The dissolution balance of organic solids is the balance between the solid part and the dissolved part:
andBalance expressionIt can be written as follows (this applies to any such reaction):
K isequilibrium constant, curly brackets representactivityBy definition, the activity of solid matter is 1.IfIonosphereThe interaction between them can be ignored (a common situation is when the concentration of solution is very low), then the activity can also be usedconcentrationReplace:
This expression means that the concentration of dissolved sugar in water is equal to K.At 25 ℃, when the standard concentration is 1mol/L, the K of sucrose is 1.971, which is the maximum amount of sucrose that can be dissolved at 25 ℃. At this time, the solution is called "saturated".If the current solution concentration is lower thanSaturation concentration, the solids will continue to dissolve until they are equal or all solids have dissolved;If the current solution concentration is higher than the saturation concentration, then the solution is "supersaturated", and sucrose in the solution will precipitate in solid form until they are equal.This process may be slow, but the equilibrium constant describes the state of the system when it is in equilibrium, not the speed at which the system reaches equilibrium.[1]
K is calledequilibrium constant, and curly brackets representactivity。The activity of solid matter, by definition, is equal to 1.WhensolutionOfconcentrationVery low, that is, when the ion activity can be regarded as 1, this expression can be rewritten as follows“Solubility product”Expression:
This expression shows that when the aqueous solution of calcium sulfate reaches equilibrium, two kinds of calcium sulfate are ionizedIon concentrationThe product of is equal to KspIs the solubility product.The solubility product of calcium sulfate is 4.93 × 10-5。If the solution contains only calcium sulfate, that is, only Ca and SO ionized from itfour, then the concentration of each ion is:
When a solute is ionized intoMeasured quantityIn case of unequal parts:
KspThe determination of can be slightly complicated.For the following ionization processes:
The relationship between solubility product and solubility is determined by the following equation:
Where: n is the total metering number on the right side of the ionization equation (for the above example, x+y), nonedimension；X is the total measurement number of all cations, dimensionless;Y is the total measurement number of all anions, dimensionless;KspIs the solubility product, (mol/kg);C is the solubility of compound A (the mass of A is higher than the mass of solution), dimensionless;MMIs the molar mass of compound A, kg/mol.
The above equation assumes that the ionization process occurs in pure solvent (no same ion effect occurs) and does not existcomplexationandhydrolysis(that is, only B and C exist in the solution), and the concentration is small enough that the ion activity can be considered equal to 1.
Isoionic effect
Isoionic effectIt refers to the basis of dissolution balanceLe Chatelier's principle The movement that occurred.In the above example, such ascalcium sulfateAdd to the solutionSulfate ion(i.e. add soluble sulfate, such assodium sulphateEtc.) will cause calcium sulfate precipitation until the product of ion concentration meets the solubility product again.
Salt effect
Salt effect refers to the existence of other salts in the solution. Even if there is no same ion, it will also affect the ionic strength, thereby affecting the ionic activity.So even if KspIf it remains unchanged, the solubility will also change (the default activity of solid is still 1).[1]
