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STEPS OF GRAVIMETRIC ANALYSIS (Step 6:Drying and Igniting (Precipitates…
STEPS OF GRAVIMETRIC ANALYSIS
Step 6:Drying and Igniting
Precipitates are dried for accurate, stable mass measurements.
Precipitates are also ignited to convert precipitate to a more suitable form for weghing
Precipitates must be in suitable form for weighing.
The temperature required to produce a suitable weighing form varies from precipitate to precipitate.
The purpose of drying or ignition is to get a material with exactly known chemical structure so that the amount of analyte can be accurately determined.
Step 3: Digestion
Definition : Quantitave analysis refers to the coagilation of a precipitate into a filterable form.
Coagulation - The process where colloidal particles 'lump' together into larger particles.
Causing :
Heating the colloid while stirring
reduces the size of the counter ion layer
decreases the number of adsorbed ions per particle.
Increase the electrolyte concentration in the solution
reduces the volume of solution
result : a smaller structure is formed
Types colloids
Hydrophilic colloid - has strong affinity for water and a solution is viscous called a gel.
Hydrophobic colloid - has little attraction for water and a solution is called a sol.
Step 2: Precipitation
2) supersaturation occur
3) nucleation and particle growth
1) adding precipitating agent
selective reagent
specific reagent (single chemical species).
Step 5: Washing
Peptization is a process of converting a fresh precipitate into colloidial particles by shaking it with the dispersion medium in the presence of a small amount of suitable electrolyte.
Coagulation is a process of agitating together the colloidal particles so as to change them into large sized particles which ultimately settle as precipitate.
Use a volatile electrolyte
washing with a solution of a volatile electrolyte helps prevent peptization
heating the precipitate will remove the volatile electrolyte during drying
To remove all the adsorbed species which will add to the weight of precipitate
Step 1: Preparation of the solution
Adjust the solution condition so as to maintain low solubility of the precipitate and to obtain it in a suitable form for filtration.
Temperature
pH
Concentration range
Presence and the concentrations of other constituents
Volume of solution
Step 4:Filtration
Definition : Precipitate is separated by filtration using either filter paper or a filtering crucible,
The common filtering medium : cellulose-based filter paper that classified according to its filtering speed, its size, and its ash content on ignition.
Precipitates tend to carry down from the solution other constituents (normally soluble), causing contamination of the precipitate.
Process of contamination :
co-precipitation.
Sources of co-precipitation :
Inclusion - Rapid crystal growth traps a pocket of solution. while the solvent can be removed, the trapped ions will remain after drying.
Surface adsorption - Unwanted material is adsorbed onto the surface of the precipitate.
Occlusion - Rapid crystal growth traps some constituent of the precipitation medium in the crystal structure.
Post-precipitation - When the precipitate is allowed to stand in the mother liquor, a second substance will slowly form a precipitate with the precipitating agent.
Mixed crystal formation - If similar ions are present, it can replace the analyze ion in the crystal lattice.