CHEMISTRY 59-320
ANALYTICAL CHEMISTRY
Fall - 2010
Lecture 10
Chapter 27: Gravimetric and
combustion analysis
Midterm exam
• Time: Oct. 21 between 8:30 and 9:40 AM.
• Location: BB121 or CHS 51, depending on
your surname (A – J write in room CHS51)
• The exam covers chapters 0-7 and chapter
27.
• It consists of (1) multiple choice (~ 25%),
(2) short answers and analysis (~15%), and
(3) calculations (~60%).
Gravimetric Analysis
• Gravimetric Analysis – one of the most accurate
and precise methods of macro-quantitative
analysis.
• Analyte selectively converted to an insoluble
form.
• Measurement of mass of material
• Correlate with chemical composition
• Why choose this technique?
-- Simple
-- Often required for high precision
How to Perform a Successful
Gravimetric Analysis
• What steps are needed?
1. Preparation of the solution
2. Precipitation
3. Digestion
4. Filtration
5. Washing
6. Drying or igniting
7. Weighing
8. Calculation
How to Perform a Successful Gravimetric
Analysis
• Precipitation Techniques
– Add precipitating reagent to sample solution
– Reacts with analyte to form insoluble material
– Precipitate has known composition or can be converted to
known composition
• Precipitate handling involves
– Quantitative collection (no losses)
– Isolation of pure product
• Measure mass of precipitate (Direct or By difference)
• Calculation of original analyte content (concentration). It
requires the knowledge of Chemistry, Stoichiometry, and
Composition of precipitate
How to Perform a Successful Gravimetric
Analysis
• Desirable properties of analytical
precipitates:
– Readily filtered and purified
– Low solubility, preventing losses during
filtration and washing
– Stable final form (unreactive)
– Known composition after drying or ignition
Filterability of Precipitates
• Colloidal suspensions
– 10-7 to 10-4 cm diameter
– Normally remain suspended
– Very difficult to filter
• Crystalline suspensions
– > tenths of mm diameter
– Normally settle out spontaneously
– Readily filterable
27-1 An example of gravimetric
analysis
• The determination of Cl- by precipitating with
Ag+ : A 10.00 ml solution containing Cl- was
treated with excess AgNO3 to precipitate 0.4368
g of AgCl. What was the molarity of Cl- in the
unknown.
• Solution: The formula mass of AgCl is 143.321
0.4368 g/ (143.321 g/mol) = 3.048 x 10-3 mol
Based on 1:1 stoichiometric relationship:
[Cl-] = 3.048 x 10-3 mol /(0.01000 L) = 0.3048
M
27.2 Precipitation
The ideal product of a gravimetric analysis
should be:
very pure
insoluble
easily filterable
known composite
Nucleation: molecules in solution come
together randomly and form small
aggregates.
Supersaturated: a solution contains more
Solute than should be present at equilibrium.
• Techniques that promote particle growth:
-- raising the temperature to increase solubility
and thereby decrease supersaturation.
-- Adding precipitant slowly with vigorous mixing
-- Using a large volume of solution so that the
concentrations of analyte and precipitant low
• Homogeneous precipitation
• Precipitation in the presence
of electrolyte.
27.3 Examples of Gravimetric
calculations
• Problem 27-12: Marie Cure dissolved 0.09192 g of RaCl2 and
treated it with excess AgNO3 to precipitate 0.08890 g of AgCl. In her
time (1900), the atomic mass of Ag was known to be 107.8 and that
of Cl was 35.4. From these values, find the atomic mass of Ra that
Marie Curie would have calculated.
• Answer: The formula mass of AgCl is
(35.4 + 107.8) = 143.2
0.08890 g/(143.2 g/mol) = 6.2081 x10-4 mol.
Cl- is 6.2081 x10-4 mol
RaCl2 is (6.2081 x10-4)/2 mol
the formula mass of RaCl2 is 0.09192/(3.10405x10-4 )
= 296.1292
the atomic mass of Ra = 296.1292 – 2x35.4 = 225.329
= 225.3
Example: A problem with two
components
• Problem 27-22:
27.4 Combustion analysis
Combustion analysis (2)
Thermal analysis
Lab 2:Gravimetric determination of
calcium
• Prep time for Experiment 2 will begin the week of
October 11, 2010.
• For students in Lab Sections 53 - 60, please attend on
your regularly scheduled lab day between 12:30 p.m. to
4:30 p.m. during the week of October 11th to prep for
Experiment 2.
• Your GA will be in the lab during these hours to assist
and answer any questions you may have regarding the
experiment.
• For students in Lab Sections 51 and 52, please attend
Friday, October 15th between the hours of 9:30 a.m. to
1:30 p.m. to prep for Experiment 2.
• If you have a problem attending the prep session on your
regularly scheduled lab day during the hours indicated,
please contact your GA for alternative arrangements.
A few tips for Lab Preparation
• Steps that have great impacts on your final
accuracy.
• Label your glassware in a way that your partner
can easily recognize, such as 52 – 2 -1 ; 52-2-2
… (e.g. section # -- group # -- crucible #).
• Standard deviation and Q test?
• Discuss with your partner. In case you have a
busy week, one can come in to clean and place
the three crucibles into the oven, while the other
weights the crucibles an hour late.
Data analysis
• (H2N)2CO + 3H2O + heat ⇌ CO2 + 2NH4+ + 2OH• Ca2+ + C2O42- + H2O ⇌ CaC2O4 · H2O
• The dried precipitate is CaC2O4 with a formula
mass of 128
• # mol CaC2O4 = ?? g/(128 g/mol) = yy mol
• # mol CaC2O4 = # mol CaO
• Mass of CaO = yy mol x 56.0790 g mol-1 = yy2 g
• % calcium = yy2/?? =