Heterogeneous Solutions
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Gravimetric analysis
• ) preparation of analyte solution • ) precipitation • ) digestion of the precipitate • ) filtration and washing of the precipitate • ) drying and weighing of the precipitate • ) determination of amount of analyte present
Properties of precipitates and precipitants
Specificity and Selectivity- should be specific and react with only one chemical species
Important points of gravimetric analysis
Ease of recovery of analyte, particle size, colloidal and crystalline suspensions, factors determining particle size
Factors determining particle size
Precipitate solubility (S) & solute concentration (Q) determine particle size which is related to a property for he system called relative supersaturation
Relative supersaturation (rs)
Q-S/S = relative supersaturation When rs is large = colloidal When rs is small = crystalline
Experimental conditions
Should be chosen to minimize Q and maximize S
Mechanism of precipitate formation
Nucleatuon & particle growth
Nucleation
A small number of particles join together to produce a small solid, which may then grow further
Solubility
Defined as the number of grams of solute necessary to form a saturated solution in one kilogram of water
Fundamental rule of solubility
Like dissolves like
Heat of dissociation
The change in enthalpy that accompanies the formation of a solution
Factors influencing dissolution
• ) structure • ) pressure • ) temperature
Structure
Solubility increases if both solute and solvent have similar polarities
Pressure
Pressure exerts a remarkable effect on only gasses, the influence of pressure on solubility of gases is best expressed by Henry’s Law
Henry’s Law
In the absence of chemical reaction between the solute and the solvent, the quantity of gases dissolved in a solution is directly proportional to the pressure of the gas above the solution
Temperature
Dissolution of solids occurs more rapidly at higher temps The solubility of gas and liquid decrease with temperature
Solubility product constant (Ksp)
Equilibrium expression for equilibrium that exists between the solute and its ions in a saturated solution of slightly soluble ionic compound
Solubility product constant for PbI2
Ksp = [Pb^2+][I^-]^2 The equilibrium is independent of the amount of undissolved solute present
Precipitation titrations C- analyte with the concentration Mc-, with AgNO3
Preequivalence points Equivalence point Postequivalence points
Preequivalence points Ag+ + C- -> AgC, Ksp = [Ag+][C-]
Up to equivalence point, C- will be present in excess. Two steps are required to calculate the preequivalence point concentration of [C-].
Preequivalence points Ag+ + C- -> AgC, Ksp = [Ag+][C-]
[C-] = ((Vc•Mc)-(Vag•Mag)/Vc + Vag) + Ksp/[C-]
Correction factor
= [C-] = [Ag+] = Ksp/ ([Ag]/[C-]) Not needed if solution is very dilute or very near equivalence point
Equivalence point
[Ag+] = [C-] = (Ksp)^1/2
Post equivalence point
[Ag+] = (((VAg•MAg)-(Va-•Mac-))/ Vc- + Vag) + Ksp/[Ag]