Notes on the Preparation of Hydrochloric and Hydrofluoric Acids by the Sub-Boiling Distillation Unit of Mattinson Sir: The sub-boiling distillation unit for volatile acids described by Mattinson ( 1 ) is impressive in simplicity and design. It is difficult to conceive a still simpler than two Teflon bottles connected a t a right angle by a bored and threaded T F E Teflon block. One bottle serves as the reservoir for the feed acid and the other as the receiver. The operation involves a nearly closed system. In one position, the receiver is continuously cleaned by acid under reflux; in a second position, distillation of the acid is accomplished. When the receiver is filled, it serves as the storage and dispensing container. Mattinson was interested in hydrofluoric and other volatile acids of low lead content. In this laboratory, in efforts toward the characterization of high-purity and special purity compounds, many trace metals are of interest. Encouraged by the simplicity of Mattinson's unit and the low level of lead achieved, similar units were prepared and studied. It was established that close attention to some practical details, notably the cleaning of the unit, is essential if acids of extreme purity are to be obtained, stored, and be applied successfully in trace analysis. It is the purpose of this paper to describe these details and to summarize some findings as to the quality of the acids that can be obtained.
EXPERIMENTAL The 32-02 narrow-mouth F E P Teflon bottles, available from Nalge Corp., used by Mattinson, and also initially by this laboratory, are apparently no longer available. Currently, bottles of the same capacity have a larger size screw-cap opening, and are supplied with unpigmented caps made of Tefzel. The former screw cap was colored blue, and was found in our laboratories to contain cobalt a t a concentration of 250 ppm and therefore was a potential source of cobalt contamination ( 2 ) .This information was communicated to the Nalge Corp. in 1971. The large screw-cap opening requires use of a larger Teflon block (2.5 in. wide, 2.5 in. high and 2.5 in. thick) with a larger bore than that described by Mattinson. This two-bottle still is made possible by Teflon. This plastic is superior to glass for storage of a general acid, and its chemical inertness permits the drastic cleaning measures necessary to remove trace contaminants. Teflon bottles received in this laboratory contain varying amounts of black particles imbedded in the plastic. Similar particles in polyethylene have been shown by Freeman (3, 4 ) t o contain such diverse elements as aluminum, copper, iron, magnesium, manganese, silicon, and zinc. Freeman traced the iron contamination of a redistilled hydrochloric acid to the Teflon F E P receiver and the iron present in such particles. Freeman suggested that removal of black particles from F E P Teflon bottles could be accomplished by boiling with nitric acid for 60 hours. Heating with aqua regia was later stated t o be faster ( 5 ) .In this laboratory, most of the black particles could not be removed by such treatment with aqua regia. Either they are too deeply imbedded or involve some material produced in the fabrication of the bottle that is resistant to oxidizing acid treatment. An all-Teflon still has been constructed by Kuehner et al. (6) using a commercially available quartz still as the model. This Teflon still was soaked in hot 8 N nitric acid for 24 hours; and then in 6 N hydrochloric acid for 24 hours, it was rinsed with high-purity water and finally with the acid to be distilled. In the following procedure, for the initial cleaning of Teflon bottles. caps, and connecting blocks, aqua regia is recommended. In ( 1 ) J. M. Mattinson, Anal. Chem., 44, 1715. ( 2 ) D. E. Robertson, in "Ultrapurity, Methods
and Techniques." M. Zief and R. Speights. Ed., Dekker, Inc., New York. N.Y., 1972, p 238. (3) D. H. Freeman, Ed., Nat. Bur. Stand. (U.S.) Tech Note, 459, 40 (1 968). (4) D. H. Freeman and W. L. Zieiinski, Ed., Nat. Bur. Stand. (U.S.) Tech Note, 509, 56 (1970). (5) D. H. Freeman, National Bureau of Standards, Washington, D.C., personal communication to M. Zief, J. T. Baker Chemical Co., 1973. (6) E. C. Kuehner, R. Alvarez, P. J. Paulsen, and T. J. Murphy, Anal. Chem., 44, 2050 (1972).
one set of experiments, a new shipment of four bottles, two were cleaned by 24-hour pickling in 6N hydrochloric acid and two using the more exhaustive aqua regia treatment. When these bottles were used concurrently for the distillation of hydrochloric acid, the distillates obtained did not differ significantly in impurity content. However, the bottles involved appeared to contain fewer black particles than those previously received. Preliminary Cleaning of Bottles, Caps, and Connecting Blocks. The purity of the product acid has been found to be highly dependent on thoroughness and persistence in the cleaning of the receiving bottle. Storage of the distilled acid in that container for a considerable period of time also requires this careful cleaning if the trace element content is not to be compromised. The following procedure for the initial cleaning of the Teflon bottles and the connecting blocks is recommended: First clean the bottle thoroughly with a detergent and water. Then leach both the inner and outer surfaces by complete immersion and heating with aqua regia (5:l hydrochloric and nitric acids, reagent grade) in the following manner. Use the bottom section of a 2-liter resin flask (or other suitable vessel) as the container for the aqua regia. Place the resin flask containing the Teflon bottle inside of a 4-liter beaker. Fill the bottle with aqua regia and then the resin flask and add distilled water to fill the space between the resin flask and the beaker. Cover the beaker with a 20-cm watch glass. Heat the beaker on a hot plate for 24 hours so that the temperature of the contents remains just below the boiling point of water. T o serve as a condenser, place a deep crystallizing dish filled with water on top of the watch glass and fill the space between that dish and the watch glass with water. After the 24-hour period, allow the beaker and the contents to cool and replace the spent aqua regia with fresh acid mixture. Repeat the heating for another 24 hours. After the acid treatment, rinse the bottle with distilled water, then immerse in distilled water, and heat just below the boiling point for 24 hours in a covered 4-liter beaker. Repeat this immersion and standing a t least 3 times, changing the distilled water each time. Our experience indicates that 4 such leachings are needed for the leachings to show a low residual chloride content (silver nitrate test). For some purposes, it may be unnecessary to remove the traces of chloride ( e . g . , with hydrochloric acid); however, our limited experience suggests that these efforts may also remove some trace metals present as soluble chlorides. Promptly after completion of the cleaning of a bottle, close it with a cleaned cap and store within a clean plastic container (see below). Clean the Teflon connecting blocks using the same sequence as for a bottle; however, the operations can be conducted for a number of blocks in a 4-liter beaker with sufficient acid added to cover them completely. Clean the Tefzel caps by prolonged soaking in 6Ai hydrochloric acid, rinse with water, and store in polyethylene bags. Storage of Acid-Filled Bottles. The Teflon bottles into which acid has been distilled can be protected by placement within a suitable cleaned container. For this purpose, non-pigmented polyethylene or polypropylene refrigerator pitchers used to store and dispense fruit juices are excellent. Each pitcher should be cleaned with a detergent and then be soaked in 6N hydrochloric acid for a period of several days and be rinsed with distilled water. The identification label is placed on the outside of the pitcher, rather than on the Teflon bottle. The use of this outer container has the added advantage t h a t it prevents the escape of acid fumes from the Tefzel screw cap. Operation of the Distillation Unit. The two-bottle still is operated as directed by Mattinson ( I ) , using a 300-watt infrared lamp. In our hands, only 45 ml of acid is collected over a period of 24 hours. The reason for such a low yield is that no effort is made to maintain maximum cooling and the distillation is deliberately slow to produce the purest product. T h e distilled water cooling batch is iced only a t the beginning and the end of the normal day and allowed to run overnight unattended. No attempt is made to add or change coolant on weekends or holidays. No other cooling alternatives have been explored as yet. The distillation requires 14 days to produce a volume of 600 ml of product acid. In a conventional fume hood, two distillation units can be accommodated simultaneously.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 9, AUGUST 1974
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T a b l e I. Hydrochloric Acid (6 normal basis), I m p u r i t y C o n t e n t in p p b b y w t (ng/g)%* Batch No. 5 HC1
Batch No. 3 HC1 Element
Feed
Product
A1
35
Ba Ca Cr co cu Fe
