Chapter 6 - Titrations
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Precipitation Titrations
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Precipitation titrations are possible because of the solubility
product (Ksp) of the insoluble substance has a small value.
The Ksp is an equilibrium constant that gives the extent to
which a slightly soluble substance dissolves. It is expressed
as the concentrations of each of the ions produced raised to
the power of their coefficients in the chemical equation. For
example, the Ksp of AgCl is for the reaction
AgCl(s) < == > Ag+(aq) + Cl (aq)
Precipitation Titrations
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Precipitation titrations are possible because of the solubility
product (Ksp) of the insoluble substance has a small value.
The Ksp is an equilibrium constant that gives the extent to
which a slightly soluble substance dissolves. It is expressed
as the concentrations of each of the ions produced raised to
the power of their coefficients in the chemical equation. For
example, the Ksp of AgCl is for the reaction
AgCl(s) < == > Ag+(aq) + Cl (aq)
is
Ksp = [Ag+] [Cl ] = 1.8 x 10 10
Precipitation Titrations
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The solubility product of a substance such as La(IO3)3
may also be written from its chemical equilibrium.
La(IO3)3 (s) < == > La+3 + 3 IO3
Precipitation Titrations
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The solubility product of a substance such as La(IO3)3
may also be written from its chemical equilibrium.
La(IO3)3 (s) < == > La+3 + 3 IO3
Ksp = [La+3] [IO3 ]3 = 6.2 x 10 12
Precipitation Titrations
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Before the end point the excess of Cl ions causes the colloidal AgCl to have a
negative charge. The Indicator has a negative charge so it is repelled by the
colloidal particle. After the end point the slight excess of Ag+ causes the
colloidal AgCl to have a positive charge which attracts the negatively charged
Indicator. The free Indicator has a different color than the adsorbed Indicator.
Precipitation Titrations
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The Indicator for the Fajan’s Method. The dinegative anion causes the
Indicator to have a -2 charge. In this form, the Indicator has a yellow-green
color. When it is adsorbed onto the colloidal AgCl (which will have a
positive charge at the endpoint) it gives a light pink color.
Precipitation Titrations
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The titration curve for the precipitation titration of a mixture of chloride
and iodide. The Ksp values are 1.8 x 1010 for AgCl and 8.3 x 10 17 for
AgI. Because the Ksp values differ (~107) so much, separate breaks are
observed, with the more insoluble AgI precipitating first.
The Common Ion Effect
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The common ion effect is whenever there are two
independent sources of the same ion within the system. For
example, consider that we have a precipitate of AgCl, and a
second source of the chloride ion such a solution of NaCl.
The effect of the chloride from the second source is to force
the following equilibrium towards the left, making AgCl less
soluble in a solution of NaCl than it would be in pure water.
AgCl(s) < == > Ag+(aq) + Cl (aq) is
< -------Ksp = [Ag+] [Cl ] = 1.8 x 10 10
The Common Ion Effect
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The best way to solve equilibrium problems is to set up a grid
that lists the concentrations of all species at initial conditions
and at equilibrium. The author shows this method for two
examples on pages 119 and 120. Sometimes the resulting
algebraic expressions may be of cubic or higher order and
require iterative methods to obtain the solutions. Iterative
methods use successive approximations until the right and left
sides of an equality are approximately equal to an acceptable
level.
Chapter 7 – Gravimetric Methods
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Chlorine content in the rings of trees as a function of time. The
analysis were done in a combustion flask shown in the next slide.
Chapter 7 – Gravimetric Methods
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The combustion flask for the analysis of tree rings.
Chapter 7 – Gravimetric Methods
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Gravimetric Analysis – using the mass of the product of
a reaction to determine the quantity of analyte in the
original sample. Developed in the early 1900’s allowing
6 figure accuracies in the determination of the atomic
masses of several elements.
Chapter 7 – Gravimetric Methods
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1.
The ideal precipitating reagent for a gravimetric
method of analysis would
1 – react with only one analyte; be specific.
2 – produce a solid that
a – has a low solubility (small Ksp value)
b – is easy to filter
c – may be washed to remove contaminants
d – is not reactive with the atmosphere
e – has a known composition after drying or
ignition
Chapter 7 – Gravimetric Methods
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Much of what is done in a specific gravimetric procedure
addresses one or more of the ways that real precipitate are form
during the analysis. We saw some of this in the Gravimetric
Chloride Determination such as
• Heating to allow growth of the crystal size to make the
precipitate so it could be easily be filtered quantitatively
• Addition of excess reagent to drive the precipitation reaction
towards completion
• Washing the ppt to remove excess reagent
• Drying or igniting at a described temperature
• Storage in the desiccator
Table 7-1
Chapter 7 – Gravimetric Methods
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Table 7-1 page 132 shows typical inorganic ions that are
analyzed by gravimetric methods.
 It is especially important to note the interfering species
list. If these species are present in the sample, they must
either be removed or their presence masked.
Chapter 7 – Gravimetric Methods
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Table 7-1 page 132 shows typical inorganic ions that are
analyzed by gravimetric methods.
 It is also important to note that the form that the product
to be weighed is not always what was precipitated,
because heating or drying may alter the product.
Chapter 7 – Gravimetric Methods
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Table 7-1 page 132 shows typical inorganic ions that are
analyzed by gravimetric methods.
 It is important to note the stoichiometric relationship
between the species analyzed and the form weighed.
For example, in the analysis of PO4-3 , it is precipitated as
Mg(NH4)PO4 . 6 H2O but weighed as Mg2P2O7
Chapter 7 – Gravimetric Methods
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Table 7-2 page 133 shows a few organic precipitating
agents; they are frequently more specific for a limited
number of ions, as is shown dimethylglyoxime for the
analysis of N, Pd, Pt. Especially interesting in this list is
sodium tetraphenylborate which is one of a very limited
reagents that precipitate the alkali and pseudo alkali metal
ions.
Table 7-1
Chapter 7 – Gravimetric Methods
Homogeneous Precipitation
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Homogeneous Precipitation the slow generation of the
precipitation reagent by a chemical reaction in situ in low
concentrations which produces purer and larger crystals of the
precipitate. For example, if OH is the precipitating reagent, it
might be prepared homogeneously by the hydrolysis of urea:
(NH2)2CO + H2O  CO2 + 2 NH4+ + 2 OH
Another example is the homogeneous generation of the hydrogen
sulfide by the hydrolysis of thioacetamide.
CH3(CS)NH2 + H2O  CH3(CO)NH2 + H2S(g)
End Friday, Sept 24, 2004
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