BKF1243 ANALYTICAL CHEMISTRY SEM 2 SESSION 20072008

1
BKF1243 ANALYTICAL CHEMISTRYSEM 2 SESSION
2007/2008

• ANALYSIS METHODS TITRIMETRY

2
CONTENTS

• Overview of Titrimetry
• Acid-Base Titrimetry
• Complexation Titrimetry
• Redox Titrimetry
• Precipitation Titrimetry

3
OVERVIEW
Overall Concept

• Identical definition of titrimetry any method
  in which the signal is a volume or change in
  volumes.
• Seven conceptual particulars in titrimetry
  titration, titrant, analyte, indicator,
  equivalence point, end point and titration error.

4
OVERVIEW
Equivalence and End Points

• Accurate titrations must achieve stoichiometric
  chemical reactions. (Example?)
• Amounts of titrant and analyte must be
  equivalent.
• Equivalence point point that stoichiometric
  mixture of both titrant and analyte is achieved,
  which its determination is as follows
• Moles of titrant Veq x CT, where
• Veq equivalence point volume
• CT concentration of titrant
• With that equation, moles of analyte can be
  determined.
• End point point that the adding process titrant
  is stopped. (Why?)
• Indicators added in analyte solution detects both
  points. (How?)
• Titration error obvious difference between both
  equivalence and end points. (Can both equivalence
  and end points overlap together? If overlap, what
  is that meant?)

5
OVERVIEW
Using Volume as a Signal

• Volumes of both titrant and analyte are
  considered in any analysis involving titrimetry.
• Mass of solution can replace its volume for
  measurement. (How?)
• Conditions involving titrimetric analysis
• Stoichiometry of all reactions involving titrant
  and analyte must be known.
• Titration reactions must occur rapidly at
  observable rate.
• A suitable titrimetric method must be available
  for determining end point with an acceptable
  level of accuracy (minimize titration errors).

6
OVERVIEW
Using Volume as a Signal

• 4 fashionable titrations are as follows
• Direct titration example for analyte Ag
• Ag SCN- (with Fe3) ? AgSCN
• Back titration example for analyte H2CO
• H2CO 3OH- I3- ? HCO2- 3I- 2H2O (1st)
  too slow
• I3- (excess) 2S2O32- ? S4O62- 3I- (2nd)
• Displacement titration example for analyte
  Ca2
• Ca2 EDTA4- ? CaEDTA2- (1st) not obvious
• Ca2 MgEDTA2- ? CaEDTA2- Mg2 (2nd)
• Mg2 EDTA4- ? MgEDTA2- (3rd)
• Indirect titration example for analyte S
• S O2 ? SO2
• SO2 H2O2 ? H2SO4
• H2SO4 2OH- ? SO42- 2H2O

7
OVERVIEW
Types of Titrimetric Methods

• Acid-base titrimetry a titrimetric method in
  which an acidic titrant reacts with basic analyte
  or vice versa.
• Complexation titrimetry - a titrimetric method
  that involves metal-ligand complexation reaction
  to obtain analyte.
• Redox (reduction-oxidation) titrimetry a
  titrimetric method that uses either an oxidizing
  or reducing agent as titrant.
• Precipitation titrimetry a titrimetric method
  in which both titrant and analyte react together
  to form a precipitate.

8
OVERVIEW
Titration Curves

• Titration curve of acid-base titrations.

9
OVERVIEW
Titration Curves

• Titration curves of complexation (top-left),
  redox (top-right), precipitation (bottom-left)
  and thermometric (bottom-right) titrations.

10
ACID-BASE TITRIMETRY
Concept

• Main practice neutralization.
• At first, determination of acidity or alkalinity
  of solutions and purity of carbonates and
  alkaline earth oxides.
• Equivalence and end points are marked by the
  colour changing of indicators, but the accurate
  determination of analysis results is difficult to
  obtain.

11
ACID-BASE TITRIMETRY
Acid-Base Titration Curves

• In acid-base titration curves, pH level
  characterizes equivalence point, either analyte
  or titrant.
• End point corresponded by pH level may or may not
  overlap with equivalence point.
• Relationship between equivalence and end points
  should be understood by knowing pH changes during
  acid-base titrations and constructing acid-base
  titration curves. (How to construct?)

12
ACID-BASE TITRIMETRY
Acid-Base Titration Curves

• 2 titration curves will be considered and
  constructed as follows
• 50.00 ml 0.100 M hydrochloric acid (what is its
  pH?) is titrated by 0.200 M sodium hydroxide.
• 50.00 ml 0.100 M acetic acid (what is its pH?) is
  titrated by 0.100 M potassium hydroxide.
• Generally, for 1st case, the equilibrial reaction
  is H3O OH- ? 2H2O (For 2nd case?)
• Generally also, moles of acid moles of base, or
  MaVa MbVb. (How to relate this equation to Veq
  (volume required to achieve equivalence point)?)

13
ACID-BASE TITRIMETRY
Acid-Base Titration Curves

• In 1st case, if 10.00 ml of NaOH is added to HCl
  solution, the current excess HCl concentration
  should be calculated using following formula
• HCl moles excess HCl MaVa MbVb
• total volume Va Vb
• (How to calculate and what its current pH?)
• At equivalence point, both moles of NaOH and HCl
  are equal and its pH is determined. (How to
  determine and what its pH result? Can end point
  be considered here?)
• When NaOH is added after the equivalence point,
  use the same formula above, but replace HCl with
  NaOH to calculate the current excess NaOH. (How
  to calculate and what its current pH?)
• Construct the titration curve of this titration
  from calculated pH results and additional titrant
  volumes (which one acts as titrant?).

14
ACID-BASE TITRIMETRY
Acid-Base Titration Curves

• For 2nd case, Veq is determined first using the
  same formula (What is its value?), following by
  calculating pH of acid and its equilibrium
  constant (Which formula is used? Why pH of acid
  should be calculated this way? Why equilibrium
  constant should be calculated also? What are
  their values?).
• Using the same formula, the current acetate and
  excess CH3COOH concentrations are calculated
  after 10.00 ml of KOH is added to CH3COOH
  solution (What are their current concentration
  and pH values?).
• Acetate concentration and its pH are also
  calculated at equivalence point. (How to
  determine and what its pH result? If different
  with 1st case, why?)
• When KOH is added after the equivalence point,
  use the same formula as in 1st case, but replace
  CH3COOH with KOH to calculate the current excess
  KOH. (How to calculate and what its current pH?)
• Construct the titration curve of this titration
  from calculated pH results and additional titrant
  volumes (which one acts as titrant?).

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ACID-BASE TITRIMETRY
Acid-Base Titration Curves

• The actual titration curve for 2nd case is as
  shown below
• In order to evaluate relationship between 2
  titration points (what are the points?), the
  related titration curves should be sketched (what
  is its objective?).

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ACID-BASE TITRIMETRY
Acid-Base Titration Curves

• Sketching titration curves during its
  construction is important for these following
  cases (Why?)
• Polyprotic acids titrated by strong base (right
  figures).
• Polyprotic bases titrated by strong acid.
• Polyprotic acids titrated by polyprotic bases or
  vice versa.
• Weak bases titrated by strong acid.
• Weak acids titrated by weak bases or vice versa.
• Number of equivalence points increases with the
  number of ions present from a molecule of
  polyprotic acids or bases.

17
ACID-BASE TITRIMETRY
Select and Evaluate End Point

• Equivalence point(s) of titration process(es) can
  also be marked in its constructed curves without
  doing calculations for it. (Why?)
• Equivalence point(s) is/are always located in
  between the inflection points in all titration
  curves. (What are inflection points and where are
  they located? Are they detectable?)

18
ACID-BASE TITRIMETRY
Select and Evaluate End Point

• End point(s) of titration process(es) can be
  found through 3 optional methods
• Visual Indicators (Are indicators take part in
  titration process?) as shown below
• Monitoring pH of titrated solution (4 types
  normal, 1st derivative d(pH)/dV, 2nd derivative
  d2(pH)/(dV)2 and Gran plot) as shown at
  following page
• Monitoring temperature (using thermometric
  titration curves) as shown at right

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ACID-BASE TITRIMETRY
Select and Evaluate End Point

• Left figure represents monitoring pH and right
  figure represents monitoring T in a titration
  process.

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ACID-BASE TITRIMETRY
Titrations in Non-aqueous Solvents

• An approach for titration using non-aqueous
  solvents (instead of water) for example solvent
  SH that autopyrolyzes (dissociates) as follows
• 2SH ? SH2 S-
• with Ks (instead of Kw) SH2 S-
• Is the determination of pH using following
  formula
• pH -log SH2 (general) and
• pHneut 0.5pKs (neutral solvent)
• Continuously, concentrations of H3O (for 90
  completion) and OH- (for 110 completion) those
  obtain pH and pOH values are also calculated.

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ACID-BASE TITRIMETRY
Titrations in Non-aqueous Solvents

• Example related An acid-base titration using
  water (a) and the same titration using a
  non-aqueous solvent (b), which obtain different
  results as shown figures. (Why?).
• Another related factor the favourable of ions
  dissociated to the solvent, which modifies the
  strength of the related titrated solution. (How?)
• Example related
• 2 equal amounts of acetic acid is dissolved in 2
  different solvents water and pure liquid
  ammonia. Which solvent gives the acetic acid
  stronger?
• 2 equal amounts of ammonia is dissolved in 2
  different solvents water and pure glacial acetic
  acid. Which solvent gives the ammonia stronger?

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ACID-BASE TITRIMETRY
Applications

• Quantitative applications of acid-base
  titrimetry
• Titrant selection and standardization (Table
  9.7)(Why and how?)
• Inorganic analysis determination of acidity,
  alkalinity and free CO2 in water and waste-water
  analyses, as shown in following figures (a) NaOH
  HCl, (b) Na2CO3 HCl and (c) NaHCO3 HCl and
  Table 9.8. (Which one acts as titrant?)
• Organic analysis Kjeldahl method in
  biochemical, agricultural, pharmaceutical and
  environmental laboratories. (Tables 9.9 and 9.10)
• Quantitative calculations
• Qualitative applications - determination of
  mixture of ionic species in a sampling solution
  by titration using either strong acid or strong
  base with mixing suitable visual indicators as
  end point detection. (Table 9.11)
• Characterization applications
• Equivalent weights characterize properties of
  analyte.
• Equivalent constants characterize dissociation
  constant in weak acids and weak bases.

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ACID-BASE TITRIMETRY
Evaluating Acid-Base Titrimetry

• How to evaluate effective acid-base titrimetry in
  chemical analysis
• Scale of operation (use various methods and
  equipment/instrument of accurate titrations)
• Accuracy (achievable 0.1 to 0.2 relative
  errors)
• Precision
• Sensitivity k (Vtitrant k x molanalyte)
• Selectivity
• Other related factors (Time, cost, equipment)

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COMPLEXATION TITRIMETRY
Concept

• Second approach of titrimetry.
• In this approach, aqueous sample is titrated
  using suitable metal-ligand complexation reagent.
  
• Historically, the earliest one is the
  determination of CN- and Cl- using respective
  titrants containing Ag and Hg2 those result a
  single stable complex mixture. (What are the
  results?)
• Ligands used in complexation titrimetry are
  classified as follows
• Monodentate ligands (CN-, Cl-)
• Multidentate ligands (EDTA (What is its full
  name?))

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COMPLEXATION TITRIMETRY
Chemistry and Properties of EDTA

• The use of EDTA is common in any analysis of
  metals using complexation titrimetry as its
  analytical approach. (In which group EDTA is?
  What are the chemical properties in EDTA?)
• The stoichiometric ratio of all metal-EDTA
  complexes is 11. (Why?)
• The formation of metal-EDTA complex has its
  constant which is determined as follows
• Kf CdEDTA2- for Cd2 EDTA4- ?
  CdEDTA2-
• Cd2EDTA4-
• EDTA obtains 6 successive pKa values. (Why and
  how?)
• For mass balance consideration involving EDTA,
  CEDTA (total concentration of unbound EDTA)
  combined concentrations of each of EDTAs forms
  (What are those concentrations?)

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COMPLEXATION TITRIMETRY
Chemistry and Properties of EDTA

• To correct Kf that may overestimate the complexs
  stability, EDTAs fraction ?EDTA4- is accounted
  as follows
• EDTA4- ?EDTA4- x CEDTA
• With the accounted fraction,
• Kf CdEDTA2-
• Cd2 x ?EDTA4- x CEDTA
• If the EDTAs pH is fixed with a buffer,
  ?EDTA4- becomes a constant, then is being
  combined together with Kf, obtaining the
  following formula
• Kf ?EDTA4- x Kf CdEDTA2-
• Cd2 x CEDTA
• which Kf conditional formation constant of
  EDTA complex.

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COMPLEXATION TITRIMETRY
Chemistry and Properties of EDTA

• EDTA should be added with a buffering agent to
  maintain its pH, and it must compete with any
  metal-ligand complex formed from any of the
  buffers component. (Example, for analyte Cd2,
  NH3 in NH4/NH3 buffer is the competitor of EDTA.
  How do they compete?)
• NH3 in NH4/NH3 buffer is called auxiliary
  complexing agent, and its effect can be accounted
  as follows
• For mass balance consideration involving Cd2,
  CCd (total concentration of unbound Cd2)
  combined concentrations of each of Cd2s forms
  (What are those concentrations?)
• To correct Kf that may overestimate the complexs
  stability, Cd2s fraction ?Cd2 is accounted as
  follows
• Cd2 ?Cd2 x CCd
• With accounted fraction, EDTAs fixed pH and
  constant NH3 concentration,
• Kf ?EDTA4- x Kf CdEDTA2-
  and
• ?Cd2 x CCd x CEDTA
• Kf ?Cd2 x ?EDTA4- x Kf
  CdEDTA2-/(CCd x CEDTA)
• which Kf new conditional formation constant
  of EDTA complex, and ?Cd2 can be replaced with
  ?M(n) which Mn represents any of ionic metals.

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COMPLEXATION TITRIMETRY
Complexometric EDTA Titration Curves

• Titration of 50.00 ml 0.005 M Cd2 with 0.01 M
  EDTA at pH 10.00 with additional 0.01 M
  ammonia. (What are the values of Kf, ?EDTA4-,
  ?Cd2 and Kf?)
• From the main basic titration formula M1V1
  M2V2, the following formulae are used for whole
  complexometric EDTA analysis.
• MEDTAVEDTA MCdVCd
• CCd (MCdVCd - MEDTAVEDTA)/(VCd VEDTA)
• Cd2 ?Cd2 x CCd
• From the formulae used, pCd value can be
  obtained. At equivalence point,
• CdEDTA2- MCdVCd /(VCd VEDTA)
• From the formula used, values of Kf, CCd (eq)
  and pCd (eq) are obtained. (What are those
  values?)

29
COMPLEXATION TITRIMETRY
Complexometric EDTA Titration Curves

• After equivalence point, CdEDTA2- and CEDTA
  are calculated first in order to obtain CCd and
  Cd2 values. (How to calculate and what are the
  values?)
• The actual titration curve for this EDTA
  complexometric titration is as shown below
• In order to evaluate relationship between 2
  titration points (what are the points?), the
  related titration curves should be sketched.
  (what is its objective?)

30
COMPLEXATION TITRIMETRY
Select and Evaluate End Points

• Equivalence point of complexometric titrations
  occurs when
• CM CEDTA where
• CM total concentration of metal ions (M)
  reacted
• CEDTA total concentration of EDTA reacted
• End point for this titration can overlap
  experimentally with the equivalence point.
  Methods of finding end point are as follows
• Use suitable visual indicators
• Use relevant electrical sensors
• Monitor T (temperature) of titration mixture
• Monitor A (absorbance) of electromagnetic
  radiation by titration mixture.

31
COMPLEXATION TITRIMETRY
Select and Evaluate End Points

• For 1st finding end point method, metallochromic
  indicators are used, based on these factors
• They show colour difference (reacted and
  non-reacted metal ions give different colours).
• Their components consist weak acids or bases.
• Their conditional formation constants depend on
  solutions pH control over their titration
  errors.
• For 2nd method, example, ion-selective electrodes
  that records potentiometric titration curves.
• 4th method is available for already coloured
  solutions containing analyte. This is useful in
  analysis of clinical samples (example blood) and
  environmental samples (example natural waters).

32
COMPLEXATION TITRIMETRY
Select and Evaluate End Points

• Example regarding 4th method, ammonia (pH
  solution adjusting agent) mixed in Cu2 solution.
  Difficulties occurring when 1st method is used to
  determine end point accurately are
• Formation of coloured Cu(NH3)42 complex.
• Other absorbing species present within Cu2
  matrix will also interfere in similar fashion
  (coloured complex ions).
• As an alternative, based on absorption difference
  of electromagnetic radiation by different
  metallic ions, 4th method is used to locate the
  equivalence and end points at a carefully
  selected wavelength.
• For Cu2, 745 nm wavelength is suitable, where
  Cu(NH3)42 complex absorbs strongly, and this
  wavelength is the maximum absorbance.

33
COMPLEXATION TITRIMETRY
Select and Evaluate End Points

• Both Cu(NH3)42 concentration and its absorbance
  decrease when EDTA is still being added, until
  the absorbance value reaches a minimum at the
  equivalence point.
• This resultant is presented in spectrometric
  titration curve, with equivalence point located
  at extrapolative intersection of two linear
  curves.
• To linearize, the following formula is used
• Acorr Ameas x (VCu VEDTA)/VCu where
• Acorr - corrected absorbance
• Ameas - measured absorbance
• VCu - volume of Cu solution
• VEDTA - volume of EDTA

34
COMPLEXATION TITRIMETRY
Applications

• Quantitative applications of complexation
  titrimetry
• Titrant selection and standardization EDTA is a
  versatile titrant for all metallic cations only.
  Unavailable for direct analysis of anions and
  neutral ligands. Other standard titrants used are
  Ag and Hg2 solutions. (Why? How EDTA, Ag and
  Hg2 solutions are prepared?)
• Inorganic analysis determination of hardness
  ions Ca2, CN- and Cl- in water and waste-water
  analyses. These analyses also use pH adjusting
  agents (basic or acidic solutions) and certain
  visual indicators. (How the complexation
  titrimetry plays its role here?)
• Quantitative calculations base on the
  conservation of electron pairs between the ligand
  (as donor) and metal (as acceptor), which obtains
  stoichiometric complexation reactions with 11
  ratio.

35
COMPLEXATION TITRIMETRY
Evaluating Complexation Titrimetry

• How to evaluate the effective complexation
  titrimetry in chemical analysis
• Scale of operation (same as acid-base one)
• Accuracy (achievable 0.1 to 0.2 relative
  errors)
• Precision
• Sensitivity
• Selectivity
• Other related factors (cost, equipment, time)
• Comparing with acid-base one, it is more
  selective. (Why?)
• Spectrometric _at_ spectrophotometric titrations are
  also part of complexation titrimetry, especially
  in case of monitoring absorbance of analytes.

36
REDOX TITRIMETRY
Concept

• Third approach of titrimetry.
• This approach based on simultaneous reduction and
  oxidation reactions.
• History related Quantitative analysis of
  chlorine water (what is its chemical elements?)
  and bleaching powder based on chlorines ability
  to oxidize solutions of indigo dye. (Where is the
  equivalence point for this reaction?)
• Glamorous titrants in this titrimetry are
• Fe2, S2O32- (reducing titrants)
• MnO4-, I2, Cr2O72- (oxidizing titrants)

37
REDOX TITRIMETRY
Redox Titration Curves

• Redox reaction has its own feature
  electrochemical potential.
• For redox titrations, its general titration
  reaction is
• Ared Toxi ? Tred Aoxi, then
• Erxn EToxi/Tred EAoxi/Ared, where
• Ared analyte in reduced state Aoxi analyte
  in oxidized state
• Tred titrant in reduced state Toxi titrant
  in oxidized state
• Erxn electrochemical potential for redox
  reaction
• At equilibrium, EToxi/Tred EAoxi/Ared. So,
  half-reaction can be used to monitor
  electrochemical potential progress for the whole
  redox titration, using Nernst equation for both
  analyte and titrant, as follows
• EAoxi/Ared EAoxiAred ((RT/nF) x ln
  (Ared/Aoxi))
• EToxi/Tred EToxiTred ((RT/nF) x ln
  (Tred/Toxi))
• F electrochemical quantitative amount in
  Faraday
• EAoxiAred, EToxiTred standard-state
  potential of half-reaction _at_ matrix-dependent
  formal potential of half-reaction.

38
REDOX TITRIMETRY
Redox Titration Curves

• For example, titration reaction of cerium
  solution and iron solution in perchloric acid as
  a matrix, which
• Fe2 Ce4 ? Fe3 Ce3
• Same basic formula is used for calculation M1V1
  M2V2, which in this case, MFeVFe MCeVCe,
  following by using Nernst equation. (What are the
  values of VCe, E, Fe2, Fe3, Ce4, Ce3
  and Eeq?)
• The actual titration curve for this redox
  titration is as shown below
• In order to evaluate relationship between 2
  titration points (what are the points?), the
  related titration curves should be sketched.
  (what is its objective?)

39
REDOX TITRIMETRY
Select and Evaluate End Point

• Equivalence point of redox titrations can be
• Symmetry if the stoichiometry of a redox
  titration is symmetrical (1 mole analyte 1 mole
  titrant)
• Asymmetry if the stoichiometry of a redox
  titration is asymmetrical (not symmetrical)
• End point for this titration can overlap
  experimentally with the equivalence point.
  Methods of finding end point are as follows
• Use suitable visual indicators
• Find potentiometrically

40
REDOX TITRIMETRY
Select and Evaluate End Point

• For 1st finding end point method, redox
  indicators (In) are used, based on reaction
  involved
• Inoxi ne- ? Inred
• 3 common redox indicators used are
• MnO4- (purple) ? Mn2 (colourless)
• I3- (colourless for used ? dark blue for excess)
• SCN- (red complex FeSCN2 from Fe3)
• Other redox indicators are in Table 9.18.
• For 2nd finding end point method, a potentiometer
  set (potentiometer unit reference and Pt
  indicator electrodes) is used. (How does
  potentiometer set play its role in redox
  titration?)

41
REDOX TITRIMETRY
Applications

• Quantitative applications of redox titrimetry
• Adjustment of analytes oxidation state the
  analyte must be present initially in single
  oxidation state. Example for iron (Fe2 and Fe3)
  and cerium (Ce3 and Ce4). In this case, all
  Fe3 and Ce3 must be adjusted to Fe2 and Ce4
  using 4 methods
• (a) Auxiliary reducing agent (from which
  cationic component?)
• (b) Auxiliary oxidizing agent (from which
  anionic component?)
• (c) Jones reductor column (What is its content,
  and how it reacts?)
• (d) Walden reductor column (What is its content,
  and how it reacts?)
• Titrant selection and standardization oxidizing
  agents are always used as titrants for majority
  redox reactions, and reducing agents used as
  titrants are limited. (Why? How all those
  oxidizing agents are prepared?)

42
REDOX TITRIMETRY
Applications

• Quantitative applications of redox titrimetry
• Inorganic analysis widely used for analyzing
  inorganic analytes. Examples related are
• (a) Chlorination of public water supplies
• (b) Determinaton of dissolved O2 in public
  health and environmental analyses through
  standard Winkler method
• (c) Determination of H2O in non-aqueous solvents
  using Karl Fischer reagent as titrant. (How this
  reagent plays its role here?)
• Organic analysis applicably used for analyzing
  organic analytes. Examples related are
• (a) Determination of COD in natural waters and
  waste-waters. (How?)
• (b) Pharmaceutical analyses using iodine as
  oxidizing titrant.
• Quantitative calculations base on the
  conservation of electrons donated and accepted
  between both oxidizing and reducing agents, which
  obtains stoichiometric redox reactions. In
  certain cases, a back titration or an indirect
  analysis is also required.

43
REDOX TITRIMETRY
Evaluating Redox Titrimetry

• How to evaluate the effective redox titrimetry in
  chemical analysis
• Scale of operation (same as previous ones)
• Accuracy (achievable 0.1 to 0.2 relative
  errors)
• Precision
• Sensitivity
• Selectivity
• Other related factors (time, cost, equipment)
• It can be extended to the analysis of mixtures,
  for example mixture of Fe2 and Sn2 titrated
  with Ce4 as shown in figure below. (What is its
  reason?)
• Different standard-state potentials of analytes
  will result in a separate individual equivalence
  points.

44
PRECIPITATION TITRIMETRY
Concept

• Fourth approach of titrimetry.
• This approach based on precipitate formation when
  the analyte(s) is/are titrated by certain
  titrants.
• History related
• Quantitative analysis of K2CO3 and K2SO4 in
  potash (the matrix) using Ca(NO3)2 as titrant.
  (which precipitate(s) will form?)
• Analysis of Ag and halide ions.
• End point of this titrimetry is easily detected
  (How?)

45
PRECIPITATION TITRIMETRY
Precipitation Titration Curves

• For example, titration reaction of silver
  solution and chloride solution, which
• Ag Cl- ? AgCl with its Ksp 1.8 x 10-10 and
  K Ksp-1
• Same basic formula is used for calculation M1V1
  M2V2, which in this case, MAgVAg MClVCl.
  (What are the values of VAg, Cl-, pCl, Ag
  and pAg?) All these determination one by one, in
  sequence before, while and after equivalence
  point
• The actual titration curve for this precipitation
  titration is as shown below
• In order to evaluate relationship between 2
  titration points (what are the points?), the
  related titration curves should be sketched.
  (what is its objective?)

46
PRECIPITATION TITRIMETRY
Select and Evaluate End Point

• Methods of finding end point of precipitation
  titrimetry are as follows
• Use suitable visual indicators
• Find potentiometrically
• For 1st finding end point method, 3 types are
  used
• Mohr method (analyze Cl- using Ag)
• Volhard method (analyze Ag using SCN- in
  acidified solution, indicated by Fe3 which forms
  red complex FeSCN2)
• Fajans method (analyze Cl- using Ag indicated by
  anionic dichlorofluoroscein dye (adsorption
  indicator) which changes its colour from greenish
  yellow (adsorbed precipitate with excess Cl-) to
  pink (adsorbed precipitate with excess Ag))
• For 2nd finding end point method, concentration
  changes for both analyte and titrant are
  monitored using an ion-selective electrode
  attached with potentiometer set, then inspect
  visually. (How does potentiometer set play its
  role in redox titration?)

47
PRECIPITATION TITRIMETRY
Applications

• Quantitative applications of precipitation
  titrimetry
• Argentometric titration (what is its definition?)
• Quantitative calculations base on the
  conservation of charges between both analyte and
  titrant. In certain cases, a back titration or an
  indirect analysis is also required.

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PRECIPITATION TITRIMETRY
Evaluating Precipitation Titrimetry

• How to evaluate the effective precipitation
  titrimetry in chemical analysis
• Scale of operation (same as previous ones)
• Accuracy (achievable 0.1 to 0.2 relative
  errors)
• Precision
• Sensitivity
• Selectivity
• Other related factors (time, cost, equipment)
• It can be extended to the analysis of mixtures,
  for example mixture of I- and Cl- titrated with
  Ag as shown in figure beside. (What is its
  reason?)
• Different precipitated analytes will result in
  separate individual equivalence points.

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CONCLUSION

• Titrimetry concept use volume as signal.
• 4 optional common types acid-base,
  complexation, redox and precipitation
  titrimetries.
• Titrimetry applications any of industries those
  deal with volumetric analytes or products.
• Requirements for evaluating titrimetry scale of
  operation, detection limit, accuracy, precision,
  sensitivity, selectivity and effects of time,
  cost and equipment used.
• Equivalence point is the point where the titrant
  volume required achieves stoichiometric reaction
  in titration.
• End point is marked through colour changes of
  visual indicators after equivalence point. (May
  it overlap with equivalence point for proof of
  accuracy?)
• All 3 strategies (direct, back and displacement
  titrations) are used in titrations depend on
  feasibilities of titration processes.