313
C O M M U N I C A T I O N S T O THE E D I T O R
Ultrasonic Absorption in Manganese Sulfate Solutions1
Sir: Manganese sulfate solutions have been the subject of extensive ultrasonic studies. Kurtze and T a m 2 have reported a relaxation frequency of approximately 3 Me and indications of a second in the 200-Me region for aqueous MnS04 solutions. Eigen and Tamma have interpreted the relaxation spectra for 2-2 electrolytes in terms of multistate association processes of the form MZ+
+ A'-
between the low and high relaxation frequencies in either solvent system. The difference between the present results and those reported by Atkinson and Kor are particularly striking. Whereas previous worker^^*^*^ have reported for the low relaxation frequency approximately 3.5 Me, the composite of the present data together with those of Smithson and Litovitz indicates a more probable value of 5 Mc. The curve for the experimental data for aqueous 0.1 M MnS04 obtained in the present work can be constructed from two component relaxation curves with relaxation frequenciesof 4.9 and 160 Me or higher.
b
a
M2+OHzOHA2-
MZ+OH&Z-
C
MZtAZ-
where only the water molecules interposed between the aquated ion-pair members are shown. The 3- and 200Mc relaxation frequencies have been assigned to steps c and b, respectively. The computations of Eigen and T a r n 3indicate step a to have a standard volume change of approximately zero and hence a separate relaxation peak would not be anticipated for this step. This assignment of relaxation frequencies has been questioned by Atkinson and Kor4 on the basis of a third intermediate relaxation at 35 Me which they as well as Smithson and Litovitzs have reported for MnS04 solutions. In the work of the latter, a 35-Mc peak is just barely discernible in 0.05 to 0.2 M MnSOr in water or methanol-water solutions, but only at 25') and not in DzO. Atkinson and Kor, however, have observed a well-defined 35-Me peak in aqueous 0.01 to 0.1 M MnS04 and an extraordinarily sharp peak in dioxane-water mixed solvent systems-in fact, far sharper than is to be expected for a relaxation process. They assigned this peak to step b and the 200-Me relaxation peak to step a. The lack of reliable data for several of the rate constants from any other types of measurements has prevented a verification of this assignment. (See Figure 1.) This situation has prompted a reexamination of the ultrasonic absorptim in MnS04 solutions to establish the validity of the 35-Mc relaxation peak. Measure ments from 0.75 to 13 Mc have been made by the Carstensen method while a send-receive pulse technique has been used from 5 to 85 Me. The results of the present measurements in aqueous 0.1 M MnSOc and in 0.05 M MnSO, in 25% (w/w) dioxane-water mixed solvent provide strong evidence against the existence of a discernible relaxation peak intermediate
IO'
106
10s
Frequency, cyoles.
Figure 1. Excess relaxational absorption per wavelength v8. frequency at 25" in (a) 0.1 M MnS04 in water: X, Kurtze and T a m (20'); Smithson and Litovitz; A, A t b o n and Kor; e, this work; (b) 0.05 M MnSO4 in 25% (w/w) dioxane-water: V, Atkinson and Kor; 0, this work.
+,
The present results imply that the 35-Me relaxation peak is an artifact arising either because of equipment dif6culties or some chemical impurity. Ultrasonic absorption measurements are particularly susceptible to consistent errors. In any event, adequate evidence does not appear to exist at present for a third intermediate relaxation peak. (1) Research partially supported by the Office of Naval Research and the Harshaw Chemical Co. through a fellowship to L. G . J. (2) G . Kurtse and K. T a m , Awmtica, 3, 33 (1953). (3) M.Eigen and K. T a m , 2.Elelctrochem., 66, 107 (1962). (4) G. Atkinson and S. Kor, J. Phys. C h m . , 69, 128 (1966). (5) J. Smithson and T. Litovitz, S. Awust. 80c. Am., 28,462 (1966).
CONDENSED STATECENTERAND DEPARTMENT OF CHEMISTRY WESTERNRESERVEUNIVERSITY CLEVELAND, OHIO
LEROYG. JACKOPIN ERNESTYEAQER
RECEIVED SEPTEMBER 2,1965
Volume 70,Number 1 January 1966
