Chapter 7
Gravimetric Analysis
7-1 Examples of Gravimetric Analysis
• Gravimetric Analysis
– Mass of product
used to calculate
quantity of original
analyte.
• Has been largely
replaced by
instrumental
methods
7-1 Examples of Gravimetric Analysis
• T.W. Richards
– Early 1900s, measured
atomic mass of Ag, Cl,
and N to six-figure
accuracy.
– Formed the basis for
accurate atomic mass
determination.
7-1 Examples of Gravimetric Analysis
• Solubility Rules
– Set of rules that determine whether species
are soluble in water.
– Precipitation allows separation for analysis
• Species form insoluble compounds
• Dried and weighed
7-1 Examples of Gravimetric Analysis
• Combustion Analysis
– Samples burned in excess oxygen. Products
measured.
– Used most often to measure C, H, N, S and
halogens.
7-1 Examples of Gravimetric Analysis
• Precipitant
– Agents that cause precipitation
• Precipitation
– In chemical reactions, the forming of solids as a
result of a chemical reaction.
7-2 Precipitation
• Gravimetric Analysis
– Particles should be large enough to be collected
by filtration.
– Colloids (scattered light, eg: milk)
• Particles that are ~1 – 500 nm
• Small enough to pass through filters
Examples of Gravimetric Analysis
• Reactions
– Precipitation reactions
Example:
Determination of Cl by precipitation with Ag
Ag+ + C1-  AgCl
Measuring the mass of solid offers moles of AgCl.
Conversions and Stoichiometry -> final value.
7-1 Examples of Gravimetric Analysis
• A 10.00 mL solution containing Cl- was treated
with excess AgNO3 to precipitate 0.4368 g of
AgCl (FM 143.321). What was the molarity of
Cl- in the unknown?
7-2 Precipitation
• Crystal Growth
– Two Phases
Nucleation
Particle Growth
• Homogeneous Precipitation
– Precipitant generated slowly by a chemical
reaction.
• Slow precipitation => More particle growth over
nucleation
• Similar to cooling rates (nucleation and grains)
• Easy to filter
7-2 Precipitation
• Homogeneous Precipitation
– Precipitant generated slowly by a chemical
reaction.
Example: Formation of Fe(III) by decomposing urea
in boiling water to make OH
7-2 Precipitation
• Homogeneous Precipitation
2OH reacts with formic acid to produce formate,
used to precipitate Fe(III).
Box 7-1 Shorthand for Organic
Structures
• Simple lines and symbols can be used to
shorthand complex structures to save time
and space on paper.
Box 7-1 Shorthand for Organic
Structures
• Simple lines and symbols can be used to
shorthand complex structures to save time
and space on paper.
Box 7-1 Shorthand for Organic
Structures
Exercise: Write the chemical formula (such as C4H8O) for each structure below.
7-3 Examples of Gravimetric Calculations
• Relating mass of product to mass of reactant.
– The piperazine content of an impure commercial
material can be determined by precipitating and
weighing piperazine diacetate:
Common organic precipitating agents