Let the

1
Chapter 7

• Let the
• Titrations Begin

2
Titration

• Titration
• A procedure in which one substance (titrant) is
  carefully added to another (analyte) until
  complete reaction has occurred.
• The quantity of titrant required for complete
  reaction tells how much analyte is present.
• Volumetric Analysis
• A technique in which the volume of material
  needed to react with the analyte is measured

3
Titration Vocabulary

• Titrant
• The substance added to the analyte in a titration
  (reagent solution)
• Analyte
• The substance being analyzed
• Equivalence point
• The point in a titration at which the quantity of
  titrant is exactly sufficient for stoichiometric
  reaction with the analyte.

4
Titration Vocabulary

• End point
• The point in a titration at which there is a
  sudden change in a physical property, such as
  indicator color, pH, conductivity, or absorbance.
  Used as a measure of the equivalence point.
• Indicator
• A compound having a physical property (usually
  color) that changes abruptly near the equivalence
  point of a chemical reaction.

5
Titration Vocabulary

• Titration error
• The difference between the observed end point and
  the true equivalence point in a titration
• Blank Titration
• One in which a solution containing all reagents
  except analyte is titrated. The volume of
  titrant needed in the blank titration should be
  subtracted from the volume needed to titrate
  unknown.

6
Titration Vocabulary

• Primary Standard
• A reagent that is pure enough and stable enough
  to be used directly after weighing. Then entire
  mass is considered to be pure reagent.
• Standardization
• The process whereby the concentration of a
  reagent is determined by reaction with a known
  quantity of a second reagent.

7
Titration Vocabulary

• Standard Solution
• A solution whose composition is known by virtue
  of the way it was made from a reagent of known
  purity or by virtue of its reaction with a known
  quantity of a standard reagent.
• Direct Titration
• One in which the analyte is treated with titrant,
  and the volume of titrant required for complete
  reaction is measured.

8
Titration Vocabulary

• Back Titration
• One in which an excess of standard reagent is
  added to react with analyte. Then the excess
  reagent is titrated with a second reagent or with
  a standard solution of analyte.

9
Titration Calculations

• Titration Calculations rely heavily on the
  ability to perform stoichiometric calculations.
• Examples

10
Titration Calculation Examples

• How many milligrams of oxalic acid dihydrate,
  H2C2O4 . 2H2O (FM 126.07), will react with 1.00
  mL of 0.0273 M ceric sulfate (Ce(SO4)2) if the
  reaction is
• H2C2O4 2Ce4 ? 2CO2 2Ce3 2H

11
Titration Calculation Examples

• A mixture weighing 27.73 mg containing only FeCl2
  (FM 126.75) and KCl (FM 74.55) required 18.49
  mL of 0.02237 M AgNO3 for complete titration of
  the chloride. Find the mass of FeCl2 and the
  weight percent of Fe in the mixture.

12
Titration Calculation Examples

• The calcium content of urine can be determined by
  the following procedure
• Step 1. Ca2 is precipitated as calcium oxalate
  in basic solution
• Ca2 C2O42- H2O ? Ca(C2O4) . H2O(s)
• Step 2. After the precipitate is washed with
  ice-cold water to remove free oxalate, the solid
  is dissolved in acid, which gives Ca2 and H2C2O4
  in solution.
• Step 3. The dissolved oxalic acid is heated at
  60oC and titrated with standardized potassium
  permanganate until the purple end point of the
  following reaction is observed
• 5H2C2O4 2MnO4- 6H ? 10CO2 2Mn2 8H2O

13
Titration Calculation Examples

• Standardization Suppose that 0.3562 g of
  Na2C2O4 is dissolved in a 250.0 mL volumetric
  flask. If 10.00 mL of this solution require
  48.36 mL of KMnO4 solution for titration, what is
  the molarity of the permanganate solution?
• Calcium in a 5.00-mL urine sample was
  precipitated, was redissolved, and then required
  16.17 mL of standard MnO4- solution. Find the
  concentration of Ca2 in the urine

14
Titration Calculation Examples

• A solid mixture weighing 1.372 g containing only
  sodium carbonate and sodium bicarbonate required
  29.11 mL of 0.734 M HCl for complete titration
• Na2CO3 HCl ? 2NaCl(aq) H2O CO2
• NaHCO3 HCl ? NaCl(aq) H2O CO2
• Find the mass of each component of the mixture.

15
Titration Calculations

• Kjeldahl Nitrogen Analysis
• Digest the organic compound in boiling H2SO4 to
  convert Nitrogen to NH4
• Hg, Cu, or Se will catalyze the digestion process
• Raise the b.p. of H2SO4 (338oC) by adding K2SO4
  to increase the rate of the reaction

16
Titration Calculations

• Kjeldahl Nitrogen Analysis
• Treat the ammonium compound with base and distill
  as ammonia into a standard acidic solution
• NH4 OH- ? NH3(g) H2O
• NH3 H ? NH4
• The moles of acid consumed equal the moles of NH3
  liberated
• H (HCl) OH-(NaOH) ? H2O

17
Titration Calculation Examples

• A typical protein contains 16.2 wt nitrogen. A
  0.500-mL aliquot of protein solution was
  digested, and the liberated ammonia was distilled
  in 10.00 mL of 0.02140 M HCl. The unreacted HCl
  required 3.26 mL of 0.0198 M NaOH for complete
  titration. Find the concentration of protein (mg
  protein / mL) in the original sample.

18
Spectrophotometric Titrations

• One in which absorption of light is used to
  monitor the progress of the chemical reaction

19
Spectrophotometric Titration Example

• 2Fe3 Apotransferrin ? (Fe3)2transferrin
• Apotransferrin is clear
• (Fe3)2transferrin is red
• Absorption increases as a result of red from Iron
  attachment to the apotransferrin
• Saturated protein no further color change
  absorption levels off
• Extrapolated intersection is the equivalence point

20
Spectrophotometric Titration Example

• Construction of Graph
• When constructing the graph of the absorbance
  values versus the concentration of Fe3 you must
  take in account the dilution factor
• Corrected absorbance (total Volume / initial
  Volume)(observed absorbance)

21
Corrected Absorbance Calculations

• The absorbance measured after adding 125 ?L of
  ferric nitrilotriacetate to 2.000 mL of
  apotransferrin was 0.260. Calculate the
  corrected absorbance that should be plotted.

22
Titration Curve

• Titration Curve is a result of plotting pX
  (pX-log X) versus the concentration of the
  titrant
• Precipitation Titration
• Concentration of analyte, concentration of
  titrant and the Ksp influence the sharpness of
  the endpoint
• Acid-Base reaction and Oxidation-Reduction
  reactions
• Have to calculate the theoretical endpoint to
  determine the indicator to be used

23
Precipitation Titration Curve

• Example AgI(s) formation
• Ag I- ? AgI(s)
• Reverse of the Ksp reaction for the dissociation
  of AgI(s) so the K for this reaction equal
  1/KspAgI(s) 1.2 X 1016
• Large K value means that the equilibrium lies to
  the right for this reaction
• Each aliquot addition of Ag titrant reacts
  virtually entirely to form AgI(s)

24
Precipitation Titration Curve

• Titration Curves typically exhibit 3 distinct
  regions for a single titrant reacting with a
  single analyte.
• Before the Equivalence Point
• At the Equivalence Point
• After the Equivalence Point

25
Precipitation Titration Curve

• Before the Equivalence Point
• Example of AgI(s) formation
• Most of the Ag reacts entirely to give AgI
• pAg
• the amount of Ag left in solution
• The amount of I- present
• From Ksp determine the amount of Ag present

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Precipitation Titration Curve

• At the equivalence point
• Stoichiometric amount of Ag as I- so all has
  precipitated out as AgI(s)
• Regular Ksp calculations

27
Precipitation Titration Curve

• After Equivalence Point
• Ag is determined by the Ag present after the
  equivalence point with the dilution factor taken
  into account

28
Precipitation Titration Curve Equations

• Calculate the Volume of Titrant needed to reach
  equilibrium
• MTVT (Mole ratio)MAVA
• Before the Equivalence Point
• A (Fraction of Titrant Remaining)(M of
  Analyte)(Original Volume of solution / Total
  Volume of solution)
• T Ksp / A
• pT -logT

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Precipitation Titration Curve Equations

• At Equivalence Point
• AT Ksp
• Calculate like in section 6-3
• After the Equivalence Point
• T (Original Concentration of Titrant)(Volume
  of excess Titrant / Total Volume of Solution)
• pT -logT

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Precipitation Titration Curve Example

• 25.00 mL of 0.1000 M I- was titrated with
  0.05000M Ag.
• Ag I- ? AgI(s)
• The solubility product for AgI is 8.3 x 10-17.
  Calculate the concentration of Ag ion in
  solution
• (a) after addition of 10.00 mL of Ag
• (b) after addition of 52.00 mL of Ag
• (c) at the equivalence point.

31
Precipitation Titration Curve Example

• 25.00 mL of 0.04132 M Hg2(NO3)2 was titrated with
  0.05789 M KIO3.
• Hg22 2IO3- ? Hg2(IO3)2(s)
• The solubility product for Hg2(IO3)2 is 1.3 x
  10-18. Calculate the concentration of Hg22 ion
  in solution
• (a) after addition of 34.00 mL of KIO3
• (b) after addition of 36.00 mL of KIO3
• (c) at the equivalence point.

32
Precipitation Titration Curve Example

• Consider the titration of 50.00 mL of 0.0246 M
  Hg(NO3)2 with 0.104 KSCN. Calculate the value of
  pHg22 at each of the following points and sketch
  the titration curve
• 0.25VT
• 0.5VT
• 0.75VT
• 1.05VT
• 1.25VT

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Titration Curve Shape

• 11 Stoichiometry of Reagents
• Equivalence point is the steepest point of a
  curve
• Maximum slope
• An inflection point
• Other Stoichiometries
• The curve is not symmetric about the equivalence
  point
• The equivalence point is not at the center of the
  steepest section of the curve
• The less soluble the product, the sharper the
  curve around the equivalence point

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Titration Curve Shape
35
Titration of a Mixture

• The less soluble product forms first
• If there is sufficient difference in solubility
  of products
• First precipitation is nearly complete before the
  second one begins
• Separation by precipitation (section 6-5)
• Coprecipitation
• Alters the expected endpoints

36
Titration of a Mixture Example
37
Chapter 7 - Homework

• Problems 2, 4, 8, 11, 12, 18, 22, 23, 28