Distillation and rectification of osmium tetraoxide solution in carbon tetrachloride
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with a 5 constant is usually correct for sufficiently dilute solutions (,5 pct). Differential distillation used to obtain several fractions of distillate allows investigation of the dependence of relative volatility on the composition of liquid in the still. Determination of a is based on the equation of material balance (Rayleigh equation[3]):
Distillation and Rectification of Osmium Tetraoxide Solution in Carbon Tetrachloride
x
A.B. MAIBORODA, I.D. TROSHKINA, and A.M. CHEKMAREV
W ln 5 W0
The solvent extraction of osmium tetraoxide with carbon tetrachloride has found application in osmium determination.[1] This process can be used for industrial processing of platinum metal concentrates in order to selectively remove osmium and ruthenium.[2] Water solutions frequently contain microconcentrations of osmium. Solvent extraction of osmium from such solutions results in the formation of very dilute solutions of OsO4 in CCl4. Our research has shown that distillation can be used for the concentration of osmium in such extracts. The large difference between the boiling temperature of components (tOsO4 5 131 7C and tCCl4 5 76.4 7C) is a condition for effective separation of this mixture by means of distillation. A distribution of osmium between liquid and vapor has been investigated using the method of differential distillation.[3] The chart of the distillation apparatus is shown in Figure 1. The still was placed into an air thermostat, where temperature was maintained at 5 to 10 degrees above the boiling temperature of solution. On the one hand, this keeps out vapor condensation on the walls of the apparatus. On the other hand, it maintains a sufficiently small rate of distillation, which is a condition of thermodynamic equilibrium state of all volume of liquid with vapor. Solutions for distillation were obtained by extraction of OsO4 with CCl4 from 3 M H2SO4. Fractions of distillate (about 10 mL) were drawn off sequentially. The concentration of osmium in them was determined by a method using thiourea.[1] The following relation is commonly used for the calculation of relative volatility: (1 2 y)x a5 (1 2 x)y
[3]
[1]
* y dx2 x
[4]
x0
where W0 5 initial liquid, mole; W 5 residual liquid, mole; and x0 5 initial mole fraction of a less volatile component in liquid. If a is a constant, the integral may be solved directly by ln
W 5 W0
~ !
1 x ln 1 x0 21 a
[5]
By substituting Eq. [3] into Eq. [5], one obtains ln
~
!
~!
~!
W 1 y 1 2 1 5 ln 2 ln W0 a x0 a
[6]
To determine vapor composition y, we had to draw off finite quantities of distillate Di with average concentration xdl . It is to be assumed that the mole fraction xdl 5 y is in equilibrium with the liquid in the still. The amount of this liquid is equal to
Σ D 2 0.5D
i21
Wi 5 W0 2
i51
i
i
[7]
After introducing the designations k5
y xd W 5 i and w 5 i x0 x0 W0
[8]
the resultant equation was obtained: ln k 5
~a1 21! ln w 1 ln ~a1!
[9]
Mole fractions of OsO4 in investigated solutions were
where x and y are liquid and vapor
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