LU3: Gravimetric Analysis
LU3: Gravimetric Analysis
Drying & Igniting
ignition in a muffle furnace
heating at about 120-150°C in an oven
Many ppt contain varying amounts of H2O & adsorbed electrolyte from wash liquid, must heat at 110 - 120°C for 1-2 hrs.
dried for accurate, stable mass measurements
ignited to convert precipitate to a more suitable form
% analyte: (weight ppt x GF x 100)/ weight sample
GF = subs sought / subs known
Gravimetric factor: combination of mole ratios & formula weights used in stoichiometric calculation.
=, saturated soln
< Ksp, no ppt
limited solubility (insoluble cpd), ions of dissolved portion exist in equilibrium with solid material.
Common ion effect
excess of 1 ion over the other, conc. of the other is suppressed (common ion effect), solubility of ppt ↓
coagulation of ppt into filterable form
Coagulation: process where colloidal particles ‘lump’ together into larger particles.
Eg: AgNO3 (aq) + NaCl (aq) → AgCl (s) + NaNO3
•NaCl is ppting agent.
•AgCl tends to form colloidal (amorphous) precipitates.
Types of colloids
strong affinity for water
Gel: soln of viscous hydrophilic colloid
little attraction for water
Sol: soln of hydrophobic colloid
↑ electrolyte conc in soln
↓ vol of soln containing sufficient ions of opposite charge to neutralize particle.
unrelated, non-interfacing ionic cpd
Heating colloid while stirring
↓ # of adsorbed ions per particle
↓ size of counter ion layer making it easier for particles to approach each other
filter paper / filtering crucible
cellulose-based filter paper
ash content on ignition
Mixed crystal formation (Isomorphous replacement)
If similar ions (i.e. same charge, sizes within 5% of analyte ion) are present, they can replace analyte ion in crystal lattice
Ppt of Mg2+ as MgNH4PO4, K+ has nearly same ionic size as NH4+ & can replace it to form MgKPO4
Once formed, impurities cannot be removed
Choice: remove interferences prior to ppt or to select a diff reagent.
When ppt is allowed to stand in mother liquor, a 2nd subs. will slowly form ppt with the ppting agent
CuS ppt in acid soln in presence of Zn2+, but eventually ZnS will ppt
carefully controlling soln conditions
interferent being masked using a suitable complexing agent, preventing its ppt
Digestion ↓ relative surface area & area available for adsorption of impurities.
Washing removes impurities bound to surface
Unwanted material is adsorbed onto ppt surface
Eg. Adsorption of nitrate ion in ppt of barium sulphate
Prevent occlusion & inclusion
Occluded or included impurities cannot be removed by washing.
best: slow things down, ie. using dilute, warm solutions during ppt gives impurities time to leave & helps break up pockets.
Digestion may help but not completely effective
Purification by reprecipitation (effective but time consuming)
Rapid crystal growth traps some constituents of ppt medium (i.e. ions of similar size & charge) in crystal structure
randomly distributed throughout ppt
Rapid crystal growth traps a pocket of soln
solvent can be removed, trapped ions will remain after drying
localized within interior of ppt
Some primary electric layers must be washed away for quantitative results
Use a volatile electrolyte
Washing with soln of volatile electrolyte prevents peptization
Heating ppt (during drying) remove volatile electrolyte
Eg: For AgCl, washes with HCl. Drying precipitate at 110°C will removes HCl
to remove all adsorbed sp which will + to weight of ppt
Stable under atm conditions
After drying &ignition, known chemical composition
ppt sufficiently insoluble, amount lost due to solubility will be negligible
ppt tend to carry some of other constituents of soln
consist of large crystals (easily filtered)
contamination should be negligible. Large crystals ↓ contamination.
Add Precipitation Agent
soln phase contain more of dissolved salt than occurs at equilibrium
A min # particles must come together to produce microscopic nuclei of solid phase
reacts with a limited # of sp.
AgNO3 ppt Cl-, Br-, I- & SCN-
reacts only with a single chem sp.
Dimethylglyoxime ppt only Ni2+
a small no. of particle (ions, atoms, mol) in soln come together randomly & form small aggregates. Spontaneous or induced.
3D addition of more particles to nucleus, form a larger crystal.
vs. saturated soln
↑ degree of supersaturation, ↑ nucleation rate
rate of nucleation < rate growth of particles, big particles (crystals) are formed (easy to filter, less impurities)
particle size of ppt is inversely proportional to RSS of soln
RSS = (Q-S)/S
RSS can be used to estimate/control type of ppt that's formed.
RSS ↑ (S ↓ and Q ↑ ), small crystals.
RSS ↓ (S ↑ and Q ↓ ), larger crystals.
keep Q low &S high
Ppt from dilute soln
Add dilute ppting reagents slowly, + stirring
Ppt from hot soln
Ppt at low pH
Presence & conc. of other constituents