Volumetric Analysis_ Amsavel

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• Volumetric Analysis
• Dr. A. Amsavel M.Sc., B.Ed., Ph.D.,

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Content

• Introduction
• Titration Basics
• Reaction, End point Indicators
• Types of Titrations
• Acid Base Theory Principles
• Acid Base titration
• Non- Aqueous Titration
• Precipitation Titration
• Complexometric Titration
• Oxidation- Reduction Titration
• Calculation
• General Information
• Errors

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Introduction

• Tirtimetric / Volumetric analysis
• Volumetric analysis is performed for
  Quantitative determination of assay / content. It
  is simple and commonly used technique in Chemical
  Industries. Analysis conducted in Aqueous and
  non-aqueous medium.
• Simple and easy
• Fast and can be done on site
• Less expensive
• Estimation of content or Assay of chemical
• Precise and accurate - depends on method and
  specificity

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Requirements of a Titration Reaction

• Reaction chose for titration must complete by
  99.9 ie lt 0.1 can be un-reacted in the
  analysis
• Reaction must be rapid analysis shall be
  performed in a reasonable time period
• The stoichiometry must be well defined
• Reaction completion shall be predicted from
  equilibrium constants
• A method must be available to determine the
  equivalence point

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Types of Titration

• 1) Precipitation Titration
• A(aq) B (aq) AB(salt)
• 2) Acid-Base Titration
• H OH H2O (strong acids or bases)
• HA OH H2O A (weak acids)
• A H H2O HA (weak bases)
• 3) Complexometric Titration
• Zn2 4NH3 Zn(NH3)42
• 4) Redox Titration (Oxidation-Reduction)
• Fe2 Ce4 Fe3 Ce3

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Primary Standard

• Importance of Primary /Reference to standard
• Primary standard is used to standardize
  Volumetric Solution (VS)
• The accuracy will be based Quality and Accuracy
  of the primary standards used
• A standard is a reference material whose purity
  and composition are well known and well defined
• Requirement of Primary Standard
• Usually solid to make it easier to weigh
• Easy to obtain, purify and store, and easy to dry
• Inert in the atmosphere
• High formula weight so that it can be weighed
  with high precision

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Endpoint Detection

• Endpoint Detection is critical it is to know
  the completion of reaction and accuracy of
  analysis
• 1 Visual indicators
• Observe a colour change or precipitation at the
  endpoint.
• Reaction completion is identified by addition
  of external or self indicator
• 2 Photometry
• Use an instrument to find out the colour
  change or precipitation
• 3 Electrochemistry
• Potentiometry Measure the potential change (
  pH electrode)
• Amperometry Measure the change in current
  between electrodes in
• Reaction solution
• Conductance Measure the conductivity changes
  of solution
• Later two method can be used for coloured,
  turbid solution and accurate end point

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Acid-base titration

• Understand the following shall be known for
  accurate analysis
• Neutralization of reaction during titration
• Neutralization Indicators
• Indicators mixed indicators
• Neutralization curve
• Non-aqueous titration

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Principles of Acid Base

• Acids
• Arrhenius acid Any substance that, when
  dissolved in water, increases the concentration
  of hydronium ion (H3O)
• Bronsted-Lowry acid A proton donor ie conjugate
  base
• Lewis acid An electron acceptor
• Bases
• Arrhenius base Any substance that, when
  dissolved in water, increases the concentration
  of hydroxide ion (OH-)
• Bronsted-Lowery base A proton acceptor ie
  conjugate acid
• Lewis acid An electron donor

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Brønsted-Lowry Theory of Acids Bases
The conjugate acid of a base is the base plus the
attached proton . The conjugate base of an acid
is the acid minus the proton.
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Lewis Theory of Acids Bases
Lewis acid An electron acceptor Lewis acid
An electron donor
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How to calculate pH ?

• A solution contains H of 0.1 mol/L or
  10-1 (pH -log(1 x 10-1) pH -1
• A solution contains H of 0.001 mol/L or
  10-3 (pH -log(1 x 10-3) pH -3
• Q1 Calculate the pH of a solution if H 2.7
  x 10-4 M
• pH -logH pH -log(2.7 x 10-4) 3.57
• Q2 Find the hydrogen ion concentration of a
  solution if its pH is 11.62.
• H 10-pH H 10-11.62 2.4 x 10-12M
• Q3 Find the pOH and the pH of a solution if its
  hydroxide ion concentration is 7.9 x 10-5M
• pOH -logOH- pOH -log(7.9 x 10-5) 4.10
• pH pOH 14 pH 14 - 4.10 pH 9.9

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An Equation for Buffer Solutions

• In certain applications, there is a need to
  repeat the calculations of the pH of buffer
  solutions many times. This can be done with a
  single, simple equation, but there are some
  limitations.
• The HendersonHasselbalch equation

conjugate base pH
pKa log
weak acid

• To use this equation, the ratio conjugate
  base/weak acid must have a value between
  0.1010 and both concentrations must exceed Ka by
  a factor of 100 or more.

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What is strong acid and strong base?
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General Knowledge pH of various solutions
Stomach juice pH 1.0 3.0 Human blood pH
7.3 7.5 Lemon juice pH 2.2 2.4 Seawater
pH 7.8 8.3 Vinegar pH 2.4 3.4 Ammonia
pH 10.5 11.5 Carbonated drinks pH 2.0
4.0 0.1M Na2CO3 pH 11.7 Orange juice pH
3.0 4.0 1.0M NaOH pH 14.0
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What is an end Point?

• Endpoint
• Point Of Neutralization Equivalence Point
• One Equivalent Of Acid reacts with One Equivalent
  Of Base

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Equilibrium Constant Ka and Kb

• The equilibrium constant for a Brønsted acid is
  represented by Ka, and base is represented by Kb.

H3OCH3COO Ka
CH3COOH
Notice that H2O is not included in either
equilibrium expression.
NH4OH Kb
NH3
pH of 1M AcoH 2.4
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Titration curve HCl Vs NaOH solution
pH curve of 100ml of HCL titrated against NaOH of
same normality
Volume of NaOH ml 1 M Sol pH 0.1M sol pH
0.0 0.0 1.0
50.0 0.5 1.5
75.0 0.8 1.8
90.0 1.3 2.3
98.0 2.0 3.0
99.0 2.3 3.3
99.5 2.6 3.6
99.8 3.0 4.0
99.9 3.3 4.3
100.0 7.0 7.0
100.1 10.7 9.7
100.2 11.0 10.0
100.5 11.4 10.4
101.0 11.7 10.7
102.0 12.0 11.0
110.0 12.7 11.7
150.0 13.3 12.3
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Titration curve HCl Vs NaOH solution
pH curve close to end point 100ml of HCL
titrated against NaOH of same normality
NaOH 1 M Sol 0.1M sol
Vol ml pH pH
98.0 2.0 3.0
99.0 2.3 3.3
99.5 2.6 3.6
99.8 3.0 4.0
99.9 3.3 4.3
100.0 7.0 7.0
100.1 10.7 9.7
100.2 11.0 10.0
100.5 11.4 10.4
101.0 11.7 10.7
102.0 12.0 11.0
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AcidBase Indicators

• An acidbase indicator is a weak acid or base.
• The acid form (HA) of the indicator has one
  color, the conjugate base (A) has a different
  color. One of the colors may be colorless.
• In an acidic solution, H3O is high. Because
  H3O is a common ion, it suppresses the
  ionization of the indicator acid, and we see the
  color of HA.
• In a basic solution, OH is high, and it reacts
  with HA, forming the color of A.

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Function of Indicators

• How phenolphthalein is behaving in different pH

• Near pH 8, Indicator dissociates and gives red
  base Human eye can detect it as a pink tinge at
  that pH
• Indicators must be carefully chosen so that their
  colour changes take place at the pH values
  expected for an aqueous solution of the salt
  produced in the titration.

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Basis of Indicator selection
Indicator shall be chosen based on the
neutralization pH and pKa/b
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Colours of indicator at different pH
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Indicators Color changes against pH
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Titration Curve Strong Acid Vs Strong Base
At the equivalence point in an acidbase
titration, the acid and base have been brought
together in precise stoichiometric proportions.
(Endpoint)
Bromphenol blue, bromthymol blue, and
phenolphthalein all change color at very nearly
20.0 mL
At about what volume would we see a color change
if we used methyl violet as the indicator?
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Titration Curve Weak Acid Vs Strong Base
The equivalence-point pH is NOT 7.00 here.
Why not??
Bromphenol blue was ok for the strong acid/strong
base titration, but it changes color far too
early to be useful here.
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Titration Curve Different combination
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Non-Aqueous Titration
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Limitation in Aqueous Titration

• Titration in water solutions has limitation
• To titrate week acids or weak bases
• To titrate separately for a mix of acids (bases)
  with near dissociation constants.
• To determine the substances which are insoluble
  in water.
• The substances, which are either to weakly acidic
  or too weakly basics to give sharp end point in
  water
• The above can be overcome by non-aqueous to
  perform easily and with accuracy

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Non-Aqueous Titration (NAT)

• Non aqueous titration Titration performed in
  solvent medium which does not contain water.
  Substance is dissolved in a solvent and titrated
  using acid or base as titrant.
• Theory is same as Acid-Base titration
• Reaction carry out in non-aqueous medium
• Extensively used for organic acids and bases
• Principle is based on Brønsted-Lowry Theory

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Where to use NAT

• NAT is applied where
• To titrate week acid or weak bases
• To titrate separately for a mix of acids (bases)
  with near dissociation constants.
• To determine the substances which are insoluble
  in water.
• The substances, which are not give sharp end
  point in aqueous solutions, can be titrated
  non-aqueous solvent ( eg too weakly acidic or
  basic)

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Solvents used in NAT

• Solvent which are used in non aqueous
  titration are called non aqueous solvent.
• Classified as four types
• Aprotic solvents Chemically neutral
• Eg. Toluene, carbon tetrachloride
• Protogenic solvents Acidic nature readily donate
  protons,
• Eg. Anhyd. HF, H2SO4
• Amphiprotic solvent Which are sly ionize and
  donate and accept protons,
• Eg Alcohols, weak organic acids.
• Acetic acid makes weak acid into storing base
• Protophilc solvents Posses high affinity for
  protons.
• Eg. Liq ammonia, Amine, Ketones
• Increases the acidic strength

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Acetic Acid

• Acetic acid slightly ionise and combine both
  protogenic and protophilic propertiesamd able to
  donate and to accept protons
• Acetic acid is slightly ionize and dissociate to
  produce protons
• CH3COOH ? CH3COO- HBut in the presence of
  perchloric acid, a far stronger acid, it will
  accept a proton
• CH3COOH HClO4 ? CH3COOH2 ClO4
• The CH3COOH2 ion can very readily give up its
  proton to react with a base, so basic properties
  of a base is enhanced, so titrations between weak
  base and perchloric acid can often be accurately
  carried out using Acidic acid .

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Levelling Solvents

• Levelling Solvents
• In general, strongly protophilic solvents are
  important to force equilibrium equation to the
  right.
• CH3COOH HClO4 ? CH3COOH2 ClO4
• This effect is so powerful that, in strongly
  protophillic solvents, all acids act as of
  similar strength.
• HB B- H
• The converse occurs with strongly protogenic
  solvents, which cause all bases to act as they
  were of similar strength.
• Solvents, which act in this way, are known as
  Levelling Solvents.

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Titration Of Bases

• The titrant should be a very strong acid. Ie
  Perchloric acid in Dioxane
• The solvent should not be basic properties
• Aprotic solvents, such as benzene, chloroform,
  carbon tetrachloride, chlorobenzene, either alone
  or mixed with glacial acetic acid may sometimes
  be used for titration with acetous perchloric
  acid
• To determine primary , secondary , tertrary
  amines, heterocyclic amines

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Titration Of Acids

• The titrant should be a solution of a strong base
  
• Solutions of quaternary ammonium hydroxides in
  organic solvents, e.g. tetra-butylammonium
  hydroxide in benzene - methanol or IPA or
  triethyl-n-butylammonium hydroxide in benzene
  methanol.
• Solution of sodium or potassium methoxide in
  benzene - methanol
• Solvent (s)
• A mixture of benzene and methanol
• very weak acids (e.g., many phenols) usually
  require a more strongly basic solvent, such as
  DMF, anhydrous ethylenediamine or butylamine
• To determine week organic acids.
• Precaution Amine may absorb carbon dioxide from
  the atmosphere

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Selection of Solvents and Titrant

• Acetic acid used for titration of weak bases,
  Nitrogen containing compounds
• Acetonitrile / with ACOH Metal ethanoates
• Alcohols (IPA, nBA) Soaps and salts of organic
  acids,
• DMF Benzoic acid, amides etc
• Perchloric acid in acetic acid
• Amines, amine salts, amino acids, salts of acids
• Potassium Methoxide in Toluene-Methanol
• Week organic acid
• Quaternary ammonium hydroxide in acetonitrile-
  pyridine
• Acids, enols, imides sulphonamides

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Endpoint Detection

• End point detection is critical for
  titration, it is to know the completion of
  reaction and accurate determination.
• 1) Visual indicators
• Observe a colour change or precipitation at the
  endpoint.
• Reaction progress checked by addition of
  external or self indicator
• Indicators Crystal violet, Methyl red,
  Thymol blue, 1-Naphthaol benzein
• 2) Electrochemistry
• Potentiometry - measure voltage change ( pH
  electrode)
• Amperometry - measure change in current between
  electrodes in solution
• Conductance measure conductivity changes of
  solution
• Later two used for coloured, turbid accurate
  end point

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USP Titrimetry lt541gt
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Precipitation Titration

• Chloride or Iodine can be titrated against Silver
  nitrate. Precipitate of Silver halides formed
  and completion is detected as end point
• Reagents used is based on Solubility products of
  precipitate
• Titration curve pAg -log M n Vs Volume
• Concentration of ions
• Eg. Ksol (AgCl) Ag X Cl - 1.2 X10-10
• Indicators
• Formation of coloured compound (precipitate
  /complex)
• Adsorption indicators

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Precipitation Titration Curve
Copy from Vogel
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Complexometric titration

• Metal Ions can be titrated with EDTA.
• M EDTA M(EDTA)
• Complex formation depend on Stability constant
  pH,
• Titration curve pM Vs Vol of EDTA
• Indicators (Metal / metal ion indicators)
• Eriochrome black T , PR , Calmagite
• M-ln EDTA M(EDTA) In
• Eg. Ca Mg estimation in water

pM -log M n
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Types of Complexometric titration

• Direct Titration
• Back titration after formation of complex
• Replacement or Substitution Titration (using
  masking and demasking, selective demasking
  agent)
• Separation by precipitation and solvent
  extraction
• Application
• Factor influence the titration, pH of solution,
  Concetration of Metal ion, amount of indicator
  etc
• Determination of almost all the metals

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Oxidation- Reduction titration

• Principle is based on Oxidation-Rduction reaction
• Reduction potential is calculated by
• Nernst equation
• E1 E 0.591/n log (ox)/(red)
• E(E1E2)/2
• Equivalence point by redox potential Vs Volume
• Example of familiar titrations
• Potasium permanganate Vs Sod. Oxalate
• Sod dichromate Vs Ferric sulphate
• iodometric titration
• iodimetric titrations

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Oxidation- Reduction Indicators
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Oxidation- Reduction Titration Curve

• Eg. Iron(II) can be titrated with Ce (IV) in dil
  Sulphuric acid medium

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Calculation/ Formula

• Normality Equivalent wt/1000ml or meq/mL
• Morality Mole/1000ml
• V1 N1 V2N2
• N1 V2N2/V1
• Normality Weight of sample x 1000 / Eq. wt x V
• Wt of sample (mg) V x N x Eq. wt
• Assay Qty estimated in sample x 100/ wt of
  sample
• Assay V x N x Eq. wt x 100/ wt of sample x
  1000
• Where V- Volume N-Normality wt- weight Eq-
  Equivalent

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Titration Error

• Possibility of error in the test method
• End point is critical in the volumetric analysis.
  Indicator or other method to determine the end is
  important
• The endpoint in the method is not identified
  exactly at the equivalence point due indicator or
  incomplete reactions
• Error Vol. at endpoint Vol. at equivalence
  point
• Negative error means endpoint is early or before
  equivalence point Positive error is due to late
  or after equivalence point

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Errors

• Know the possible error in Volume and Weight
  measument which affects the accuracy of analysis
• 10 ml titre volume 100
• If difference in volume is 0.1ml, then error is
  1
• 5ml titre volume 100
• If difference in volume is 0.1ml, then error is
  2
• Choice to reduce the error or optimum level is
  25ml
• 25 ml titre volume 100
• 0.1ml 0.4 error

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Volumetric apparatus

• As per USP
• Burette selection
• NLT 30 nominal volume (15ml consumption in 50ml
  burette)
• Micro burette for lt 10ml
• Limit of error
• Volumetric flask 25ml, 50ml, 100ml is 0.03,
  0.05 0.08ml
• Pipets5, 10, 25 ml is 0.01, 0.02 0.03ml
• Burets10, 25, 50ml is 0.02, 0.10.1ml
• Tips out flow NMT 500µl per second for precise
  analysis

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Operational Personal Error

• The following variables may affect the accuracy
  of measurement
• Eg. When 10mL volumetric pipette used in the
  analysis
• Drain time and angle of drain
• Possible beads on the inner surface
• Temperature
• Meniscus level
• Touching off last drop
• Rinsing of the pipet with the solution used
• Pipette calibration and etc.

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Possible Error in Weighing

• Misreading of the balance,
• Balance not level,
• Not cleaning the surface of the balance first,
• Touching the weighed object with moist hands,
• Leaving the balance doors open during weighing,
• Using a miscalibrated balance,
• Not cooling the sample down to near room
  temperature,
• Not removing a static charge from the sample,
• Excess vibration or air currents from people or
  nearby equipment, and
• Prolonged time sample left on pan adds/loses
  moisture.

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Possible Contamination

• Possible contamination in the Laboratory
• Contaminate a sample during weighing by placing a
  contaminated spatula
• Placing the sample on or into a contaminated
  holder during weighing,
• Dropping some lint/hair/skin or sneeze into the
  sample while weighing,
• Opening up a bottle of chemicals near the sample
  being weighed.
• When performing trace analysis, it is possible
  for just a microgram even massive fingerprint!

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Units of measurement
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Reference

• Vogel's Text Book of Qualitative Inorganic
  Analysis 6th Edition
• Qualitative Chemical Analysis Danial C. Harris
• United States Pharmacopoeia
• Metrohm Manual

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• Thank You