Page 75 - PC 101 Interactive practical book

MANSOURA NATIONAL UNIVERSIY
PHARM D- CLINICAL PHARMACY LEVEL I PHARM. ANAL. CHEM. I (PC 101)

Introduction on Solubility Product (Ksp):

♦ K sp is calculated only for sparingly soluble salts (ex.: AgCl) and it is

constant for a certain salt at a specified temperature.

(ex.: K sp of AgCl = 1.8x10 , AgBr
-10
♦ Meaning of K sp:

+
-
For AgCl (for example) : K sp = [Ag ][Cl ]……at saturated solution (or at equilibrium).
♦ K sp values for a certain salt can be known from reference tables.

♦ Significance of K sp:

 For the ppt of AgCl (for example) to be formed in a solution:

+
[Ag ][Cl ] must be  K sp of AgCl.
-
 For different sparingly soluble salts:

 Solubility product (K sp) ≡  Solubility ………. & vice versa.

 If different precipitates can be formed:

 K sp ≡ the salt is precipitated first (i.e: the salt is more insoluble or less soluble).

≡ more stable ppt.

N.B.: This rule is not always obeyed in case of non-symmetrical salts

(i.e. salts in which the valence of the cation radicle  the valence of the anion

radicle) such as Ag 2CrO 4 , PbI 2 ......etc.

{although K sp of Ag 2CrO 4 is less than K sp of AgCl but Ag 2CrO 4 is more

soluble than AgCl…………refer to Mohr's method}.

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