CHAPTER 3: GRAVIMETRIC ANALYSIS (STEPS ( Digestion (Produce larger &…
CHAPTER 3: GRAVIMETRIC ANALYSIS
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.
Wide range of sample concentration
No calibration needed
Accurate to better than +0.01% (atomic weight)
Analyte is separated from solution as precipitate and converted to compound of known composition. Can be weigh.
Analyte separated from sample by conversion of gas of known composition.
The analyte is separated by deposition on electrode by electrical current. Mass of product provides a measure of analyte concentration.
Produce larger & purer particles
Filtration & washing precipitates
To remove adsorbed impurites & mother liquor
Remove water/absorbed electrolyte/volatile species
Repeat process of heating & weighing until constant weight obtained
GF= (mass of substance sought) / (mass of precipitate)
REQUIREMENTS FOR PRECIPITATES
Large size for easy filtration
Free of impurities (pure)
Unreactive to atmosphere
Known composition after drying
FACTORS THAT INFLUENCE THE PROPERTIES OF PRECIPITATES
Increasing the pH will decrease solubility
Solubility of precipitates
Chosen for their ability to be selective and to form highly insoluble precipitates.
Not very selective
Form slightly soluble salts
useful for metals
If temperature increases, Ksp will also increases
PROCESS OF PRECIPITATION
Analyte + Precipitating agent
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
soluble compounds carried out of solution by precipitation
Soluble compound carried out of solution on the surface of a precipitate
Interferences are incorporated into precipitating crystals
Occlusion & mechanical entrapment
Two crystals grow together and trap a species in between them
Foreign compound precipitate on top of desired precipitate because stand too long before being filtered