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Date Submitted: 08/04/2012 04:06 AM
Gravimetric methods of analysis
based upon determining the mass of pure compound to which the analyte is chemically related.
Types of gravimetric methods of analysis
1. Precipitation methods Formation of ppt: Ca2+ + C2O42- CaC2O4(s) Conversion of ppt to a form suitable for weighing: CaC2O4(s) CaO(s) + CO(g) + CO2(g)
2. Volatilization methods NaHCO3(aq) + H2SO4(aq) CO2(g) + H2O(l) + NaHSO4(aq) The mass of CO2 is determined by connecting a weighed tube with absorbent to the reaction vessel.
Formation of precipitates
A precipitate is formed when the diameter of an aggregate of atoms, ions or molecules is greater than 10-4 cm. ions in solution colloidal particles precipitate (10-8 cm) (10-7 to 10-4 cm) (> 10-4 cm)
Colloidal particles are electrically charged and resist combining to form larger particles that will settle out of solution.
Mechanism of precipitate formation
1. Nucleation: a process in which a minimum number of atoms, ions or molecules join together to give a stable solid. a. Homogeneous: nuclei form due to fluctuation of ion concentration. b. Heterogeneous: particles form on foreign solids like dust particles, rough sides of beaker 2. Particle growth: growth on the existing nuclei to form particles sufficiently large enough to precipitate.
Particle size
If rate of nucleation < rate of particle growth, then fewer particles are finally produced which are relatively large and purer. If rate of nucleation > rate of particle growth, then many fine particles are finally produced.
In a precipitation process, it is desired to form large particles because they are easily filtered large particles have smaller surface area exposed to contamination than smaller particles
Von Weimarn
proposed that the initial rate of precipiation is proportional to the relative supersaturation, VWR
VWR Q S S
Von Weimarn Ratio
whereQ = concentration of a species at any instant S = equilibrium solubility A low VWR (Q is low...