Solubility of Oxides and HydroxidesIf pure solid oxide or hydroxide is in equilibrium with free
ions in solution, for example,Me(OH) 2 (s) Me^2 ^ 2OH^ ^ (18)MeO(s) H 2 O Me^2 ^ 2OH^ ^ (19)the conventional (concentration) solubility product is given byc*Ks 0 [][](Me^2233 OH mole liter),
(20)where the subscript “0” refers to solution of the simple, uncom-
plexed forms of the metal ion.
Sometimes it is more appropriate to express the solubility
in terms of reaction with protons, for example,Me(OH) 2 (s) 2H^ ^ Me^2 ^ 2H 2 O (21)MeO(s) 2H^ ^ Me^2 ^ H 2 O. (22)In the general case for a cation of charge z, the solubility
equilibrium for Eqs. (21) and (22) is characterized bycc
K Kz
zzK
ws zs
0*()()[] 110
[][
Me
Hmole liter , (23)where K w is the ion product of water. This constant and also a
number of solubility equilibrium constants relevant to natu-
ral waters are given in Table 3.
Equation (23) can be written in logarithmic form to
express the equilibrium concentration of a cation Me^ z ^ as a
function of pH:log[ ] log
Mezc* pH
Ks
0. (24)Equation (24) is plotted for a few oxides and hydroxides in
Figure 5.pKH 2 CO 3 pK 1 pK 2H+
H 2 CO 3CTCO 3
OH–TRUE H 2 CO 3HCO 3Plog CONCENTRATION (MOLAR)a pH-1-2-3-4-5-6-7-8 4 5 6 7 8 9 10 11*-2FIGURE 4 Logarithmic concentration—pH equilibrium diagram for the aque-
ous carbonate system open to the atmosphere. Water is equilibrated with the at-
mosphere (pCO 2 = 103.5 atm) and the pH is adjusted with strong base or strong
acid. Eqs. (14), (15), (16), (17) with the constants (25C) pKH 1.5, pK 1 6.3,
pK 2 10.25, pK(hydration of CO 2 ) 2.8 have been used. The pure CO 2 solu-
tion is characterized by the proton condition [H] [HCO 3 ] 2[CO 32 ]+[OH]
see point P) and the equilibrium concentrations log[H] log[HCO 3 ]
5.65; log[CO 2 aq] log[H 2 CO 3 ] 5.0; log[H 2 CO 3 ] 7.8; log[CO 32 ]
8.5. Ref.: Stumm, W. and J. Morgan, Aquatic Chemistry, Wiley-Interscience, New
York, 1970, p. 127.1262 WATER CHEMISTRY
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