1180
ANALYTICAL CHEMISTRY
going from a monovalent anion to a divalent anion over a pH range of 1 to 1.5 units. In the attempt to minimize such changes in ionic strength, tables of buffer composition for the commonly used McIlvainetype phosphate-citrate buffer system have been developed, which are helpful in the preparation of buffer solutions of uniform ionic strength throughout the normal buffering region of this system. Potassium chloride is added to the buffer compositions described in the literature, so as to keep the ionic strength constant at any desired level-e.g., 0.5M. These tables have been used in the authors’ laboratories for several years in connection with studies of the polarographic behavior of organic compounds, and should be useful in other areas-e.g., investigation of reaction kinetics and spectrophotometric determination of pK values-in which ionic strength is a pertinent variable. The essential data are given in Table I for McIlvaine buffers of constant ionic strengths of 0.5 and l.0M; the amount of potassium chloride to be added for other ionic strength levels can be readily calculated on the basis of the ionic strength of the buffer system itself. Obviously, equivalent weights of other 1 to 1 electrolytes such as lithium chloride could be substituted for the weights of potassium chloride specified. The specific ionic strength to which the buffer is brought will affect the actual pH of the solution to a slight extent. For this reason, the data in Table I are given only to the nearest 0.1 pH
unit. The pH of the buffer solution as well as that of the final test solution should always be checked with a suitable pH meter. ACKNOWLEDGMENT
The authors wish to thank the U.S.Atomic Energy Commission, which helped support the work described. LITERATURE CITED (1) Bates, R. G., “Electrometric pH Determinations,” Chap. 5, Wiley, New York, 1954. (2) Elving, P. J., Komyathy, J. C., Van Atta, R. E., Tang, C. S.. Rosenthal, I., ANAL.CHEX23, 1218 (1951). (3) Elving, P. J., Tang, C. S., J. Am. Chem. SOC.74, 6109 (1952). (4)
(5) (6) (7) (8)
Hodgman, C. D., ed., “Handbook of Chemistry and Physics,” 36th ed., pp. 1617, 1624, Chemical Rubber Publ., Cleveland, Ohio, 1954. Kolthoff, I. XI., Laitinen, H. A., “pH and Electro Titrations,” Chap. 111, Wiley, New York, 1941. Kortum, G., Bockris, J. O’M., “Textbook of Electrochemistry,” vol. 11, pp. 737-44, Elsevier, Amsterdam, 1951. Lange, N. A , , ed., “Handbook of Chemistry,” pp. 9 3 8 4 0 , Handbook Publ., Sandusky, Ohio, 1952. Lingane, J. J., “Electroanalytical Chemistry,” pp. 54-6, Interscience, Kew York, 1953.
RECEIVED for review January 16, 1956. Accepted March 6, 1956.
Techniques for Using Polytrifluorochloroethylene Plastic in the Chemistry laboratory M. E. RUNNER and GEORGE BALOG Department of Chemistry, lllinois Institute o f Technology, Chicago 76, 111.
Apparatus of Fluorothene or Kel-F plastic, a polymer of trifluorochloroethylene, is very useful to the chemist in many cases where glass apparatus is unsatisfactory. A brief description of the useful properties of this plastic is given, along with some techniques of fabrication. A simplified technique for molding vessels from tubing ia presented.
I
N CASES There fabrication of laboratory apparatus with
glass is undesirable because of special problems of flexibility, fragility, corrosion, surface activity, and thermal or electrical insulation, polytrifluorochloroethylene plastic may be used. This material is known by the trade names Fluorothene (Bakelite Co. registered trade-mark) and Kel-F (M. W. Kellogg Co. registered trademark). Often it is desirable to use Fluorothene (used in this text for all further reference to polytrifluorochloroethylene plastic) plastic instead of metals where high temperatures will not be used and transparency is important. Fluorothene plastic cannot be fabricated into useful laboratory equipment by ordinary means; however, various techniques successfully applied by the authors are set forth here. PROPERTIES OF FLUOROTHENE
One of the most important properties of Fluorothene is its chemical inertness. As a polymer of monochlorotrifluoroethylene, its inertness is similar to that of Teflon, the completely fluorinated polymer. No effect has been observed after prolonged exposure to concentrated sulfuric, hydrofluoric, and hydrochloric acids, strong caustic, fuming nitric acid, aqua regia, and other vigorous oxidizing materials. Fluorothene is equally
resistant to most organic solvents, but is slightly swelled and plasticized by highly halogenated materials and some aromatics (S). Other useful properties are high electrical resistance, thermal insulation, and stability. Dimensional stability is maintained over a temperature range from -200’ to 190” C. Vessels of a/d-inch diameter or smaller, of approximately l/la-inch wall thickness, will withstand a high vacuum at 90’ C. without collapsing. Fluorothene has much greater resistance to cold flow than Teflon. Although Fluorothene may deform slightly under applied pressure, it returns to its original shape when the pressure is released. It is relatively hard, having a Rockwell hardness of 111-115 (R- scale), and it can be machined into almost any desired form (S). Care must be taken to avoid excessive heating
B
Figure 1.
Fluorothene fittings for tubing connections
during machining, Fluorothene, when cooled slowly from high temperature, has a rather cloudy appearance, but attains transparency if quenched rapidly from 213” C. The material will decompose slightly above 270” to 300” C., depending upon its ZST value ( 2 ) . [Zero strength time (ZST) is the time in seconds required to break a standard notched strip of heated polymer weighted with a small static load. This test, developed by the M. W. Kellogg Co. ( 2 ) , provides a means of determining the ap-
V O L U M E 28, N O . 7, J U L Y 1 9 5 6
1181
parent molecular weight of trifluorochloroethylene polymer and a basis for the grade designation of No. 270 and No. 300 for unplasticized material. ] Fluorothene decomposes slowly a t 290" C. At this temperature possibly 15 to 30 minutes are required for serious decomposition; as the temperature is raised the rate of decomposition increasesthat is, a t 320' C. extensive decomposition may require only a minute or less. Teflon does not decompose until heated above 450" C. (6) and does not soften or melt below this temperature. The decomposition of Teflon proceeds at first with little visible change, there being no melting or darkening. Under vacuum a gas evolution may be observed a t 389" C. ( 4 ) ,but when Teflon is heated in the air a temperature of 450" C. mav be reached before decomDosition Figure 2. Fhorocan be detected by odor of the vapor or texture of the surface. The deing ring and metal compression nut composition products are harmful and prerautionary measures should be used. Fluorothene plastic is exceptionally resistant to wetting by water. It is unaffected by high humidity, and water vapor transmission through unplasticized film above 0.002 inch in thickness could not be detected ( 3 ) . Mercury wets the surface of Fluorothene much less than it nets glass. The surface of Teflon is more waxlike and its use as a packing gland, valve-stem guide, or valve seat permits ease of rotation when the rotating member is made of Fluorothene. The relative softness of Teflon makes it an excellent gasketing material in conjunction with Fluorothene or metallic surfaces.
$
~
~
FABRICATION
Fluorothene is available from the manufacturer in the form of extruded rod and tubing and molded disks and sheets (supplied by Flax Corp., Hartford, Conn., which recently sold its Fluorothene processing equipment to Westlake Plastics Co., Lenni Mills, Pa.). Tubing Connections. The most convenient method of joining lines to vessels is by use of flared fittings similar to those used for copper tubing in refrigeration lines. Various types of machined Fluorothene fittings are shown in Figure 1, A .
One-quarter-inch tubing was flared, using a conventional flaring tool. The tool was heated to approximately 150" C. and after the flare was formed the tool was not released until it had cooled to room temperature. When brass flare nuts are tightened on Fluorothene threads, care must be used to avoid stripping the threads. Glass tubing may be connected to plastic by means of a tapered Teflon packing ring sealed around the end of the glass tubing by a compression nut, as in Figure 1, B. A previous application of this principle employs a Teflon packing ring to connect glass tubing to metal (1). Valves. The use of Fluorothene combined with Teflon in the construction of needle valves resulted in easy-turning, leakproof, and corrosion-free characteristics. A diagram of such a valve is shown in Figure 2. Metal parts were used to give rigidity, but in no case was metal permitted to come into contact with the material transported. Such valves are useful where greaseless stopcocks are desirable, in handling of organic solvents, and on vacuum lines for anhydrous hydrogen fluoride and other corrosive liquids. Vessels. Vessels up to 1 inch in diameter were machined from extruded rod. Caps from solid stock of larger diameter were machined and threaded to the open end of the vessel. During the machining of Fluorothene plastic it was found that a slow feed and sharp tool provided the smoothest surface. At high speeds the plastic appeared to soften at the point of contact with the tool and a clean cut was not obtained. The slowest speed used to obtain a smooth surface was 50 r.p.m. and a feed of 0.185 inch per minute. In order to ensure a leak-tight joint a compression seal was made rather than sealing through the threaded joint itself as in conventional plumbing practice with iron pipe. Examples of compression seals are shown in Figure 3, A , B , and C. Lengths of tubing can likewise be sealed at both ends to construct leaktight vessels. When the vessel is to be immersed in liquid nitrogen, thermal stresses may cause leakage at the threaded joints. This condition is much more serious when one of the joined members is made of metal. Fluorothene plastic will withstand liquid nitrogen temperatures without cracking and rapid warming does not appear to be harmful. Polyethylene, on the other hand, is entirely unsatisfactory in this respect.
~
~
'
~
~
'
~
~
STEEL PISTON
MG T I P
'-HEATER
A
Figure 4.
7
C
Figure 3. Fluorothene vessels constructed with compression seals
B
Plastic mold assembly
F'essels without threaded seals were constructed from tubing by a simplified molding technique. The open end of a tube could be closed by forcing it into a heated mold constructed as shown in Figure 4. By utilizing Teflon gaskets in the manner shown, it was possible to confine the heat to the very end of the plastic tube. In this way, pressure could be exerted on the upper cool end of the tube to force the tube into the mold body. Figure 4, A , shows the mold body for forming a flat bottom and Figure 4, B , the mold used for flanging the top of the vessel. The flsnging operation should be done first, and best results are obtained with tubing to 2 inches in diameter.
1182
ANALYTICAL CHEMISTRY
I n operation, an appropriate length of tubing was placed in the flanging mold (Figure 4, B). The magnesium (or aluminum) cap was slowly heated with a microburner until a temperature of 260' =t5" C. was reached. (The plastic used corresponded to M. W. Kellogg Co.'s grade 270. For grade 300 a maximum temperature of 290" C. is desirable.) During the period from 225' to 260' C. pressure was maintained upon the tube by turnin the stud at the top of the yoke (Figure 4, -4). The end of the t u t e in contact with the heated cap was thus made to flow into the flanging mold. The 260" temperature was maintained for approximately a half hour, during which time the pressure was kept a t a maximum. To prevent the upper m-all of the tube from softening and collapsing, powdered dry ice was placed around the tube in the lower part of the yoke. The excellent insulating properties of the Teflon gaskets permitted the maintenance of a temperature gradient of about 200" between the heated cap and the upper part of the mold body. Finally, the entire assembly m s rapidly quenched in ice water. The mold (Figure 4, A ) for forming the bottom of the vessel was used next. This was handled in much the same way as the flanging mold. however, it was necessary to employ an electrically heated Lip on the steel piston. The heater consisted of a 0.5-ohm Nichrome wire embedded in Sauereisen cement and it was operated at 4 volts. When set up initially, the end of the plastic tube extended far enough below the piston tip to provide sufficient material for completely filling the mold when the position s h o m in the diagram had been reached. I t was very important to clean the mold thoroughly after use and also to maintain the temperature below 270" C. Discoloration and decomposition resulted from excessive temperatures. (The darkening of Fluorothene is usually due to contamination by other organic materials or from plastic sources other than virgin material. The authors have felt that there may be an effect due to the magnesium surfaces of their mold.) I t was equally important to keep the final temperature above 250' C. to prevent ridging and cracking of the plastic during its flow through the mold. Vessels so constructed were capped by a Fluorothene (or hlonel) disk, which was held tightly in place by a slightly modified pipe union fitting (see Figure 3, D). The vessels showed no sign of cracks, leaks, or deterioration after repeated immersions in liquid air under pressures in the range 0 to 2 atm. Some have been in continuous use for 3 years. Capillaries. Capillaries of fine bore could not be prepared by drilling Fluorothene rod. However, it was possible to prepare capillaries of less than 0.1 mm. bore by the following method.
A 0,018-inch hole was drilled in a a/s-inch rod to a depth of 1 inch. The rod was then rotated slowly above a niicroflame until the plastic became transparent and pliable. A capillary was then drawn out in the same manner as with glass tubing. The plastic must be drawn very slowly and the center portion of smallest diameter should be permitted partially to harden before the drawing is completed. Otherwise, the softened rod may be broken before it has been stretched sufficiently. Small biilhs may be bloivn by first drilling and softening~. the plastic rod as for capillary preparation. Sealing. Electrode Leads. Electrode leads of datinum wire or other metal may be sealed into Fluorothene b> first drilling a hole in the plastic a few thousandths of an inch undersize. By welding a wire of smaller diameter to the end of the electrode lead, one has a means of pulling the larger diameter wire into the plastic. The smaller diameter wire is slipped through the hole in the plastic, and by carefully heating the lead wire while pulling the mire is forced into the hole as the plastic softens. A wire above 0.050 inch in diameter may be successfully sealed into Fluorothene by employing a tapered Teflon packing ring, which is squeezed around the wire by a packing nut in a manner similar to the seal around the glass tubing as shown in Figure 1, B. Test tubes and beakers of Fluorothene are available comniercially for laboratory use. Furthermore, there is on the market a Kel-F grease which is a lower molecular weight polymer of trifluorochloroethylene. This can be used for lubrication of stopcocks and other surfaces exposed to corrosive reagents. ACKNOWLEDGMENT
The authors wish to acknowledge the financial support of this work by the U. S. Atomic Energy Commission under Contract KO,AT(l1-1)-90, Project No. 3, Chicago Operations Office. LITERATURE CITED (1) Brown, R. A., Skahan, D. J.. ANAL.CHEM.26, 788 (1954). (2) Kaufman, H. S., Kroncke, C. O., Jr., Giannotta, C. R., Modern Platies 31, 234 (October 1954). (3) Kellogg Co., M. W., Jersey City 3, N. J., Tech. Bull. 1-1-55. (4) Rlonk, J. W., "Outgassing of Naterials in a Vacuum," Atomic Energy Commission MDDC-1307. ( 5 ) Schildknecht, C. E., "Vinyl and Related Polymers," p. 487, Wiley, New York, 1952. RECEIVED for review November 18, 1955. Accepted February 29, 1956. Abstracted in part from the M . S. thesis of George Balog, Illinois Institute of Technology, February, 1954.
Simple Indicator Method for Determination of Aluminum R. V. PAULSON
and
J.
F. MURPHY'
Kaiser Aluminum and Chemical Corp., Spokane 69, Wash.
A simple volumetric method was needed for control of aluminum concentration in certain solutions used in finishing aluminum. Such a method was developed, which is applicable in the presence of fluoride and affords accuracy and precision for dilute solutions.
T
HE method for aluminum developed by Bushey ( 2 ) involves the titration of an alkaline solution (Region A , Figure 1) to the point a t which the free hydroxyl is neutralized (Region B, Figure l), using pH measurements to determine the end point. Acid is then added through the region of precipitation of aluminum hydroxide to the point at which the precipitate is just redissolved (Region C, Figure 1). Potassium fluoride is added to precipitate cryolite in the acid solution, and the excess acid is titrated with standard base using phenolphthalein to the appearance of a pink color which remains for 15 seconds. The volume 1
Present address, General Electric Co., Schenectady 5 , N. Y.
of hydrochloric used in the determination corresponds to the distance from B to D in Figure 1. Because this determination is not affected by the presence of fluorides, an attempt was made to simplify it without undue loss in accuracy or precision. Preliminary experimental work with mixed indicators led to the conclusion that a determination based on the titration of an alkaline aluminate solution with standard acid to an end point in the pH range above 9 was feasible. The mixed indicator described by Kolthoff and Rosenblum (S), which has two color changes in the chosen pH range, was tested. The first method attempted involved the use of two samples. Potassium fluoride solution was added to one sample to precipitate aluminum as cryolite. Both samples were then titrated to the green end point of the mixed indicator at pH 9.80. HOWever, it was found that better precision and greater sensitivity were obtainable by using a single sample, titrating to the green color (pH 9.80), adding potassium fluoride solution a t this point, and then titrating the hydroxide liberated from the aluminate ion by the fluoride to a yellow color at pH 9.35. The volume of
