Question about solubility and pH and my answer

[Zellmer, Robert]

A fellow student asked about solubility and pH this morning.  How does pH
affect solubility and when?

If the anion in the salt can act as a base the salt will be more soluble in acidic
solution than in pure water.

Something like AgF would be more soluble in acidic solution than water since the
F- can act as a base and react with acid.

AgF   =>   Ag+   +   F-

H+   +   F-   =>   HF

So, the rxn of H+ with F- to form the weak electrolyte, HF, will remove the F- from
the first equilibrium making it proceed to the right toward product.  More AgF
dissolves, increasing its solubility.  It is more soluble in acidic solution than water.

For AgCl this won't happen because the Cl- is the conj. base of the strong acid
HCl so it won't act as a base.

Hydroxides and oxides will always be more soluble in acidic solution than in pure
water since OH- and O^2- are strong bases.  For the vast majority of hydroxides
their solubility decreases as pH increases (solution becomes more basic) and
eventually will precipitate out of solution once enough base as been added.
The amount of base needed (the conc of OH- needed) to cause precipitation
will depend on the metal cation and OH- conc.

For Fe(OH)3, it will be more soluble in acidic soln than in pure water.  It will be
less soluble in basic solution than pure water.  If you have Fe^3+ in solution
(by dissolving something like FeCl3) the solution would be pretty acidic due to
the Fe^3+ ion.  If you start adding NaOH the pH rises and eventually Fe(OH)3
will ppt out (when depends on the conc of the Fe^3+ and the OH-).   So, if the
conc of Fe^3+ is 0.10 M to start with, at a high pH (let's say pH=13.00) Fe(OH)3
will ppt.  The Ksp for Fe(OH)3 is 6.8 x 10^-38.  The Q when [Fe^3+] = 0.10 M
and [OH-] = 0.10 M (pOH = 1.00) will be,

Q = (0.10) * (0.10)^3 = 10^-4

Q > K so a ppt forms

As you add acid to the solution and lower the pH, say to 7.00, (pOH also
equals 7.00) then

Q = (0.10) * (1 x 10^-7)^3 = 1 x 10^-22

Q > K so solid remains (ppt still forms at this pH)  (but it is more soluble than
above at a pH = 13.00 so some of the Fe(OH)3 would dissolve)

Add more acid and lower the pH to 1.00 (pOH = 13.00) then

Q = (0.10) * (1 x 10^-13)^3 = 1 x 10^-40

Q < K so no ppt forms (the Fe(OH)3 dissolves)

So, solubility of the Fe(OH)3 is increasing as the pH is lowered (add acid).


This is a little different for amphoteric hydroxides.  They are more soluble
in both acidic and basic solutions than pure water.  As given in the book
and my lecture notes, the main hydroxides (and oxides) which are
amphoteric are those of Al^3+, Cr^3+, Zn^2+ and Sn^2+.  The solubility
of the hydroxides and oxides of these cations increases as the solution
becomes more acidic and as the solution becomes more basic.  When
solubility is graphed versus pH you get a U-shaped curve, as shown in
the notes and EOCE 17.11.

Hopefully, this provides further explanation for this.

Dr. Zellmer

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