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Quantitative Chemical Analysis 7e

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Title: Quantitative Chemical Analysis 7e Author: Daniel C. Harris Last modified by: kjwkor Created Date: 12/24/2002 1:08:46 AM Document presentation format – PowerPoint PPT presentation

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Title: Quantitative Chemical Analysis 7e


1
Harris Quantitative Chemical Analysis, Eight
Edition
CHAPTER 26 GRAVIMETRIC ANALYSIS, PRECIPITATION
TITRATIONS, AND COMBUSTION
2
THE GEOLOGIC TIME SCALE AND GRAVIMETRIC ANALYSIS
3
In the 1800s, geologists understood that new
layers (strata) of rock are deposited on top of
older layers. To measure Pb, he fused (Section
28-2) each mineral in borax, dissolved the fused
mass in acid, and quantitatively precipitated
milligram quantities of PbSO4.
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26-1 Examples of Gravimetric Analysis
Ag Cl- ? AgCl(s)
(27-1)
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A few common organic precipitants (agents that
cause precipitation) are listed in Tabled 27-2.
8
???? ??? mechanism ??? ??? particle size
?? ?? ????? ?? ??.                1)
??                      2) ???
solubility                 3) reactants
concentrations     4) ???? ?? ?? ??        ?  ?
??? ?? ? ??? ?? ? ? relative supersaturation (RS)
? ?? ??? ??
  ????? ?? ? ??? ?? ?                
     ? ????? supersaturate ( Q gt S
)                 ? ?? ??? ??? ?
precipitate ??
??? ??? average relative supersaturation? ???
?? (Q S)/S ? ? colloidal solid ?? (????
?)                ? ? crystalline solid
??(???? ?)
9
Mechanism of Precipitate Formation
?? ??? ??? RS? ?? ? precipitation mechanism ??
  ??? ? nucleation   ? particle growth ? ???
?? ??   freshly formed precipitate? ????? ?, ??
?? ?? ??? ?? ??
nucleation ?      ? ???? ??, ?? ?? ??? ?? ???
????(nuclei) ? ?? ??? ?? suspended solid?
????? nuclei? ??      ? further precipitate ? ??
?? ??? ???? nuclei?
?? ??? deposition ?? ??
10
nucleation ?? ??? ??? ??, ?? ?? ?? ??
particle growth ?? ?? ?? ?, ?? ? ?
nucleation? ?? RS? ?? ??????? ??   particle
growth? ?? RS? ????? ??      at high RS ?
nucleation rate particle growth ??? ?? ?? ??
??      at low RS ? nucleation rate particle
growth ? crystalline suspension ??
11
Experimental Control of Particle Size
  minimize supersaturation ((Q-S)/S ? ? ?? ??
?) ??    1) elevated temperature ( to increase
S)    2) dilute solution (to minimize Q)    3)
slow addition of ppt. agent with good stirring
(to lower the Q)     environment? acidity? S
???? ??? ?? ? pH ?? ? ?? ?? ?? S? ? ? size
?     ex) CaC2O4 acidic environment ? ?? ???
??         ??? NH4OH ? ? ?? ?? S is
very small (QS) ? colloidal suspensions ??
        ex) Fe(OH)3 , Al(OH)3 , Cr(OH)3,  most
heavy metal? sulfides ? colloid.
12
Homogeneous Precipitation
In homogeneous precipitation, the precipitant is
generated slowly by a chemical reaction (Table
27-3).
13
3HCO2- Fe3 ? Fe(HCO2)3 nH2O(s)?
(27-4)
Fe(III) formate
14
Precipitation in the Presence of Electrolyte
To understand why, we must discuss how tiny
colloidal crystallites coagulate (come together)
into larger crystals.
15
Demonstration 26-1 Colloids and Dialysis
Colloids are particles with diameters of 1-500
nm. You can demonstrate the size of colloidal
particles with a dialysis experiment in which two
solutions are separated by a semipermeable
membrane that has pores with diameters of 1-5
nm.3 (Collecting biological samples by
microdialysis was discussed at the opening of
Chapter 25.)
16
-Some impurities can be treated with a masking
agent to prevent them from reacting with the
precipitant.
Ca2 2RH ? CaR2(s)?
2H Analyte N-p-Chlorophenyl-
Precipitate cinnamohydroxamic
acid Mn2 6CN- ?
Mn(CN)64- Impurity Masking agent
Stays in solution
Postprecipitation Impurities might collect on
the product while it is standing in the mother
liquor. (it usually involves a supersaturated
impurity that does not readily crystallize.) Th
breaking up of the product is called peptization,
results in loss of product through the filter.
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18
Product Composition
19
A hygroscopic substance is one that picks up
water from the air and is therefore difficult to
weigh accurately. Ignition (strong heating) is
used to change the chemical form of some
precipitates. In thermogravimetric analysis, a
substance is heated, and its mass is measured as
a function of temperature.
20
26-3 Examples of Gravimetric Calculations
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Grams of Mg in analyte
Grams of Mg2P2O7 formed
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26-4 Combustion Analysis
A historically important form of gravimetric
analysis was combustion analysis, used to
determine the carbon and hydrogen content of
organic compounds burned in excess O2 (Figure
27-4).
31
Gravimetric Combustion Analysis
32
Combustion Analysis Today9
A key to elemental analysis is dynamic flash
combustion, which creates a short burst of
gaseous products, instead of slowly bleeding
products out over several minutes.
33
The sample is thermally decomposed (a process
called pyrolysis) in the absence of added O2.
34
26-5 Precipitation Titration Curves
35
26-6 Titration of a Mixture
36
26-7 Calculating Titration Curves with a
Spreadsheet
37
26-8 End-Point Detection
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