A Spectrophotometric Study of the Complexes Formed between Uranyl and Thiosulfate Ions in Water and W a te r-Et h a noI So Ivents SHARON LYNN MELTON and EDWARD
S. AMlS
Chemistry Department, University o f Arkansas, Fayetteville, Ark.
b Using the method of continuous variations a t 25" C., the complex UO-,S203 was proved to exist in dilute aqueous solutions and in dilute solutions in 30-, 60-,and 90-volume per cent ethanol. The complex (UO,),(S,O,):' was found by the same method in more concentrated aqueous solutions. The equilibrium constants for the dissociation of UO&O3 in the various solvents were determined a t 25" C. and the free energy of the process was calculated. In water and 60 volume per cent ethanol the equilibrium constant for the dissociation of this complex has been determined at three or more temperatures and the thermodynamic functions AFo, AH', and ASo have been calculated. The limit of identification and the limit of dilution for the detection of uranyl ion by a thiosulfate spot test have been investigated, and some of the ions that interfere with the test listed.
I
the activation energy for the reaction between iodide and persulfate ions in the presence of different salt., Indelli and Amis (5) noted an accelerating effect of iiranyl ion u p m the reaction. These observers also noted that uranyl ion markedly catalyzed the rcaction between persnlfate and that the spectrum of U02+2 ion was strongly modified by the presence of thiosulfate. They attributed these effects to the formation of a complex b e h e e n uranyl and thiosulfate ions. N STUDYING
Toble 1.
0 0 80
60 60 60 60 60 60 60 60 90 00
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EXPERIMENTAL
The instruments and procedures used for determining the empirical formulas and equilibrium constants were those described by Hefley and Amis (d), and the uranyl perchlorate solutions n ere prepared and analyzed in the same manner as described by those authors. The NaC104 was hfallinckrodt Analytical Reagent which was dried and used n ithout further purification. The Raker Analytical Reagent grade S a n & 0 3 n as uwd n itliout further pirification but n as standardized using KI03. The compounds used in determining the cation and anion interferences Kith the color given by the complex were eithcr Baker or Fisher Analytical Reagents and nere used nithout further purification. Solutions of the abave compounds
I5 20 25 25 20 25 30 40 20 25 30 40 25 25
325-345 325-345 325345 320-450 320-350 320-350 320-350 320-350 420-430 420-430 420-430 420-430 350-370 430-465
Av. value of 3 . 3 6 =t0 . 0 8 X 5.14 =t0.36 x 9.06 f 1.52 x 5.17 i 0.29 X 1.07 i 0.05 X 1.45 ='c 0 . 0 7 X 2.00 f 0.00 x 3 . 9 9 f 0.13 X 1.19 f 0 . 0 5 X 1.83 f 0 . 0 4 x 3.10 ='c 0.10 X 9.04 f 0.00 X 3.91 i 0.51 X 5.54 i 1.40 X
ANALYTICAL CHEMISTRY
K
10-6
10-5 lo-'
AFo cnl./mole 3290 3070 2750 4480 6680 6600 6520 6300 6610 64GO 6250 5800 7380 8540
Table II. Equilibrium Constants for the Dissociation of UO&Os in 60 Voiurne Per Cent Ethanol at 25" C. as Function of Wavelength
x 106 1.34 1.51 1.51 1 42 Av. 1.45 + 0.075 420 1 .80 424 1.80 430 1.90 Av. 1 . 8 3 i 0.04; Precision numbers are average deviation from average. X
in
m p
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