VOLUME
2 8,
NO.
7,
JULY
1956
1179
0.04M in disodium phosphate, relative to that in pure water, was determined with the rotating electrode by comparing a solution 0.02M in citric acid and 0.04M in disodium phosphate, saturated with air, with a solution of 50% of 0.04 M citric acid and 0.08M disodium phosphate, saturated writh nitrogen, and 50% of pure water saturated with air. After subtracting the diffusion current for a solution saturated with nitrogen, the diffusion current in the first solution was 6% less than twice that in the latter solution. LITERATURE
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; catecholase units would be 20 times that shown for undiluted enzyme
CITED
(1) Baruah, P., Swain, T., Biochem. J. 55, 392 (1953). (2) Brackett, F. S., Olson, R. A., Crickard, R. G., J. Gen. Physiol. 36, 529 (1953). (3) Ingraham, L. L., Makower, B., Anal. Chem. 27, 916 (1955). (4) International Critical Tables, vol. Ill, p. 258, McGraw-Hill, New York, 1928. (5) Laitinen, . A., Kolthoff, I. M., J. Phys. Chem. 45, 1061 (1941); Science 92, 152 (1940). (6) Longmuir, I. S., Biochem.. J. 57, 81 (1954). (7) Marsh, G. A., Anal. Chem. 23, 1427 (1951). (8) Miller, W. H., Dawson, C. R., J. Am. Chem. Soc. 63, 3375
(1941).
Miller, W. H., Mallette, M. F., Roth, L. J., Dawson, C. R., Ibid., 66, 514 (1944). (10) Olson, R. A., Brackett, F. S., Crickard, R. G., J. Gen. Physiol. (9)
used to prepare potato enzyme by Baruah and Swain (1), except that 0.5% Pectinol (Rohm & Haas Co.) which contained no polyphenol oxidase activity was added to eliminate the pectin. The solubility of oxygen in the solution 0.02J1Í in citric acid and
32, 681 (1949). (11) Warshowsky, B., Schantz, E. J., Anal. Chem. 26, 1811 (1954). Received tor review October 8, 1955. Accepted March 29, 1956.
Preparation of Buffer Systems of Constant Ionic Strength PHILIP J. ELVING, JOSEPH M. MARKOWITZ, and ISADORE ROSENTHAL University of Michigan, Ann Arbor, Mich.
Directions are given for the preparation of Mcllvaine buffer systems of constant ionic strength, including a table of data. importance of using adequately buffered solutions for the types of chemical phenomena has long been recognized. The preparation of a large variety of buffer systems covering the usual range of pH is described in many reference and textbooks (1, 4~S). 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 polarographie half-wave potentials of certain types of organic compounds have been shown to be markedly dependent on the ionic strength of the test solution (£, 3). The diffusion currents are also affected, although to a much lesser degree, and the slope of the polarographie 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 ± 1. normal buff ering range of the system corresponding to Bates (I) 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 to keep the ionic strength constant. The usual directions for preparing these types of buffers 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 present, the effect might be serious in
THE study of many
Table I.
pH
Desired
at 2ó° C.
2.2 2.4 2.6 2.8
Preparation of Constant Ionic Strength Mcllvaine Buffered Solutions
Composition, G./Liter Solution NaiHPOt. HsCellsO:. 12H20 1.43 4.44
7.80 11.35
3.0 3.2 3.4 3.6 3.8
17.7 20.4
4.4
29.7 31.6 33.4
4.0 4.2 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
14.7 21.5
25.4 27.6
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
HzO
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
Buffer System
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 to Produce Ionic Strength of l.OAf 0.5 M
74.5 72.7 71.5 70.2 68.7 67.6 66.2 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
ANALYTICAL
1180
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 Mcllvainetype 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 an}-· 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 Mcllvaine buffers of constant ionic strengths of 0.5 and 1.0AÍ; 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
CHEMISTRY
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 (1)
(2) (3) (4)
(5)
(6) (7) (8)
CITED
Bates, R. G., “Electrometric pH Determinations,” Chap. 5, Wiley, New York, 1954. Elving, P. J., Komyathy, J. C., Van Atta, R. E., Tang, C. S., Rosenthal, I„ Anal. Chem. 23, 1218 (1951). Elving, P. J., Tang, C. S., J. Am. Chem. Soc. 74, 6109 (1952). Hodgman, C. D., ed., “Handbook of Chemistry and Physics,” 36th ed., pp. 1617, 1624, Chemical Rubber Publ., Cleveland, Ohio, 1954. Kolthoff, I. M., Laitinen, . A., "pH and Electro Titrations,” Chap. Ill, Wiley, New York, 1941. Kortüm, G., Bockris, J. O’M., “Textbook of Electrochemistry,” vol. II, pp. 737-44, Elsevier, Amsterdam, 1951. Lange, N. A., ed., “Handbook of Chemistry," pp. 938—40, Handbook Publ., Sandusky, Ohio, 1952. Lingane, J. J., “Electroanalytical Chemistry," pp. 54-6, Interscience, New York, 1953.
Received for review January 16, 1956.
Accepted March 6, 1956.
Techniques for Using Polytrifluorochloroethylene Plastic in the Chemistry Laboratory .
E.
RUNNER and GEORGE BALOG
Department of Chemistry, Illinois Institute of Technology, Chicago 16, III.
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 is presented. CASES where fabrication of laboratory apparatus with
INglass 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 trade-mark). 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 Vv-inch diameter or smaller, of approximately Vie-mch 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 (3). Care must be taken to avoid excessive heating
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 ($). [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. (3), provides a means of determining the ap-
