CORRESPONDENCE
Tris(hydroxymethy1)aminomethane-A Useful Secondary pH Standard SIR: Buffer solutions composed of tris(hydroxymethy1)aminomethane (“tris,” “THAM”) and its hydrochloride have been shown to be useful as secondary pH standards and for the control of acidity in the pH range 7 to 9, important for studies of physiological media (I), sea water ( 2 ) , and the like. In a recent publication, Durst and Staples (3) report the assignment of conventional ~ U values H to buffer solutions composed of tris and its hydrochloride in a molar ratio of 1:3. This buffer ratio was chosen in order that the pH of the solutions would match closely that of human blood. Also reported were similar data for tris buffers brought to a total ionic strength of 0.16 (close to that of isotonic saline) by the addition of sodium chloride. There is likewise a need for a secondary standard composed of tris and its hydrochloride in the more conventional concentration range. It is the purpose of this communication to report pa^ values from 0 to 50 “C for the equimolal buffer composed of tris and tris hydrochloride, each at a molality of 0.05 mol kg-l. The results given in Table I were calculated from emf data already published ( 4 ) . The convention for the single ionic activity coefficient (9,also utilized by Durst and Staples, should be applied only at ionic strengths of 0.1 or below. An uncertainty of 10.005 unit is estimated. Inasmuch as the original emf data were for solutions in which the buffer ratio was not exactly 1, it was necessary to subtract a small correction (0.007 unit) from each “observed” value of P U R . This is the value of log (MT/MTH*), where T represents the tris base, in the buffer solutions for which emf data were obtained ( 4 ) . The ~ U values H are given as a function of temperature ( t in “C)by the equation (1) G. Gomori, Proc. SOC.Exptl. Biol. Med., 62. 33 (1946). (2) W. H. Smith, Jr., and D. W. Hood, in “Recent Researches in the Fields of Hydrosphere, Atmosphere, and Nuclear Geochemistry,’’Maruzen Co. Ltd., Tokyo, Japan, 1964. (3) R. A. Durst and B. R. Staples, Clin. Chem., 18,206(1972). (4) R. G. Bates and H. B. Hetzer, J.Pliys. Cliem., 65,667 (1961). ( 5 ) R. G. Bates and E. A. Guggenheim, Pure Appi. Cliem., 1, 163 ( 1960).
Table I. Conventional ~ U HValues of the Buffer Solution Composed of Tris and Tris Hydrochloride, Each at a Molality of 0.05 Mol Kg-1, from 0 to 50 “C t , “C 0 5 10 15 20 25
PaH 8.946 8.774 8.614 8.461 8.313 8.173
t , “C 30 35 37 40 45 50
PaH
8.036 7.904 7 . 851a 7.777 7.654 7,537
Calculated by the equation relating paH with f .
PUH =
8.1715 - 0.02804(t - 25)
+ 0.00011(t - 25)’
(1)
with an average deviation of 0.002 at the 11 temperatures. The temperature coefficient is large enough to make temperature control important if the highest precision is to be obtained. This standard buffer solution is prepared by weighing (in air) 5.970 grams of tris and 7.768 grams of tris hydrochloride, dissolving the buffer substances in carbon dioxidefree water, and diluting to a total volume of 1 dm3 at 25 “C. There is some evidence (6) that linen-fiber junctions may be responsible for abnormal liquid-junction potentials in tris buffers. Hence, this type of junction should be avoided. ROGERG . BATES R. A. ROBINSON Department of Chemistry University of Florida Gainesville, Fla. 32601 RECEIVED for review July 28, 1972. Accepted October 20, 1972. (6) C. C . Westcott and T. Johns, Appl. Res. Tech. Rept. 542, Beck-
man Instruments, Inc., Fullerton, Calif.
On the Problem of Nonlinear Diffusion at Heterogenous Electrodes SIR: In a recent paper by Rolando Guidelli and Giovanni Piccardi ( I ) , reference is made to evidence for nonlinear diffusion phenomena occurring in the electrochemical system investigated. The data are interpreted by means of a formula which Smythe deduced for the electrical conduction through a single metal cylinder having at one end a coaxial circular disk electrode, the radius of which is less than the radius of the metal cylinder ( 2 ) .
Rolando Guidelli and Giovanni Piccardi, ANAL.CHEM..43, 1639 (1971). (2) W. R. Smythe, J . Appl. Phys., 24, 70 (1953).
(1)
420
We would like to point out that the possibility of applying Smythe’s approximations to the problem of nonlinear diffusion to a heterogeneous electrode occurred to us several years ago (3). Qualitative agreement with Smythe’s equations was found for the rotating disk electrode and for chronopotentiometric experiments. Mixed graphite electrodes were used in these experiments (3). Any real heterogeneous electrode corresponds to an irregular arrangement of a large number of active patches of various sizes and shapes. Before applying (3) R. Landsberg and R. Thiele, Electrochim. Acta, 11, 1243 (1966).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 2, FEBRUARY 1973
