boiling flask containing 150 ml. of perchloroethylene, the trap is connected, and cold water is circulated through the condenser. It is important to fill the return line of the t.rap with solvent before distillation starts, in order t o prevent water from entering the small diameter tubing. The content of the flask is quickly brought to boiling, and maintained a t a vigorous boil. It is necessary to have a large flow of condensed solvent through the return line. If the flow is small, the solvent in the portion of the return line inside tube A may boil and the water layer may fall into the return line and completely upset the correct operation of the trap. Small droplets that sometimes remain on the condenser and glass surface of the trap a t the completion of the distillation will unite with the condensed liquids if the condenser is lifted and touched with a small rod (1 mm. in'diameter) wetted with Tergitol 4 or 7. An excess of wetting agent will make it difficult to read the meniscus between the solvent and water, because of formation of a cloudy layer. When the condenser is replaced and the contents of the trap are gently swished, the glass surfaces will drain. Since the condenser maintains the water layer and condensed solvent near room temperature, the water volume may be read as soon as boiling has ceased, thereby shortening by several minutes the total time of the determination. The trap was test,ed for water recovery by placing 0.50 ml. of water in the boiling flask with 150 ml. of perchloroethylene and boiling for 30 minutes. The water recovery was 0.48 ml. Cause for the discrepancy of 0.02 ml. was not determined, but is probably water held on the surfaces of the trap and condenser. \Then determining the moisture content of a foodstuff, the incomplete recovery of t\-ater may be neglect'ed, because decomposition of the sample a t the high temperature of the boiling solvent requires an arbitrary adjustment of the total distillation time to produce agreement between the distillation method and a reference method. For whole egg powder, with perchloroethylene as the solvent, a distillation time of 25 minutes was found to give results comparable to those obtained by the A.O.A.C. ( 2 ) and the faster high-vacuum methods (11). LITERATURE CITED
(1) Alexandcr, H. €3.. I s u . Exti. CHEM.,ANAL.ED.,8, 314 (1936). ( 2 ) Assoc. Official Agr. Clleni., Official and Tentative Methods of
Analysis, 5th ed., pp. 30s-9 (1940). (3) Bailey, L J ,i , IsD, , Exti. CHEJc,, .IsaL. ED,,9,568-9 (1937).
ENG,CHEM,,17, 147 (4) Bidwell, G. L.,aIld Sterling, IT,F., (1925). I N D . ESG. C H E M . , A N A L . E D . , ( 5 ) Cleland. J. E., alld FetZer, Jv. R., 14, 27-30 (1942); 13,855, Si58 (1941). ( 8 ) Dean, E. K., and Stark, D. D., J. IND. ENG.C H E Y . , 12, 486 (1990). (7) Fetzer, TT-. R., E v a n s , J . V-., and Longenecker, J. B., ISD. EXO. CHEJI.,.&SAL. ED.,5, 81 (1933). (8) Griffiths, C. W., Ibid., 10, 394 (1938). (9) Kingman, W. A , , IND.ESO.CHEM.,18, 93-4 (1926). (10) Langeland, E. E., and Pratt, It. W., IND. ENQ.C H E ~ ~ . . ED.,10, 400-2 (1938). (11) Lindsay, W ,K.. and Slansfield, T., I b i d . , 16, 628-30 (1944). (12) Rice, E . W.. I b i d . , 1, 3 1 (1929).
DESCRIPTION OF T R A P
Tube A (17 nim. in diameter) is; connected to the boiling flask with a cork or ground-glass joint. Vapors from the boiling flask pass through A into the 25-mm. diameter section of the trap containing an immersion condenser, C, the shoulder of which rests
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