K4 can be expected to be greater than KZ or K3 and, because [F-] is of the order of loe5while KEF= 7.7 X lo2,Equation 28 can be simplified to
~-
- KI[A13+][H+] [F-1
dt
+ KHF[A13+l[F-I
X
which leads to
KEFKIK~ [Ala+][F-I2 (30) K4
It will be noted that Equation 30 is of the same form as the experimental rate equation. On comparing the two equaKmKiKa KHFKS * K3 tions, we note that KII = and KIII = -
K4 K4 ' Assuming that KI and K4 are essentially diffusion controlled, we can estimate K3 as 1.3 X l o 2from the experimentally determined value of KIIIand inserting this in the equation for KII, K5 for the ratio - which should be we get a value of 4.9
x
K4
equal to the first hydrolysis constant of aquo aluminum ion.
The value compares favorably with the literature value of 8 x at 25 "Cand p = 0. Finally, Brosset (6), Biedermann (7)) and Sillen (8) are of the view that the hydrolysis of aquo aluminum ion yields polymeric hydroxy complexes. On the other hand, the work of Frink and Peech (9) favors the existence of the simple A10H2+ species in solution. In the mechanism proposed here, AIOHz+is postulated only as a reaction intermediate and not necessarily as the final equilibrium product of the hydrolysis of aluminum ion. While the present method does not unambiguously distinguish among such mechanistic nuances, it should be apparent that the fluoride ion-selective electrode provides an effective and elegant means of studying the kinetics of even reasonably fast reactions involving changes in free fluoride concentration and should be useful for the continuous monitoring of fluoride activity in changing systems.
RECEIVED for review May 8, 1968. Accepted July 12, 1968. Investigation supported by grants from the National Science Foundation, National Institutes of Health, and the Office of Saline Water. (6) C . Brosset, Acfa. Chern. Scand., 6,910 (1952). (7) C. Brosset, G. Biedermann, and L. G. Sillen, ibid., 8, 1917 (1954). (8) L. G . Sillen, Quarr. Rev. (London),13,146 (1959). (9) C. R. Frink and Michael Peech, Znorg. Clzern., 2,473 (1963). ~~~~~
~
Correction Atomic Fluorescence Flame Spectrometric Detection of Palladium, Titanium, Zirconium, Chromium, and Aluminum Using a Hot Hollow Cathode In this article by J. I. Dinnin [ANAL.CHEM., 39,1491 (1967)l the fluorescence responses attributed to titanium, zirconium, and aluminum are in error. Scattering of helium light by undissociated particles of the refractory oxides was mistaken for fluorescence. Additional investigation of the behavior of the vapors of these elements stems from questions raised by T. S. West and his associates ( I ) who point out that no resonance lines of these elements exist at the wavelengths indicated (387C-3895 A) and that it is unlikely that free atoms of the metals would be produced in the relatively cool, oxidizing, hydrogen-air flame used in this study. Although scattering of the 3888.6 A line was considered in the original study and was used to explain the apparent fluorescence signal encountered in the 3900 A region for water, scattering was not adequately taken into account in evaluating the strong responses in the 3900 A region encountered for aluminum, titanium, and zirconium. A simple means for testing for scatter has been suggested by Walter Slavin (2). In questioning the fluorescence attributed to titanium and zirconium, he recommended that concentrated solutions of other refractory elements be substituted for the element under study, under the same conditions of excitation. If the response was caused by fluorescence, no significant signal should be excited in vapors of
( 1 ) T. S. West, R. M. Dagnall, D. N. Hingle, E. F. Kirkbright, and
K. C. Thompson, Imperial College of Science and Technology, University of London, personal communication,January 1968. (2) W. Slavin, The Perkin-Elmer Corp., personal communication,
December 1967.
other refractory elements; if the response was caused by scatter, a strong signal should be reflected from aspirated solutions of other refractory elements. The scattering test was applied at 3900 A, 2-mm slit, using the demountable hot hollow cathode lamp with helium atmosphere previously used for titanium, zirconium, and aluminum. The titanium lamp excited a signal of approximately the same magnitude in vapors of concentrated solutions of titanium, aluminum, and zirconium; aluminum and zirconium lamps excited similar signals from vapors of all three elements. The inescapable conclusion is that the signal observed in the 3900 A region is not caused by fluorescence but is the result of scattering of the 3888.6 A line of helium emitted by the hot hollow cathode. Additional proof is provided by the fact that no signal is observed at 3900 A with argon atmosphere in the hollow cathode. The sensitive fluorescence response reported for palladium in the 3450 A region is probably caused by the 3404.6 A line of palladium and is much more intense than that found in the 2500 A region. In correspondence ( I ) , the signal obtained at 2500 A where no resonance line of palladium is known to exist has been questioned. However, substitution of solutions of other refractory elements yields no scattering signal for either palladium line, thus indicating that the fluorescence may be real. The following corrections have also been noted ( I ) : In Table I, the wavelength noted for bismuth should be 3068 A; for cadmium, 2288 A; and for indium, 4102 A. I also extend my apology to Dr. K. C. Thompson for omitting his name from citations to reference ( 2 ) of the original article. VOL. 40, NO. 12, OCTOBER 1968
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