A New Moisture Tube H. B. ALEXANDER Hercules Powder Company, Wilmington, Del.
A
LTHOUGH numerous devices utilize the principle of
distillation with a n immiscible solvent for the measurement of moisture in many classes of materials, the one described here is new and possesses certain advantages. It is designed for use with a water-immiscible solvent, such as carbon tetrachloride, which has a greater density but a lower boiling point than water. It has been used during the last seven years in the Hercules Powder Company laboratories for the routine determination of water in dynamite. The method of operation of the apparatus, as shown in Figure 1, is obvious. The apparatus is constructed of Pyrex glass. The bottom of the moisture tube fits into a 500-cc. round-bottomed flask, and a n Allihn water condenser is fitted into the top. (This moisture tube may be secured from Otto R. Greiner Co., 55 Plane Street, Newark, N. J.) T o maintain a constant minimum hang-up of water in the condenser, the tip of the condenser either should be beveled a t 45" to 60" or else be constricted and provided with a vapor inlet as indicated. The latter arrangement is preferable. Because the return arm is surrounded by a continual stream of hot vapors, there is no tendency for water to separate and accumulate a t any point in the liquid-return system. This important advantage makes possible higher accuracy than usually is obtained by other distillation methods. This moisture tube has the advantage of being very coinpact; thus, a supply of moisture tubes can be conveniently kept in acid-potassium dichromate solution when they are not in use. This with the requirement that all forms of apparatus in which small volumes of water are measured must be kept scrupulously clean.
COND€NSU
W
FIGURE1. HERCULES MOISTURE TUBE It is important that the ratio of length A t o length B be
the same as the ratio of density of solvent t o denslty of water. If carbon tetrachloride is used as the solvent, A / B should be 1.6. Dimensions given are approximate. The measuring tube is graduated in 6 or 7 1-00. divisions, numbered as indicated and marked with 0.05-eo. subdivisions.
Fill the U-tube with carbon tetrachloride, then add about 0.25 gram of distilled water. Again add carbon tetrachloride dropwise, until a drop falls through the water layer. This produces an upper meniscus of uniformly reproducible curvature. Record the volume between the bottomOf the meniscus and the top of the lower meniscus- this is a zero reading. Then add 2 to 3 grams of water, weighed to the nearest 0.005 gram, and repeat the readings as described. The weight of water, added as the second portion, will correspond to the increase in volume measured on the graduated scale, if the tube is accurately graduated. Repeat this procedure to check other portions of the scale.
amount of water is always present in the carbon tetrachloride at equilibrium and a small amount may adhere to the condenser. Therefore, a n additive apparatus correction constant must be applied to compensate for all the water Present in the apparatus which is not actually measured. This additive correction constant was established as 0.11 gram, by making 45 determinations with 7 different moisture tubes and known amounts of water. The following procedure was followed for determining this additive correction constant F~~~grams of a dry absorbent material-Kieselguhr, for example-were in the distilling flask. Then a known amount of water was added, followed by the regular charge of 200 cc. of carbon tetrachloride. The absorbent material was placed in the flask to absorb the water and to simulate usual working conditions. The distillation was conduct,ed in the usual may until a maximum amountof water had been collected. The amountof water added less the amount of water collected equals the apparatus correction constant, expressed in grams.
Determination of Apparatus Correction Constant
Procedure for Determination of Moisture
The amount of water collected in the moisture tube is measured by reading the bottom of the upper meniscus and the top of the lower meniscus, This chosen nlanner of reading the tube does not include the water actually in the tube which is above the bottom of the upper meniscus and below the top of the lower meniscus. Furthermore, a small
Weigh a sample of suitable size into a 500-cc. round-bottomed flask. Add 200 CC. of carbon tetrachloride and connect the flask to a moisture tube and 30-cm. (12-inch) Allihn condenser, both of which are clean and dry. Heat the flask at such a temperature that the carbon tetrachloride falls from the tip of the condenser in a steady stream for a period of 3 hours or until the volume of water in the measuring tube reaches a constant maximum value. By means of a pipet, add carbon tetrachloride dropwise until a drop j q t falls through the water layer. Immediately read the position of the bottom of the upper meniscus and the top of the lower meniscus. The difference between these two readings, plus a constant correction of 0.11, is the amount of water present expressed in grams.
Calibration of Moisture Tube
WITH K~~~~ A~~~~~~ T~~~~~ 1. R~~~~~~OF D~~~~~~~~~~~~~ O F W S T E R PRESENT
Determination 1
2 3 4 5
6 7 8 9 10
11
Water Present
Water Found
Grams
Grams
Gram
0.50 0.50 1.00 1.00 1.03 2.00
0.51 0.49 0.95 0.97 0.98
$0.01 -0.01 -0.05 -0.03 -0.05 +O.Ol -0.04 -0.01 +0.03 -0.03 +O.Ol
2.00
3.00 3.00 4.00
4.00
2.01
1.96 2.99 3.03 3.97 4.01
Error
'
The results given in Table I were obtained with the moisture tube just described and serve to show the accuracy and precision of the method. It is believed that this method will find useful application in many analyses where the rapid determination of moisture by distillation with a solvent such as carbon tetrachloride is desired. RECEIVEDApril 28, 1936.
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