ferent methods in the very interesting paper of Kohn and Furda (4) would affect as follows the results of Ref 1 : a) In Figure 6 the curves at low concentrations would be almost unaffected. At higher concentrations the deviation from linearity would be even more pronounced than drawn, and the slopes would indeed exceed the Nernstian value. + ~ This may possibly indicate that the calculated Y ~ is ~incorrect at high ionic strengths, and that y* agrees better with experimental results. b) In Figures 9-11 the theoretical curves would approach even closer to the experimental results, and thus emphasize the point made that the Orion and the solid polymer membranes behave indeed as perfectly ion-specific membranes in the concentrations of biological interest, while the paraffin membranes appear to be completely nonspecific. The question of yca* has been discussed in more detail in a separate paper (14). Finally, I agree with Dr. Frant that generally it is preferable to use a specific electrode instead of an inert (calomel) electrode. However, Reference 1 dealt with the behavior of CaCL in a large excess of other chlorides whose concentration varied by orders of magnitude. Thus the emf obtained by a chloride-specific electrode might contribute excessively to the total potential obtained, thus possibly masking the behavior of the calcium-specificelectrode.
ADAM SHATKAY Isotope Department The Weizmann Institute of Science Rehovoth, Israel
SIR:I feel that Dr. Shatkay’s reply has made explicit a point that concerned me in writing my earlier letter-that the unusual choice of mean ionic activity for evaluating single ion activity electrodes was made primarily because it gave the best fit to his electrode, and not because the two are directly proportional. He may be correct in his belief that mean ionic activities are the more reliable, but he did not do the crucial experiment I described earlier, and which could have been done easily in preparing the data for Figure 6. I am quite willing to leave the experimental resolution of this question to others, although my own experience has been that one gets better than 98 of the theoretical slope with the Ross (15) electrode using either Debye-Huckel values or mean ionic activities in a cell without liquid junction, and only 9 2 x using Dr. Shatkay’s techniques for plotting the data. At the time
this is written, the experimental details for the preparation of Dr. Shatkay’s electrode were still unpublished [Ref. 13 in (2)] or unobtainable. Our correspondence emphasizes the need for a generally accepted pCa scale. This has been done for pH, despite the problems of making absolute single ion activity measurements, to which Dr. Shatkay refers. The availability of electrodes which respond to calcium activity should make a universal pCa scale experimentally possible. A single yardstick for evaluating new electrodes is obviously a necessity as the literature increases. MARTINS. FRANT Orion Research, Inc. Cambridge, Mass. 02139
SIR: In Dr. Frant’s last communication there is a notesuch as the references to “the unusual choice of mean ionic activity” and to “my” electrode-which appears to me subjective and unfortunate. The solid polymer membrane is not mine, and the use of mean ionic activity is common, and was employed also by Tendeloo, whose results I examined in the paper under discussion. I shall deal, however, only with the more pertinent points: In my note to Dr. Frant’s remarks I did not say that “mean ionic activities are more reliable.” On the contrary, I said that they are merely more convenient “instead of the possibly more accurate” single ion activities. As to the use of the cell without liquid junction, I should perhaps emphasize what has already been stated in the experimental part of my paper (1): while the solid membrane electrodes were measured with two calomel electrodes (for the reasons given in my note), the Orion electrode was measured with an arrangement of Ag/AgCl electrode in the standard inner solution, and a calomel electrode in the mixed outer solutionas advised by the manufacturers and as measured by Ross in the paper cited by Dr. Frant (15). Incidentally, Ross refers there to the difficulties discussed by us: “A rigorous evaluation of electrode selectivity in solutions of practical interest is difficult” writes Ross, “owing to uncertainties in the reference electrode liquid junction potential, and the questionable ability of the Debye-Huckel equations to predict ionic activity coefficients in mixed ion solutions at high ionic strength.” ADAMSHATKAY
(14) A. Shatkay, “Activity Coefficients of Calcium Ions in Mixed Solutions,” J. Phys. Chem., 71, 3858 (1967). (15) J. W. Ross, Science, 156, 1378 (1967).
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ANALYTICAL CHEMISTRY
The Weizmann Institute of Science Rehovoth, Israel
