LECTURE 2
Titration method
Buffer solutions.
ass. prof. I. R. Bekus
Titrimetric analysis- is a method of quantitative analysis
used to determine unknown concentration of known
substance.
 You
must know definition of some useful
terms:
 Titrant or Standard solution – a solution of
accurately known concentration.
 Titration – the process of determining
unknown concentration by adding the small
increments of standard solution until the
reaction is just complete.
Titration setup: the titrant drops from the burette into the analyte
solution in the flask. An indicator present then changes color
permanently at the endpoint.
Overview of Titrimetry:
Titrimetric methods are classified into four groups based on
the type of reaction involved.
1)
2)
3)
4)
These groups are
acid–base titrations, in which an acidic or basic titrant
reacts with an analyte that is a base or an acid;
complexometric titrations involving a metal–ligand
complexation reaction;
redox titrations, where the titrant is an oxidizing or
reducing agent;
precipitation titrations, in which the analyte and titrant
react to form a precipitate..
 Titrant
- the reagent added to a solution containing
the analyte and whose volume is the signal.
A
reagent, called the titrant, of known
concentration (a standard solution) and volume is
used to react with a solution of the analyte, whose
concentration is not known.
 equivalence
point - the point in a titration where
stoichiometrically equivalent amounts of analyte
and titrant react.
End point - the point in a titration where we stop adding
titrant.
 Indicator - a colored compound whose change in color
signals the end point of a titration.
 Titration error - the determinate error in a titration due to
the difference between the end point and the equivalence
point.
 A primary standard is a standard that is accurate enough
that it is not calibrated by or subordinate to other
standards. A primary standard in chemistry is a reliable,
readily quantified substance.

 Secondary
reagent - a reagent whose purity must
be established relative to a primary reagent
A
burette (also buret) is a vertical cylindrical
piece of laboratory glassware with a volumetric
graduation on its full length and a precision tap,
or stopcock, on the bottom (or calibrated glass
tube).
 Even
the thickness of the lines printed on the
burette matters; the bottom of the meniscus of the
liquid should be touching the top of the line you
wish to measure from.
Equipment for Measuring
Volume
Analytical chemists use a variety of glassware to
measure volume: beaker; graduated cylinder;
volumetric flask; pipette; dropping pipette.
Burette

A burette is a device used in
analytical chemistry for the
dispensing of variable,
measured amounts of a
chemical solution. A
volumetric burette delivers
measured volumes of liquid.
Piston burettes are similar to
syringes, but with precision
bore and plunger. Piston
burettes may be manually
operated or may be
motorized.
(a)
(b)
(c)
(d)
Common types of pipettes and syringes: (a) transfer
pipette; (b) measuring pipette;
(c) digital pipette; (d)
syringe.
chemical dropper

A pipette, pipet, pipettor
or chemical dropper is a
laboratory tool
commonly used in
chemistry, biology and
medicine to transport a
measured volume of
liquid, often as a media
dispenser.
Laboratory flasks

1.
2.
3.
4.
5.
6.
There are several types of laboratory
flasks, all of which have different
functions within the laboratory.
Flasks, because of their use, can be
divided into:
Reaction flasks
Multiple neck flasks
Schlenk flask
Distillation flasks
Reagent flasks
Volumetric flask
Volumetric flask

A volumetric flask
(measuring flask or
graduated flask) is a piece of
laboratory glassware, a type
of laboratory flask,
calibrated to contain a
precise volume at a
particular temperature.
Volumetric flasks are used
for precise dilutions and
preparation of standard
solutions.
Graduated cylinder

A graduated cylinder,
measuring cylinder or
mixing cylinder is a
piece of laboratory
equipment used to
measure the volume of a
liquid. Graduated
cylinders are generally
more accurate and
precise than laboratory
flasks and beakers
Funnels (laboratory)

Laboratory funnels are
funnels that have been made
for use in the chemical
laboratory. There are many
different kinds of funnels
that have been adapted for
these specialized
applications. Filter funnels,
thistle funnels (shaped like
thistle flowers), and
dropping funnels have
stopcocks which allow the
fluids to be added to a flask
slowly. For solids, a powder
funnel
Burette – kind of laboratory glass for exact
measurement of volume of solution used. Burette is
graduated and has a burette tap or stopcock at one
extreme end to control the flow of titrant.
Equivalence point. The point in a titration at which the
amount of titrant added is chemically equivalent to
the amount of substance titration.
End point. The point at which the completion of a
reaction is practically observed. When using an
indicator, the end point occurs when enough titrant
has been added to change the color of the indicator.
Three important precautions are needed when
working with pipettes and volumetric flasks.
First, the volume delivered by a pipette or contained
by a volumetric flask assumes that the glassware is
clean.
 Second, when filling a pipette or volumetric flask, set
the liquid’s level exactly at the calibration mark.
 Sird, the liquid’s top surface is curved into a
meniscus, the bottom of which should be exactly
even with the glassware’s calibration mark.

Burette filling instruction
 Always
use a small funnel to fill a burette
 To fill a burette, close the stopcock at the bottom.
You may need to lift up the funnel slightly, to
allow the solution to flow in freely
 Fill the burette past the zero mark
 Check the tip of the burette for an air bubble. To
remove an air bubble you must lift up tip of
burette and then open stopcock. If an air bubble is
present during a titration, volume reading may be
in error!
 Take the funnel out of the burette so that drops of
solution from the funnel will not fall into the
burette.
When you burette is filled, with no air bubbles, you
must level of the liquid to exactly the zero mark. Read
the bottom of the meniscus. Be sure your eye is at the
level of meniscus, not above or below
After filling burette, a known volume of the unknown
concentration solution should be taken with the pipette
and placed into the conical flask, along with a small
amount of the indicator.
Slowly release known solution from the burette into
the conical flask, while swirling the mixture.
The solution should be let out of the burette until the
indicator changes colour and value on the burette should
be recorded.
Types of Titration
Neutralisation
(Acid-Base) titration
Precipitation titration
Reduction-Oxidation (Redox)
titration
Complexometric titration
Acid-Base Titration
As the second step in this investigation you are now going to
compare two solutions (an acid and a base) using a method
called "titration".
In the first procedure you are simply going to add an acid
solution to a basic solution. Each solution will be of a different
"strength", or concentration, or amount, and you will simply
observe the relative results.
In the second procedure you are going carry out a number of
titrations in which an acid solution is carefully added to a basic
solution. In each case you have to find the "end point", which
is the point at which you have added just enough of the acid
solution to exactly neutralize all the base that was in the
original solution.
The properties of the acid solution are standardized, and fully
known. So, by finding the exact amount of acid that
neutralizes a known solution of base, it is possible to carry out
a calculation and find out the molecular weight of the base.




These titrations are based on the neutralization reaction
that occurs between an acid and a base, when mixed
in solution.
A neutralization reaction in aqueous solution is a
reaction of an acid and a hydroxide base to
produce a salt and water
An acid-base titration is the determination of
the concentration of an acid or base by
exactly neutralizing the acid/base with an acid
or base of known concentration. This allows
for quantitative analysis of the concentration
of a unknown acid or base solution.
An acid-base titration in which a base is
titrated with a standard solution of an acid is
called Acidimetric
An acid-base titration in which an acid is
titrated with a standard solution of an alkali
(a base) is called Alkalimetric
Precipitation Titration
Precipitation Titration it is a volumetric titration method where the reaction between
the titrant and sample solution yield precipitate (low solubility, usually ionic
compounds)
The most important precipitating reagent is silver nitrate.
Titrimetric methods based upon silver nitrate are sometimes termed argentometric
methods.
Argentometry, where the titrant is a standard AgNO3 solution is the most common
precipitation titrimetric method, because
 silver precipitates are usually highly insoluble
 many species form steichiometric precipitates with Ag+ (e.g. Cl-, Br-, I-, F-, CN-,
SCN-, CrO42-, PO43- etc.)
 these precipitates are formed quickly
Titrant is a standardized AgNO3 solution. The titrant needs to be stored in a dark
(brown) container.
Argentometry is most often used for determination of chloride ions, but it can be used
for other halides (bromide, iodine).
There are 3 techniques of end point determination:
 method of Mohr (indicator: potassium chromate)
 method of Volgard (indicator: ferric salt)
 method of Fajans (indicator: fluorescein)
The most often used Mohr method
Mohr method
Mohr titration is used for determination
of halide in a solution.
Potassium chromate can serve as an
indicator for the determination of
chloride, and bromide ions by
reacting with silver ion to form a
brick-red silver chromate (Ag2CrO4)
precipitate in the equivalence-point
region.
Mohr titration has to be performed at a neutral or weak
basic solution of pH 7-9 (or 6-10), because silver
hydroxide forms at high pH, while the chromate forms
H2CrO4 at low pH, reducing the concentration of
chromate ions and delaying the formation of the
precipitate.
If Ag+ solution is add to a Cl- solution containing of small
quantity of CrO4-, then AgCl will firstly precipitated,
while Ag2CrO4 has not yet, and concentration Ag+
increases progressively until solubility product of the ions
reach the value of Ksp Ag2CrO4 (2,0·10-12) to form
brick-red precipitate.
Before titration small amount of sodium or potassium
chromate is added to the solution, making it’s slightly
yellow colour. During titration, as long as chlorides are
present, concentration of Ag+ is too low for silver
chromate formation. Near equivalence point concentration
of silver cations rapidly grows, allowing precipitation of
brick-red silver chromate which signals end point.
Reduction-Oxidation (Redox) Titration
A redox titration is based on an oxidation-reduction reaction between
analyte and titrant.
In this experiment you will use a standard solution of potassium
permanganate (KMnO4) to determine the of iron (as Fe2+) in an
unknown solution.
Permanganate ion reduces to a manganese (II) ion in the acidic
solution. This reaction requires 5 electrons and 8 hydrogen ions:
MnO4-+ 8H+ + 5 e- = Mn2+ + 4H2O
Only one electron is necessary to reduce Fe (III) to Fe (II)
Fe3+ + e- = Fe2+
Therefore, 1 mole of MnO4-(the oxidizing agent) reacts with 5 moles
of Fe2+ (the reducing agent) to form 5 moles of Fe3+ and 1 mole of
Mn2+. Thus, in net ionic form:
MnO4- + 5Fe2+ + 8H+ = 5Fe3+ + Mn2+ + 4H2O
Reactions in which electrons are transferred from one
species to another are known as redox reactions,
or oxidation-reduction reactions.
2 Na + Cl2  2 NaCl
A redox reaction is made up of two reactions:
reduction -- gain of electron(s)
oxidation -- loss of electron(s)
Writing Redox Equations
In a redox reaction, the number of electrons
lost by the species being oxidized must
balance the number of electrons gained by
the species being reduced.
In a balanced redox reaction equation:
* the number of atoms of each element
must be balanced
* the total charge on the ions on the left
hand side of the equation will equal the
total charge on the ions on the right hand
side of the equation
In the redox titrations, we need a
chemical species that can change
colour in the potential range
corresponding to the sharp change at
the end point. A chemical substance,
which changes colour when the
potential of the solution reaches a
definite value, is termed as an
oxidation-reduction or redox indicator.
Inox +
ne → Inred
colour A
colour B
Permanganatometry
Potassium permanganate is a very strong oxidizing agent and
is employed in the estimation of reducing agents like ferrous
salts, oxalic acid, arsenious oxide, etc.
The permanganate ion, MnO4-, gets reduced to Mn2+ ion in
acidic medium and to MnO2 in neutral and alkaline media.
Titrations involving potassium permanganate are usually
carried out in acidic medium.
Since MnO4– is intense purple while Mn2+ is colour less, the
reaction mixture at equivalence point is colour less and even a
single drop of the permanganate would impart sufficient pink
colour to the solution acting as self indicator.
The reducing agent in the titration to be discussed is oxalic
acid here. The composition of it is H2C2O4·2H2O. In spite of
being a dehydrate it is a good primary standard as its
composition is unchanged during storage or weighing.
This redox reaction can be split apart in two parts- one
showing the oxidation and the other reduction
This titration is carried out in warm conditions
(temperature about 60 C). The reaction at room
temperature is slow because of the equilibrium
nature of this reaction. CO2 is highly soluble in
water and thus heating removes all dissolved carbon
dioxide out of the solution.
While noting the burette readings, it should be taken
into account that the solution is so intensely
coloured that the lower meniscus of the solution
may not be clear. Thus for permanganate titrations
the upper meniscus in the burette is noted.
Erio - T indicator or
Eriochrome Black-T
Complexometric titration
indicator is used in this
titration.
EDTA is a versatile chelating
agent. A chelating agent is a
substance whose molecules
can form several bonds to a
single metal ion. Chelating
agents are multi-dentate
ligands. A ligand is a
substance that binds with a
metal ion to form a complex
ion. Multidentate ligands are
many clawed, holding onto
the metal ion to form a very
stable complex. EDTA can
form four or six bonds with
a metal ion.
The picture on the left shows the color of the indicator
before titration.
This color change from wine red to violet to
blue is due to the compact nature of the
complex. The statement "the compact nature
of the complex" means when the indicator is
added to the hard water, the indicator Erio-T
forms a complex with the Ca+2 ions that is
pink in color. As EDTA is added to the
solution, the EDTA forms a complex with the
Ca+2 leaving the indicator Erio-T
uncomplexed, which is blue in color.
pH scale
ACID
NEUTRAL
BASE (ALKALINE)
0-----------------------------7--------------------------------14
Techniques of titrimetric
analysis.
 Washing
up and drying ware
 Preparation of standard solutions
 Sample preparation
 Titration:
- Measurement of volumes
- An indicator choice
 Calculations
Acid - Base indicators
Acid - Base indicators (also known as pH indicators) are
substances which change colour with pH. They are
usually weak acids or bases, which when dissolved in
water dissociate slightly and form ions.
The acid and its conjugate base have different colours.
At low pH values the concentration of H3O+ is high
and so the equilibrium position lies to the left. The
equilibrium solution has the colour A. At high pH
values, the concentration of H3O+ is low - the
equilibrium position thus lies to the right and the
equilibrium solution has colour B.
Methods to determine the end point

visual indicators:

Colour change: In some reactions, the solution changes colour without
any added indicator. This is often seen in redox titrations, for instance,
when the different oxidation states of the product and reactant produce
different colours.
Precipitation: If the reaction forms a solid, then a precipitate will form
during the titration. A classic example is the reaction between Ag+ and
Cl- to form the very insoluble salt AgCl. This usually makes it
difficult to determine the endpoint precisely. As a result, precipitation
titrations often have to be done as "back" titrations (see below).

Physical and chemical methods with the subsequent analysis of

curves of the titration showing changes which occur in the course of
titration (change of physical and chemical parametres standard
solutions)
Types of titrimetric
determinations.
Titration can be:
 direct titration
 back-titration (on residue)
 substitute-titration (displacement titration)
 revertive titration
direct titration – titrant add to an analyte solution and
react with determined substrance







Requirements to reactions in direct titration
reaction involving the titrant and analyte must be of
known stoichiometry, quantitatively
the titration reaction must occur rapidly
a suitable method must be available for determining the
end point with an acceptable level of accuracy
Reactions should proceed by room temperature
Titration should not be accompanied by collateral
reactions which deform the results of the analysis
Reactions should be specific
a suitable indicator is available
А + Т = product
Back titration. A titration in which a reagent is
added to a solution containing the analyte, and the
excess reagent remaining after its reaction with the
analyte is determined by a titration.
This titration is used, when:

the titration reaction is too slow,

a suitable indicator is not available,

there is no useful direct titration reaction

the standard solution lacks of stability (fugitive)
А + Тexcess = product1 + Тresidue
Тresidue + Тpadding = product2
displacement titration. A titration in which the
analyte displaces a species, usually from a
complex, and the amount of the displaced species
is determined by a titration.
This titration is used, when:

the analytes are unstable substance

It is impossible to indicate the equivalent (end) point in
direct reaction

Analyte doesn’t react with titrant

reaction involving the titrant and analyte mustn’t be of
known stoichiometry, quantitatively
Most commonly used indicators in acid-base titration are:
Calculate concentration of primary
standard
m
C 
MV
M
m
C 
N E V
m
T  m/V
Establishment of secondary standard
concentration
А) a measured volume of another
primary standard solution
CN 2
CN1  V1

V2
where:
CN2 and V2 are concentration and volume
of secondary standard solution
CN1 and V1 are concentration and volume
of primary standard solution
B) a weighed quantity of a primary standard
m 1000
CN 
Em  V
where:
CN and V are concentration
and volume of secondary standard
solution
m and Em are mass and
equivalent weight of primary
standard
N1V1=N2V2
From the total volume of known solution needed to react the end point, the
concentration of the unknown solution can be calculated.
 N1 – normality of solution with known concentration
 V1 – volume of solution with known concentration
 N2 – normality of solution with unknown concentration
V2 – volume of solution with unknown concentration
Example:
Problem.
30 ml of 0.10N NaOH neutralised 25.0 ml of hydrochloric acid. Determine the
concentration of the acid.
Solution.
N1- normality of NaOH = 0,1 mol-equiv/l
V1 - volume of NaOH = 30 ml
V2 - volume of HCl = 25 ml
N2 - normality of HCl - ?

N1  V1 0,1  30
N2 

 0,12 mol  equiv / l
V2
25
BUFFER SOLUTIONS
Buffers are solutions which can
resist changes in pH by addition
of acid or alkali.
Buffers are mainly of two types:
(а) mixtures of weak acids with their salt
with а strong base
(b) mixtures of weak bases with their salt
with а strong acid.
А few examples are given below:
Н2СО3 / NаНСО3 (Bicarbonate
buffer;carbonic acid and sodium
bicarbonate)
СН3СООН / СН3СОО Na (Acetate
buffer; acetic acid and sodium acetate)
Na2HPO4/ NaH2PO4 (Phosphate buffer)
1)
Acid–Base Concept
The Arrhenius theory
ACID - a substance that provides H+ ions in water
BASE - a substance that provides OH- ions in water
2) The Brønsted-Lowry Theory
All Brønsted–Lowry bases have one or more lone pairs of
electrons:
3) The Lewis Acids and Base theory
LEWIS ACID An electron-pair acceptor
LEWIS BASE An electron-pair donor
Acids
There are many acids
present in our
everyday lives.
Lemon juice contains citric acid, and
vinegar contains ethanoic acid.
Some strong acids are hydrochloric acid,
sulphuric acid and nitric acid.
Some weak acids are ethanoic acid,
citric acid and carbonic acid.
Alkalis
Many everyday substances are alkalis.
They feel soapy.
They are corrosive.
When the oxides of
some metals dissolve
in water they make an
alkali solution.
Alkalis react with acids
and neutralise them.
Alkalis
Alkalis are present in many
cleaning substances in use in
our homes.
Kitchen cleaners are alkaline
because they contain
ammonia or sodium
hydroxide, which attack
grease.
Calcium hydroxide and sodium hydroxide are strong alkalis.
The most recognisable and common weak alkali is ammonia.
An acid (НА) reacts with а base (Н2О) to form
the conjugate base of the acid (А-) and the
conjugate acid of the base (H3O+).
Accordingly, the acetate ion (СН3СОО- ) is the
conjugate base of acetic acid (СН3СООН), and the
ammonium ion (NH4+ ) is the conjugate acid of
ammonia (NН3). The acid-base reaction is frequntly
abbreviated НА = Н+ + А- with the participation of H2O
implied.
The relationship between the pH of а solution and the
concentrations of an acid and its conjugate base is easily
derived.
[НА]
[Н+]= K ---------[А-]
Taking the negative log of each term
[А-]
рН = - log К + log --------[А-]
[А-]
рН = pК + log --------[А-]
This relationship known as the Henderson-Hasselbalch equation.
Factors Affecting pH of а Buffer
The pH of а buffer solution is determined by
two factors:
 1. The value of pK: The lower the value of
pK, the lower is the pH of the solution.
 2. The ratio of salt to acid concentrations:
Actual concentrations of salt and acid in а
buffer solution may be varied widely, with
по change in рН, so long as the ratio of
the concentrations remains the same.
Buffer Capacity
On the other hand, the buffer capacity is
determined by the actual concentrations
of salt and acid present, as well as by
their ratio. Buffering capacity is the
number of grams of strong acid or alkali
which is necessary for а change in pH of
one unit of one litre of buffer solution.
 The buffering capacity of а buffer is,
definеd аs the ability of the buffer to resist
changes in pH when an acid or base is
added.

Buffers Act




When hydrochloric acid is added to the acetate buffer, the salt
reacts with the acid forming the weak acid, acetic acid and its
salt. Similarly when а base is added, the acid reacts with it
forming salt and water. Thus, changes in the pH are minimised.
СН3СООН + NaOH = СН3COONa + Н2О
СН3СООNа + HCI = СН3СООН + NaCI
The buffer capacity is determined by the absolute
concentration of the salt and acid. But the рН of the buffer is
dependent on the relative proportion of the salt and acid (see
the Henderson - Hasselbalch's equation). When the ratio
between salt and acid is 10:1, the pH will be one unit higher
than the pKa. When the ratio between salt and acid is 1:10, the
pH will be one unit lower than the pKa.
Mechanisms for Regulation of pH
1. Buffers of body fluids,
 2. Respiratory system,
 3.
Renal excretion.
These mechanisms are
interrelated.

Acidic solutions have a high H+
concentration. Base solutions have a low
H+ concentration. The pH scale is used
to indicate the acidity or alkalinity of a
solution. Pure water with an equal
number of hydrogen and hydroxide ions
has a pH of 7.
Thank you for attention