Handbook of Solvents - George Wypych - ChemTech - Ventech!

21.3 Oxide solubilities in ionic melts 1491
Since for the same oxide σis constant, the increase of the precipitate square should result
in the increase of precipitate solubility.
Similar considerations were included in well-known Ostvald-Freundlich equation. 45
For substances having 1:1 dissociation, a similar equation can be written in the following
form:
RT
M
s1
RT P1
σ⎛
1 1⎞
ln = ln = ⎜ −
s 2 M P2 d ⎜
⎟
⎝r2
r ⎟
1 ⎠
[21.3.16]
where:
s1,s2 the solubility of crystals with radii r1 and r2, respectively
P1,P2 corresponding solubility products
M molecular weight
d density
From this equation, it follows that the increase of crystal size reduces its solubility.
Oxide particles deposited from more concentrated cation solution, should possess larger
sizes due to the so-called “deposit ageing”, than those obtained from mere diluted solutions.
Therefore, results of Delimarsky et al. 29 obtained from 0.01 mole/kg solutions should be
higher that those obtained in other works 11,33,38 using 0.05 mole/kg solutions. In the first case
solubility was greater approximately by half-order of the magnitude than in second one. Oxide
formed from more concentrated solution should have less surface and, hence, less solubility.
Data obtained in works 11,33,38 practically coincide although we 33,38 have used NaOH as
titrant similarly to other study. 29
Sedimentational titration results in formation of fine dispersed oxide which immediately
begins to age because of the recrystallization, i.e., the growth of larger crystals and disappearance
of smaller ones, this process leads to the surface energy, σS, decrease.
Continuous holding at high temperature and high solution concentration favors this process.
In particular, samples used for isothermal saturation technique studies exposed to high temperatures
for a long time, and tablets obtained are held in the contact with the melt for some
hours. High concentration solutions favor transfer of substance from small to large crystals
(diffusion). The differences between data in references 8 and 9 may be explained by different
conditions of the powder calcination. During the annealing of the oxide sample
recrystallization processes occur, which lead to the reduction of surface square of the powder
and the decrease of solubility. The differences in data for KCl-NaCl are caused by the
differences in crystal sizes of solid oxide being in equilibrium state with the saturated solution.
21.3.3.3 Other chloride-based melts
ZrO2 solubilities in molten mixtures KCl-KPO3 at 800 o C have been investigated. 46 Zr IV concentration
in pure KPO3 was 1.34 wt%, and in the equimolar mixture KCl-KPO3 concentration
was 3.25 wt%. A reason for solubility increase is in the depolymerization of PO 3- which
may be schematically described by the following equation:
( )
n−
−
3 n
3
PO = nPO
[21.3.17]