TMHsiung2007 142

1
Chapter 07 Let the Titrations Begin
2
Contents in Chapter07

• Overview of Titrimetry
• Grades of Chemicals
• Titration Calculations
• Precipitation Titrations
• 1) Titration curve
• 2) Argentometric titration
• Establish Precipitation Titration Curves with a
  Spreadsheet

3

• Overview of Titrimetry
• Define Titration
• Titrations (or titrimetric method) are based on
  measuring the amount of a reagent of known
  concentration that reacts with the unknown.
• A general equation can be expressed
• aA tT ? products
• A analyte
• T titrant

4

• Revisiting Keywords of Titration
• Equivalence point The point (e.g., volume of
  titrant) in a titration where (theoretically)
  stoichiometrically equivalent amounts of analyte
  and titrant react.
• Indicator A colored compound whose change in
  color signals the (experimental) end point of a
  titration.
• End point The point (e.g., volume of titrant) in
  a (experimental) titration where we stop adding
  titrant in an experiment.
• Titration error The determinate error in a
  titration due to the difference between the end
  point and the equivalence point.

5

• Type of Titrations based on Chemical Reactions
• Acid-Base Titrations, example
• H OH ? H2O K 1/Kw
• Precipitation Titrations, example
• Ag(aq) Cl(aq) ? AgCl(s) K1/Ksp
• Redox Titrations
• 5 H2O2 2 MnO4 H ? 5 O2 2 Mn2 8H2O
• Complexometric Titrations, example
• EDTA Ca2 ? (CaEDTA)2

6

• Type of Titrations based on Measuring Techniques
• Volumetric titrimetry Measuring the volume of a
  solution of a known concentration (e.g., mol/L)
  that is needed to react completely with the
  analyte.
• Gravimetric (weight) titrimetry Measuring the
  mass of a solution of a known concentration
  (e.g., mol/kg) that is needed to react completely
  with the analyte.
• Coulometric titrimetry Measuring total charge
  (current x time) to complete the redox reaction,
  then estimating analyte concentration by the
  moles of electron transferred.

7
5) Type of Titration Curves
8

• 2. Grades of Chemicals
• Terms and Definitions
• Reagent Grade The reagents which meets or
  surpasses the latest American Chemical Society
  specifications.
• Primary standard The reagent which is ready to
  be weighted and used prepare a solution with
  known concentration (standard).
• Requirements of primary reagent are
• - Known stoichiometric composition
• - High purity
• - Nonhygroscopic
• - Chemically stable both in solid and solution
• - High MW or FW

9

• Secondary standard A standard which is
  standardized against a primary standard.
• Certified reference materials (CRM) A reference
  material, accompanied by a certificate, which has
  been analysed by different laboratories to
  determine consensus levels of the analyte
  concentration.
• NIST Standard Reference Material (SRM) A CRM
  issued by NIST that also meets additional
  NIST-specific certification criteria and is
  issued with a certificate.
• Standardization The process by which the
  concentration of a reagent is determined by
  reaction with a known quantity of a second
  reagent

10

• Titration Calculations
• Terms and Definitions
• Blank Titration Titration procedure is carried
  out without analyte (e.g., a distilled water
  sample). It is used to correct titration error.
• Back titration A titration in which a (known)
  excess reagent is added to a solution to react
  with the analyte. The excess reagent remaining
  after its reaction with the analyte, is
  determined by a titration.

11
2) Standardization
Example To standardizing a KMnO4 stock solution,
the primary standard of 9.1129 g Na2C2O4 is
dissolved in 250.0 mL volumetric flask. 10.00 mL
of the Na2C2O4 solution require 48.36 mL of KMnO4
to reach the titration end point. What is the
molarity (M) of MnO4 stock solution? (FW Na2C2O4
134.0)
Solution
5C2O42(aq) 2MnO4(aq) 16H(aq) ? 10CO2(g)
Mn2(aq) 8H2O(l)
Ans
12
3) Unknown Analysis with a Blank Correction
Example A 0.2865 g sample of an iron ore is
dissolved in acid, and the iron is converted
entirely to Fe2. To titrate the resulting
solution, 0.02653 L of 0.02250 M KMnO4 is
required. Also a blank titration require 0.00008
L of KMnO4 solution. What is the Fe (w/w) in
the ore? (AW Fe 55.847)
Solution MnO4(aq) 5Fe2 8H(aq) ? Mn2(aq)
5Fe3 4H2O(l)
Ans
13
4) Back Titration
Solution mL x M mmol Total mmol Ag added
(mmol Ag consumed by SCN) (mmol Ag consumed
by H3AsO4)
14
Ans
15

• Kjeldahl Analysis for Total Nitrogen (TN)
• i) KD description

16
Example A typical meat protein contains 16.2
(w/w) nitrogen. A 0.500 mL aliquot of protein
solution was digested, and the liberated NH3 was
distilled into 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. Solution
17
Ans
18
6) Titration of a Mixture
19
Ans
Ans
20

• Precipitation Titrations
• A titration in which the reaction between the
  analyte and titrant involves a precipitation.
• 1) Titration curve
• i) Guidance in precipitation titration
  calculation
• Find Ve (volume of titrant at equivalence point)
• Find y-axis values
• - At beginning
• - Before Ve
• - At Ve
• - After Ve

21

• Example For the titration of 50.0 mL of 0.0500 M
  Cl with 0.100 M Ag. The reaction is
• Ag(aq) Cl(aq) ? AgCl(s) K 1/Ksp
  1/(1.81010) 5.6 x 109
• Find pAg and pCl of Ag solution added
• 0 mL (b) 10.0 mL (c) 25.0 mL (d) 35.0 mL
• Solution

22
(a) 0 mL Ag added (At beginning) Ag 0, pAg
can not be calculated. Cl 0.0500, pCl 1.30
(b) 10 mL Ag added (Before Ve)
23
(c) 25 mL Ag added (At Ve) AgCl(s) ? Ag(aq)
Cl(aq) Ksp 1.81010 s
AgCl Ksp 1.81010 s2 AgCl1.3
5x105 pAg 4.89 pCl 4.89
24
(d) 35 mL Ag added (After Ve)
25
ii) Construct a titration curve Example
Titration of 50.0 mL of 0.0500 M Cl with 0.100 M
Ag
26
iii) End point determination
27
iv) Diluting effect of the titration curves
25.00 mL 0.1000 M I titrated with 0.05000 M Ag
25.00 mL 0.01000 M I titrated with 0.005000 M
Ag
25.00 mL 0.001000 M I titrated with 0.0005000 M
Ag
28
v) Ksp effect of the titration curves
25.00 mL 0.1000 M halide (X) titrated with
0.05000 M Ag
29
vi) Titration of a mixture (uncertainty
concerned)

• 40.00 mL of 0.0502 M KI 0.0500 M KCl, titrated
  with 0.0845 M Ag
• 20.00 mL of 0.1004 M KI titrated with 0.0845 M
  Ag

30
Example A 25.00 mL solution containing Br and
Cl was titrated with 0.03333 M AgNO3.
Ksp(AgBr)5x1013, Ksp(AgCl)1.8x1010.

• Which analyte is precipitated first?
• The first end point was observed at 15.55 mL.
  Find the concentration of the first that
  precipitated (Br or Cl?).
• The second end point was observed at 42.23 mL.
  Find the concentration of the second that
  precipitated (Br or Cl?).

Solution
(a) Ag(aq) Br(aq) ? AgBr(s) K 1/Ksp(AgBr)
2x1012 Ag(aq) Cl(aq) ? AgCl(s) K
1/Ksp(AgCl) 5.6x109 Ans AgBr precipitated first
31
(b)
Ans
(c)
Ans
32

• Argentometric Titration
• General information
• Define Argentometric Titration A precipitation
  titration in which Ag is the titrant.
• Argentometric Titration classified by types of
  End-point detection
• Volhard method A colored complex (back
  titration)
• Fajans method An adsorbed/colored indicator
• Mohr method A colored precipitate

33
2) Volhard method A colored complex (back
titration). Analysing Cl for example
Step 1 Adding excess Ag into sample Ag Cl
? AgCl(s) left Ag Step 2 Removing AgCl(s) by
filtration/washing Step 3 Adding Fe3 into
filtrate (i.e., the left Ag) Step 4 Titrating
the left Ag by SCN Ag SCN ?
AgSCN(s) Step 5 End point determination by red
colored Fe(SCN)2 complex. (when all Ag has been
consumed, SCN reacts with Fe3) SCN Fe3 ?
Fe(SCN)2(aq)
Total mol Ag (mol Ag consumed by Cl)
(mol Ag consumed by SCN)
34

• Fajans Method An adsorbed/colored indicator.
  Titrating Cl and adding
• dichlorofluoroscein for example

Cl
Before Ve (Cl excess) Greenish yellow solution
AgCl(s)
1st layer
Ag
In pink
AgCl(s)
After Ve (Ag excess)
1st layer
35
4) Mohr Method A colored precipitate formed by
Ag with anion, other than analyte, once the Ve
reached. Analysing Cl and adding CrO42 for
example
Precipitating Cl Ag Cl ? AgCl(s) Ksp 1.8
x 1010 End point determination by red colored
precipitate, Ag2CrO4(s) 2Ag CrO42 ?
Ag2CrO4(s) Ksp 1.2 x 1012
36
5) Applications of argentometric titrations
37

• Establish Precipitation Titration Curves with a
  Spreadsheet
• Example
• M(aq) X(aq) ? MX(s) K 1/Ksp

Titrant
Analyte
???
???
???
???
???
???
???
???
???
???
???
???
???
???
38
1) Derive the equation
moles of M in precipitate
moles of X in precipitate
Mass balance The moles of an element in all
species in a mixture equal to the total moles of
that element delivered to the solution.
39
2) Apply the equation
40

• Example Construct a titration curve for the
  titration of 50.0 mL of 0.0500 M Cl with 0.100 M
  Ag. (Ksp of AgCl 1.81010)

41
EXCEL 961_Ch07_Titration_X_with_Ag
42
Homework Problem 7-30/p.139, Due
2007/11/7 ?????????? Examples All Exercise A-E
, G Problems 1-7, 11-14, 31, 36
End of Chapter07