V O L U M E 28, NO. 7, J U L Y 1 9 5 6
1179
0.04X in disodium phosphate, relative to that in pure water, was determined x-ith the rotating electrode by comparing a s o h tion 0.02M in citric acid and 0.04X in disodium phosphate, saturated with air, yith a solution of 50% of 0.04M citric acid and 0.08M disodium phosphate, saturated with nitrogen, and 507, of pure water saturated with air. After subtracting the diffusion current for a solution saturated with nitrogen, the diffusion current in the first solution \vas 69& less than tn-ice that 111 the latter solution.
4 t
LITERATURE CITED
Enzyme
Figure 4. Rate of oxygen absorption measured by rotating platinum electrode at various amounts of polyphenol oxidase from prunes Enzyme amounts are milliliters of enzyme per 100 ml. of reaction mixture; catechulase units would be 20 times t h a t shown for undiluted enzyme
used to prepare potato enzyme by Baruah and Swain (I), except that 0.5% Pectin01 (Rohm gS Haas Co.) which contained no polyphenol oxidase activity was added to eliminate the pectin. The solubility of oxygen in the solution 0.02111 in citric acid and
(1) Baruah, I?., Swain, T., Biochem. J . 55, 392 (1953). (2) Brackett, F. S.,Olson, R. A., Crickard, R. G., J . Gen. P h ~ s i o l . 36, 529 (1953). (3) Ingraham, L. L., Makower, B., ANAL.CHEm 27, 916 (1955). (4) International Critical Tables, vol. 111, p. 258, RIcGraw-Hill, Kew York, 1928. ( 5 ) Laitinen, H. A, Kolthoff, I. hI., J . Phys. Chem. 45, 1061 (1941); Science 92, 152 (1940). ( 6 ) Longmuir, I. S., Biochem. J . 57, 81 (1954). (7) Marsh, G. A,, ANAL.CHEW23, 1427 (1951). (8) Miller, W. H., Damon, C. R., J . Am. Chem. soc. 63, 3375 (1941). (9) Miller, W. H., AIallette, I f . F., Roth, L. J., Dawson, C. R., Ibid.. 66. 514 (1944). (10) Olson, R. A.,Brackett, F. S., Crickard, R. G., J . Gen. Phvswl. 32, 681 (1949). (11) Warshowsky, B., Schantz, E. J., ANAL. CHEx 26, 1811 (1954).
RECEIVED for review October 8, 1955. Accepted March 29, 1Q56.
Preparation of Buffer Systems of Constant Ionic Strength PHILIP J. ELVING, JOSEPH M. MARKOWITZ, and ISADORE ROSENTHAL University o f Michigan, A n n Arbor, M i c h .
Directions are given for the preparation of McIlvaine buffer systems of constant ionic strength, including a table of data.
THE
importance of using adequately buffered solutions for the qtudy of many types of chemical phenomena has long been recognized. The preparation of a large variety of buffer systems rovering the usual range of pH is described in many reference and textbooks (1,4-8). For certain purposes it is necessary to maintain the ionic strength of the solution relatively constant while varying the pH by varying the composition of one buffer system, as well as by using different buffer systems. For example, the polarographic half-wave potentials of certain types of organic compounds have been shown to be markedly dependent on the ionic strength of the test solution ( 2 , 3). The diffusion currents are also affected, although to a much lesser degree, and the slope of the polarographic wave is in some cases sensitive to ionic strength. In the case of simple buffer systems such as those involving acetic acid-sodium acetate and ammonia-ammonium chloride, it is relatively simple to keep the ionic strength constant over the normal buffering range of the system corresponding to pK, f 1. Bates ( 1 ) has described the preparation of a number of monobasic weak acid and monoacid weak base buffer systems of a definite ionic strength. However, in the case of more complicated buffer systems, such as those involving citrate and phosphate, it is much more difficult t o keep the ionic strength constant, The usual directions for preparing these types of bufiers result in large variation of ionic strength over the normal buffering range. Because the ionic strength depends upon the square of the charges on the ions uresent. the effect miaht be serious in
Table I.
Preparation of Constant Ionic Strength RIcIlvaine Buffered Solutions
Composition, G /Liter PH Solution Desired NazHP04. HsCsH10i a t 2 j 0 C. 12H20 Hz0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5,6 5.8 6.0
6.2 6.4 6.6 6.8 7.0 7.2 7.4 7.6 7.8 8.0
1.43 4.44 7.80 11.35 14.7 17.7 20.4 21.5 25.4 27.6 29.7 31.6 33.4 35.3 36.9 38.4 40.0 41.5 43.3 45.2 47.5 49.6 52.1 55.4 58.9 62.3 65.0 67.2 68.6 69.6
20 6 19.7 18.7 17 7 16.7 15.8 15.0 14.2 13.6 12.9 12.3 11.7 11.2 10.7 10.2 9.75 9.29 8.72 8.32 7.74 7 12 6.47 5 72 4.79 3.70 2.74 1.91 1.35 0.893 0.589
$ :;:
~~~i~ Strength,
M 0 0108 0.0245 0.0410 0.0592 0.0771 0.0934 0.112 0.128 0.142 0 157 0.173 0.190 0.210 0.232 0.256 0,278 0,302 0.321 0.336 0.344 0.358 0,371 0.385 0.392 0.427 0.457 0.488 0.516 0.540 0.559
G. KC1 Added per Liter of Solution t o Produce IonicStrength of 1.OM 0.5.M 74.5 72.7 71.5 70.2 68.7 67.6 6G.2 64.9 64.0 62.8 61.7 60.4 58.9 57.2 55.5 53.8 52.1 50.6 49.5 48.9 47.9 46.9 45.8 44.5 42.7 40.4 38.2 36.0 34.3 32.9
37.2 35.4 34.2 32.9 31.4 30 3 28.9 27.6 26.7 25.5 24.4 23.1 21.6 19.9 18.2 16.5 14.8 13.3 12.2 11.6 10.6 9.62 8.50 7.23 5.44 3.10 0.488
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 a t 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 a t 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-
