Chemistry

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...