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CHAPTER 3: GRAVIMETRIC ANALYSIS (STEPS (Digestion (Produce larger &…
CHAPTER 3: GRAVIMETRIC ANALYSIS
PROCESS
Analyte is selectively coverted to an insoluble form.
Quantitatively measure the mass of a pure compound to which the analyte is chemically related.
Uses the mass of a product to calculate the quantity of analyte.
FEATURES
Traditional method
Slow
Labour Intensive
Wide range of sample concentration
No calibration needed
Accurate to better than +0.01% (atomic weight)
TYPES
PRECIPITATION
Analyte is separated from solution as precipitate and converted to compound of known composition. Can be weigh.
VOLATILIZATION
Analyte separated from sample by conversion of gas of known composition.
ELECTROGRAVIMETRY
The analyte is separated by deposition on electrode by electrical current. Mass of product provides a measure of analyte concentration.
STEPS
Sample dissolution
Precipitation
Digestion
Produce larger & purer particles
Filtration & washing precipitates
To remove adsorbed impurites & mother liquor
Drying precipitates
Remove water/absorbed electrolyte/volatile species
Weighing
Repeat process of heating & weighing until constant weight obtained
Data calculations
GF= (mass of substance sought) / (mass of precipitate)
REQUIREMENTS FOR PRECIPITATES
Low solubility
Large size for easy filtration
Easily washed
Free of impurities (pure)
Unreactive to atmosphere
Known composition after drying
FACTORS THAT INFLUENCE THE PROPERTIES OF PRECIPITATES
pH
Increasing the pH will decrease solubility
Solubility of precipitates
Precipitating agents
Chosen for their ability to be selective and to form highly insoluble precipitates.
Type
Inorganic
Not very selective
Form slightly soluble salts
Organic
useful for metals
Temperature
If temperature increases, Ksp will also increases
PROCESS OF PRECIPITATION
Analyte + Precipitating agent
Supersaturation
Primary nucleation
Crystals growth (secondary nucleation)
FACTORS DETERMINE PARTICLES SIZE OF PRECIPTATES
Relative Supersaturation = (Q - S) / S
Q= conc. of reactants before precipitation
S= solubility of precipitation at equilibrium
(Q - S) = degree of supersaturation
How to get small (Q - S)?
Precipitate from diluted solution
Add dilute precipitate slowly with stirring
Precipitates from hot solution
IMPURITIES IN PRECIPITATES
Co-Precipitation
soluble compounds carried out of solution by precipitation
3 Types
Surface adsorption
Soluble compound carried out of solution on the surface of a precipitate
Mixed-crystal formation
Interferences are incorporated into precipitating crystals
Occlusion & mechanical entrapment
Two crystals grow together and trap a species in between them
Post Precipitation
Foreign compound precipitate on top of desired precipitate because stand too long before being filtered