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.
-
4-5
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
204
ANALYTICAL CHEMISTRY
frequently used, these two electrodes might be sealed into a single T14/20 joint. Thus, on1 one $14/20 female joint would have to be sealed into the alapter cap. ,Probably the most convenient modification is the stopcock, S,which is sealed in the base of the flask as shown. This enables the operator to drain off the excess liquid which accumulates in the flask after each titration without disassembling the apparatus. Exposure to atmospheric moisture is completely avoided, as the displacing air passes through the drying tube, D, as the liquid is drained out. Thus, in a series of routine titrations, the excess liquid can be removed without varying the end point, and the operator can pass from one sample to the next without going through the usual intermediate step of bringing the system back t o the end point with Karl Fischer reagent.
fi
14/20
The over-all modifications which were applied t o the original Mitchell and Smith design are the interchangeable electrodes, E, the relocation of the breather tube, B, fitted to take the drying tube, D, and the stopcock, S, in the base of the titration vessel. ACKNOWLEDGMENT
The writer gratefully acknowledges the suggestions and aid given by William Wilt, glassblower, and Anders Laurene, who prepared the drawing. This work was supported in part by Atomic Energy Commission Contract KO..4T( 30-1)-562.
Modified Poth Carbon Dioxide Generator. C. E. Childs and V. A. Moore, Parke, Davis & Co., Detroit, Mich. time ago Poth (3)described an excellent generator for proS ducing a continuous source of pure carbon dioxide. Since OME
then valuable improvements have been made by him and others ( 1 , 9, 4, 6-7). In our hands a simplified carbon dioxide generator has been evolved that has all the advantages of the original Poth generator but is much easier t o clean and less trouble to charge. The apparatus as pictured consists of a 3-liter flask, A , for the potassium bicarbonate solution connected by a ground-glass joint to a 2-liter flask, B, that contains the sulfuric acid solution. The carbon dioxide formed by the acid drotying into the bicarbonate solution is released through the bub e trap and gas delivery tube, C. The acid delivery tube, D, in the upper flask is constructed as a siphon beaker. The lower tip is about 1 mm. in diameter. The ring seal a t the bottom of flask B could be the weak joint in this generator, and should be watched for cracks. However, if it has been annealed properly no trouble should arise. Flask B is connected to the top piece, E, by a ground-glass joint and is held firmly in place with springs G-G. The mercury manostat, F , is attached by means of pressure tubing. The apparatus is mounted on a cork ring and ring stand and is supported by a clamp to E.
Assuming that the generator is empty and completely separated, it is charged as follows: A warm solution of potassium bicarbonate (900 grams in 2000 ml. of water) is introduced through a funnel into the lower flask, A , which is in an upright position. The upper flask, B, is fitted into the lower one with a liberal amount of heavy stopcock grease or Kronig cement. Next 500 ml. of the hot 1 to 1 sulfuric acid solution is added to flask B. The top iece is fitted into place with grease or cement, and the springs 8-G are connected. The complete system is evacuated a t this point with a filter pump (asprator) through side arm H with the stopcock a t C closed. A manometer should be included a t this point to permit measurement and control the pressure inside the generator. The vacuum is continued for about 15 minutes while the carbon dioxide and air are evolved from the bicarbonate solution. Meanwhile, a pinch clamp is placed on the tube leading from the filter pump to H . At the end of the 15-minute period the clamp is tightened, and the end of the tube to the pump is removed. Then the clamp is carefully released for short periods, allowing small amounts of air to flow into the upper flask which in turn allows the reaction to start. This is continued until atmospheric pressure is established in the system. The small amount of air drawn in will not affect the purity of carbon dioxide evolved from the lower flask. There should be some caution to avoid allowing too much air to go into the system, as a very high positive pressure could result in a looseningof the warm sealed joints. This charging cycle may be re eated several times to be sure t i a t the carbon dioxide in the lower flask is completely ure. Finally, t i e rubber tube is removed, manostat F is connected, and pinch clamp I is tightened. T h e manostat should maintain a pressure of about 1 pound (5 em. mercury) when in operation. The generator is placed in operation by a slight momentary vacuum applied throu h the gas delivery tube, C. T i i s will allow the sulfuric acid to be drawn over to start thereaction. When the generator is in use a Z-tube should be connected to the gas delivery tube with rubFigure 1 ber tubing allowing some flexibility at that point. An alternative method of charging could be that of either Pickard (3)or Poth ( 4 )which should prevent any air from getting into the system. This would be the preferred method if it is necessary that the generator stand for any prolonged period of time. In cleaning, the pressure is released through pinch clamp I , and the apparatus is simply taken apart and washed out. The whole operation of cleaning and recharging can be completed in 1 hour. This generator has been in continual use in this laboratory for about 2 years with micro-Dumas determinations. No air blank has been necessary, and even with everyday use the apparatus will last about 5 months. ACKNOWLEDGMENT
The authors are indebted to E. J. Poth and W. H. Rauscher for their criticisms and suggestions. LITERATURE CITED
(1) Lowe, E. W., and Guthmann, W. S., IND.ENG.CHEM.,ANAL.
ED.,4,440 (1932). ( 2 ) Pagel, H. A., Ihid., 16,
344 (1944).
(3) Pickard, P. L., Ibid., 21, 1015 (1949). (4) Poth, E. J., Ibid., 3, 202 (1931). ( 5 ) Ibid., 11, 518 (1939). ( 6 ) Rauscher. W. H., Ibid., 12, 694 (1940). (7) Shelberg, E. F., Ibid., 10, 704 (1938).
