How do concentration of analyte and titrant vary during the titrationTitration curve

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Titration Curves (Precipitation)

• How do concentration of analyte and titrant vary
  during the titration?--Titration curve
• Understand the chemistry during the titration
• What parameters control the quality of the
  titration
• What could affect the titration?
• pH
• Ksp

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Titration Curves

• The titration curve is a plot of how the
  concentration of a reactant varies with titrant
• We usually plot this as a power (p) function
  because of the wide concentration ranges
  encountered
• A titration curve has three distinct regions
  before, at, and after the equivalence point

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Titration Curves

• If you titrate 25.00 mL of 0.1000 M I- with
  0.05000 M Ag and monitor Ag
• This is the reverse of the equilibrium
• Because K for the titration reaction is large
  (1/Ksp 1.2x1016), products are favored, so wed
  expect Ag to be nearly used up during the
  titration until the equivalence point when Ag
  will increase strongly
• The expected equivalence volume of titrant Ve

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Before the Equivalence Point

• What is Ag in flask once 10.00 mL of the Ag
  titrant have been added?
• The initial mol of I- was
• The current mol of I- is (11 rxn with Ag)
• Concentration of Ag in equilibrium

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Shortcut Calculation

• You can reduce the complexity of calculations a
  bit by using a shortcut to calculate I-
• When 10 mL of the Ve (50 mL) have been added
• I- is simply a function of the portion of the
  titration left to go and how much the solution
  has been diluted.

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Example (Before Equivalence)

• Calculate pAg when VAg is 49.00 mL
• Even with the titration 98 complete, the
  concentration of Ag is negligible

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At the Equivalence Point

• At the equivalence point, we have added the exact
  amount of Ag needed to react with I-
• pAg is found by solving for x from Ksp

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After the Equivalence Point

• After equivalence, pAg is simply determined by
  calculating the excess Ag
• If weve added 52.00 mL of Ag solution (77.00
  total mL)

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Shortcut Calculation

• Again, you can shorten the process a bit
• 2.00 mL of excess Ag have been added
• Ag is simply a function of how much excess has
  been added and the dilution factor

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Shape of the Titration Curve
If all the data is plotted, you get a
titration curve that looks similar to this. The
equivalence point is the steepest point of the
curve (1st derivative) and an inflection point
(2nd derivative0). This is always true for
systems with 11 stoichiometry for the reactants,
whether it is a precipitation, acid-base,
compexation, or redox titration. With other
stoichiometries, the curve will not be symmetric
at the equivalence point. When we set
conditions for a titration, we try to make the
steepest part of the curve representative of the
equivalence point.
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Effect of Ksp on the Curve
The less soluble the product of a
precipitation titration, the steeper the
titration curve will be at the equivalence point.
We have here the titration curves for Ag with
three halides, I-, Br-, and Cl-. Each
titration curve is for a solution of 25 mL of
0.10 M X- titrated with 0.050 M Ag.
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Example

• 25.00 mL of 0.04132 M Hg2(NO3)2 was titrated with
  0.05789 M KIO3
• Ksp for Hg2(NO3)2 is 1.3x10-18
• Calculate the concentration of Hg22 in solution
  (a) after addition of 34.00 mL KIO3, (b) after
  addition of 36.00 mL KIO3, and (c) at the
  equivalence point

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Step 1

• Volume of iodate needed to reach the equivalence
  point (Ve)

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Example A

• If V34.00 mL, Hg22 precipitation is not complete

Conci
Frxn Remaining
Dilution Factor
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Example B

• If V36.00 mL, the precipitation is complete and
  we have gone past the equivalence point

Dilution Factor
Conci
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Example C

• At the equivalence point, there is exactly enough
  IO3- to react with Hg22

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Titration of a Mixture

• With a mixture of two ions that are titrated, the
  less soluble precipitate will form first
• When the equivalence point of the less soluble
  precipitate is obtained, the concentration of
  titrant suddenly increases and then the more
  soluble salt begins to precipitate
• Therefore, we should observe 2 breaks in the
  titration curve, one corresponding to each
  equivalence point

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Titration Curve of Mixture
Titration curve for AgNO3 vs. I- and Cl-.
AgI precipitates first while Cl- remains in
solution. As Ag increases, than AgCl
precipitates. The system used 0.0502 M KI
and 0.050 M KCl with 0.0845M AgNO3 titrant.
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Titration of a Mixture

• Comparing the mixture curve to the pure curves
  for I- and Cl-, we find the endpoint for AgI is
  slightly higher (more volume) for the mixture.
• Random error (this could be positive or negative)
• Coprecipitation (systematically positive)
• In coprecipitation some Ag precipitates with Cl-
  early, carried out on the surface of AgI

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Technique to Monitor Titration
The technique used to monitor the mixture
titration of Ag with I- and Cl- was
electrochemistry. An electrode specific to Ag
was used to monitor Ag The principles of
electrochemistry and electrode operation will be
discussed in Chs. 14-15. As discussed, this is
just one method of monitoring a titration.
Spectroscopy is another.
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Detection of End Point

• Methods for determining the end point for
  precipitation titrations
• Indicators
• Electrodes (Ch. 15)
• Light scattering
• Well discuss indicators used for the Ag Cl-
  titration equilibrium
• Titrations with Ag are called argentometric
  titrations

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Indicators

• Volhard titration
• Uses the formation of a soluble colored complex
  at the end point
• Fajans titration
• A colored indicator is adsorbed onto the
  precipitate at the end point

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Volhard Titration

• Used as a procedure for titrating Ag
  determination of Cl- requires a back-titration
• First, Cl- is precipitated by excess AgNO3
• Excess Ag is titrated with KSCN in the presence
  of Fe3
• When Ag has been consumed, a red complex forms
• The Volhard titration can be used for any anion
  that forms an insoluble salt with silver

red
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Fajans Titration
The technique uses an adsorption indicator.
Prior to the equivalence point, there is excess
Cl- in solution. Some is adsorbed on the surface
of the crystal, giving a partial negative charge.
After the equivalence point, there is excess Ag
in solution. Some adsorbs to the surface
imparting a partial positive charge to the
precipitate. Choosing an indicator with a
partial negative charge will cause it to adsorb
to the surface.
Dichlorofluorescein is green in solution but pink
when absorbed on AgCl
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Common Precipitation Titrations
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Light Scattering Detection

• In a precipitation reaction, the mixture gets
  cloudier
• Particles in solution scatter light (turbidity).
• Turbidity ceases to change after the equivalence
  point because no more precipitate is formed.
• Scattering is particle-size dependent, so
  conditions much be very well controlled
• Methods
• Turbidimetry-measures transmittance of light
• Nephelometry-measures light scattering