LU3: Gravimetric Analysis

  1. Preparation
    of soln

Consider factors:

Temp.

Vol. soln

pH

Conc. range

Presence & conc. of other constituents

  1. Precipitation

Conditions

Add Precipitation Agent

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.

Steps

Reagent

Precipitate Formation

Selective

Specific

reacts with a limited # of sp.

reacts only with a single chem sp.

AgNO3 ppt Cl-, Br-, I- & SCN-

Dimethylglyoxime ppt only Ni2+

Supersaturation

Nucleation &
Particle
Growth

soln phase contain more of dissolved salt than occurs at equilibrium

A min # particles must come together to produce microscopic nuclei of solid phase

Nucleation

Particle
Growth

3D addition of more particles to nucleus, form a larger crystal.

a small no. of particle (ions, atoms, mol) in soln come together randomly & form small aggregates. Spontaneous or induced.

Supersaturated
vs. saturated soln

Supersaturation

↑ degree of supersaturation, ↑ nucleation rate

rate of nucleation < rate growth of particles, big particles (crystals) are formed (easy to filter, less impurities)

Favorable
conditions

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

Q low

S high

Ppt from dilute soln

Add dilute ppting reagents slowly, + stirring

Ppt from hot soln

Ppt at low pH

  1. Digestion

coagulation of ppt into filterable form

Coagulation: process where colloidal particles ‘lump’ together into larger particles.

Types of colloids

Eg: AgNO3 (aq) + NaCl (aq) → AgCl (s) + NaNO3
•NaCl is ppting agent.
•AgCl tends to form colloidal (amorphous) precipitates.

hydrophilic

hydrophobic

strong affinity for water

little attraction for water

Gel: soln of viscous hydrophilic colloid

Sol: soln of hydrophobic colloid

Colloid coagulation

↑ electrolyte conc in soln

Heating colloid while stirring

↓ vol of soln containing sufficient ions of opposite charge to neutralize particle.

↓ # of adsorbed ions per particle

↓ size of counter ion layer making it easier for particles to approach each other

unrelated, non-interfacing ionic cpd

smaller structure

  1. Filtration

filter paper / filtering crucible

cellulose-based filter paper

filtering speed

size

ash content on ignition

Contamination
(Co-ppt)

Mixed crystal formation (Isomorphous replacement)

Post Precipitation

Surface adsorption

Prevent occlusion & inclusion

Inclusion

Occlusion

Rapid crystal growth traps a pocket of soln

solvent can be removed, trapped ions will remain after drying

localized within interior of ppt

Rapid crystal growth traps some constituents of ppt medium (i.e. ions of similar size & charge) in crystal structure

randomly distributed throughout ppt

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)

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

most common

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

minimized

carefully controlling soln conditions

interferent being masked using a suitable complexing agent, preventing its ppt

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.

  1. Drying & Igniting
  1. Washing
  1. Calculation

Some primary electric layers must be washed away for quantitative results

Preventing Peptization

to remove all adsorbed sp which will + to weight of ppt

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

ignition in a muffle furnace

heating at about 120-150°C in an oven

600-1200°C

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

stoichiometry

General formula

% 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.

Ppt equilibria

Common ion effect

excess of 1 ion over the other, conc. of the other is suppressed (common ion effect), solubility of ppt ↓

Ksp

limited solubility (insoluble cpd), ions of dissolved portion exist in equilibrium with solid material.

=, saturated soln

Ksp, ppt

< Ksp, no ppt