Apparatus for Purification by Sublimation - Analytical Chemistry (ACS

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Apparatus for Purification by Sublimation. E. 31. Abrahamson, 60 Sutton Place South, X e n York 22, Ii. T. analytical procedures require spccial precautioiis in preparing the chemicals that go into the rwgents. Purification ran be accomplished by many methods, such as distillation and crystallization, but a great m:tny subPtances do not lend themselves to the ordinary proccsscs of purification. Their differential solubility in hot and cold solvents may not be great enough to make crj&allization practicable. Then, too, there is the difficulty of separat,ing the crjst.als from the mother liquor completely. Distillation is usually very effective, but some subst’ances of rather high melting point yield a solid distillate, which must then he melted out of the receivcr and ground to obtain pieces of convenient eize. ASY

CONDENSER

smaller tube fused into it to car current of air. This extends downward to about 2 inches from the bottom of the evaporator when the apparatus is assembled. The receiver rests in a suit,able vessel of water with a layer of glass beads or marbles to support it (not shown in the illustration). The condenser is a short-necked flask (Pyrex 4260) of 500-ml. capacity, fitted with inlet and outlet tubes for the coolant (tap water). The operation is simple. The evaporator containing the crude material is heated on the hot plate until va ors appear in its upper portion. Then a slow current of air is howm through the small side tube. This sweeps the vapors into the receiver, where they condense on the wall of the flask and drop to the bott,om of the receiver. The flask engages the flare of the receiver tightly enough to prevent. significant loss of the sublimate and loosely enough to permit the air to escape. After the rate of heating and the air current have been adjust’ed, the apparatus will operate with little or no attention. After sublimation is complete, the apparatus is dismantled and t.he sublimate can be easily removcd. When subliming substances of high boiling point, it may be necessary to lag the evaporator and side tube of the receiver with asbestos paper or cloth to prevent crystallization in the tube. The lagging is conveniently fixed to the side tube by clipping it in place p i t h a wire test tube holder. If, despite this precaution, the side tube clogs with crystals, these can be dislodged by alloning the flame of a Bunsen burner to play on it. The crystals ndl soon melt and drop back into the evaporator. The recently introduced electrical heating tape can also be wrapped around the side tube t o prevent crystallization there. This apparatus has hcen uscd to purify such diverse substances as iodine, 2-naphthol, naphthalene, and arsenic trioxide. The process is rather prctty to watch, the sublimate falling into the ieceiver in a miniaturc snowfall. Obviously, when toxic substances are suhlimcd, the process should be performed under a fume hood. The receiver was blown from borosilicate glass by E. .\Iachlett, R; Sons, New York, S . I*.

RECEIVER

Modified Karl Fischer Titration Cell. Donald E. Campbell, Walker Laboratory, Rensselaer Polytechnic Institute, Troy, N. Y. , ~ C L , O S E D titration

cell for use in electrometric Karl Fischer titrations has been described by Mitchell and Smith (Mitchell, J., and Smith, D. I f . , “ilquametry,” p. 80, ?;el\- York, Interscience Publishers, 1948). In the diagram is shown a modification of their design which has proved to be convenient in routine water determinations.

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The basic design of the cell is the same as the original, being constructed from a 250-ml. Erlenmeyer-type flask (a 250-ml. suction flask was found suitable) fitted by means of a 3-34/45 joint to an adapter containing the electrodes and the buret tips. More space in the adapter cap was made available by employing a magnetic stirrer rather than a motor-propeller stirrer, and by changing the location of the breather tube. It was found convenient to locate the breather tube, B , in the upper part of the titration flask, so that it could also be used as a port for introducing liquid samples by pipet. This eliminates the necessity of removing the titration vessel for the introduction of the sample and thus exposing the system to atmospheric moisture. After the sample is introduced, and during the titration, the system is protected from the moisture in the air by a drying tube, D , connected to the breather tube by a $10/30joint. Rather than sealing in a set of electrodes two 3-14/20 female joints were sealed into the cap, into which suitably adapted electrodes are fitted. This feature gives some flexibility to thesystem -for example, either polarized platinum or a platinum-tungsten couple can be employed. Furthermore, damaged electrodes can readily be replaced. ,J, Mitchell (privat.e communication) has suggested that because the platinum-platinum electrode system is the pair most

HOT P L A T E

Sublimation is an excellent method for securing small crystals of high purity. The usual practice involves heating the solid raw material under a cooled flask on whose outer surface the crystals collect. Unfortunately, the sublimate often does not adhere Tyell to the flask, but drops back into the melt. An apparatus was designed to permit the purification of substances by sublimation in reasonably large amounts.

It is assembled from standard laboratory glassware except for one part. It consists of an evaporator (Pyrex flask 5120) of 500-ml. capacity in which the crude material is placed. This joins, by a standard-taper joint, the receiver which is blown from a lipless tall beaker of 1-liter capacity (Pyrex 1040). This beaker has a side tube blown into i t as depicted. The side tube has a 203