The poorly soluble compound M m A n in the aqueous solution is slightly dissolved in water and dissociated. The solid compound M m A n (s), the dissolved M m A n (1) molecule and the corresponding ion in the solution establish the following balance:
(1)
When saturation (equilibrium) is reached, the salt M m A n has equal chemical positions in different states. Since the solid phase chemical position is constant at constant temperature, it can be derived:
(2)
In the formula, with The activity of ions M n + and A m + respectively, the equilibrium constant K sp is called the solubility product, which reflects the thermodynamic tendency of precipitation.
Since the solubility product of the poorly soluble compound is small and the activity coefficient is close to 1, the concentration can be used instead of the activity. The solubility product can be expressed as:
(3)
In the formula, [M n + ] and [A m - ] are the concentrations of M n + and A m - ions at the time of dissolution equilibrium, respectively. If a concentration product (strictly the activity product) function Q c is introduced , its relationship with the ion concentration is similar to the expression of the solubility product K sp , ie Q c =[M n + ] m in any case [ A m - ] n , but unlike the expression of the solubility product, the concentration here is any concentration. According to the solubility product rule, for a saturated solution, that is, when the dissolution equilibrium is reached, Q c = K sp ; and when Q c > K sp is a supersaturated solution, there is a thermodynamic tendency for precipitation to occur, and the dissolution equilibrium will form a precipitate. The direction moves and the new M m A n (s) continually precipitates until a new dissolution equilibrium is established. After precipitation the solution remaining in the transition of metal ions calculated by the following formula:
(4)
Obviously, increasing the concentration of the anion A m - in the solution, for example, adding another salt containing A m - , it is known from the above formula that the concentration of residual metal ions in the solution is lowered lower, which is the same ion effect.
Solubility is often used to measure the ease with which a substance dissolves. Solubility c is defined as the maximum number of grams of water that can be dissolved in 100 grams of water. To simplify the calculation, the molarity can be used to indicate solubility. Introducing the solubility c into the solubility product expression gives:
(5)
This gives the following relationship between solubility and solubility product:
(6)
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