2434
Communications to the Editor
librium or near-equilibrium voltages. This fundamental belief has never, in fact, been adequately tested, in that the standard state potential difference is not included in calculations. This factor cannot be omitted since most cells are 30-40% nonaqueous by weight. In tissues where this factor is included, the necessity for electrogenic pumps has di~appeared.2-~ With the continuing demise of such pumps, the term “membrane potential” may attain a clearer meaning in the future. However, until the situation in electrophysiology is corrected, the designation of the voltages we measured as “membrane potentials” can only lead to greater confusion. References and Notes
0.4 o’6
(1) M. Goldsmith, D. Hor, and R. Damadian, J. Phys. Chem., 79, 342 (1975). (2) G. Ling, J. Gen. Physiol., 43, 149 (1960). (3) N. Joseph, M. Engel, and H. Catchpole, Nature (London). 191, 1175 (196 1). (4) N. Joseph, M. Engel, and H. Catchpole, Nature (London), 203, 931 (1964). (5) H. Catchpole, N. Joseph, and M. Engel, Fed. Proc., 25, 1124 (1966). Department of Medicine and Program in Biophysics State University of New York at Brooklyn Brooklyn, New York 11203 Received May 3, 1976
Michael Goldsmith
Ferrioxalate Actinometry. A Warning on Its Correct Use
Sir: While recalibrating the ferrioxalate actinometer,lS2 we found it to be erratic and irreproducible, a complaint we have subsequently heard from numerous photochemists. We have traced the problem to an apparently previously unrecognized slow photodegradation of the 1,lO-phenanthroline (phen) solutions which makes the development time dependent on the age of the phen solution and the order of addition of reagents; using published procedures errors can be >40%. We followed the procedures of Hatchard and Parker1 using reagent grade chemicals. Two bottles of phen (G. Frederick Smith Chemical Co.) and several preparations of potassium ferrioxalate and buffer1 were used. Our original data were taken with the phen solutions stored in thin-walled clear Pyrex bottles under fluorescent room lights. Typical development data for a uv photolysis of 0.006 M potassium ferrioxalate are given in Figure 1.The slow development of the phen first solutions and the dual terminal absorbances arose only with the aged phen solutions and were insensitive to the preparation or age of the other components. When aged phen was added first, longer delays before addition of the buffer produced lower initial absorbances and longer development times, Addition of Fez+ to premixed phen-buffer solutions yielded behavior like the buffer first results. The problems were common to all Fez+ source^.^ Addition of NH4F, recommended to accelerate color d e ~ e l o p m e n twas ,~ without effect on the development times or the presence of two final absorbances. Where two “terminal” absorbances resulted, the buffer first value was the correct one, although it took up to -5-6 h to develop.5 Phen solutions prepared within a few weeks of use, however, always yielded immediate The Journal of Physical Chemistry, Vol. 80, No. 21, 1976
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TIME, min
Figure 1. Development of longwave uv photolyzed 0.006 M ferrioxalate developed with: (A) fresh phen solution;(6)7-week old phen solution; (C) 8-month old phen solution. (-) Buffer added to ferrioxalate followed by phen. (- - - -) Phen added to ferrioxalate followed by buffer. In A the phen first curve is shifted upward by 0.01 for visualization. Absorbances cannot be compared for different samples of phen because of differing quantum doses. Each solution was diluted to 25 ml and contained 1 ml of irradiated ferrioxalate, 2 ml of 0.1 % phen solution, and 1 ml of acetate buffer. All measurements were in a 1-cm cell. Phen solutions were stored in thin-walled Pyrex bottles under fluorescent room lights.
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