Transcript 02. Titration method

LECTURE 2
Titration method
ass. prof. Ye. B. Dmukhalska
• Titrimetry, in which we measure the volume
of a reagent reacting stoichiometrically with
the analyte, first appeared as an analytical
method in the early eighteenth century.
Overview of Titrimetry:
• Titrimetric methods are classified into four
groups based on the type of reaction involved.
• 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;
and precipitation titrations, in which the
analyte and titrant react to form a precipitate..
Typical instrumentation for performing an
automatic titration.
Equivalence Points and End Points
• For a titration to be accurate we must add a stoichiometrically
equivalent amount of titrant to a solution containing the analyte.
We call this stoichiometric mixture the equivalence point.
Unlike precipitation gravimetry, where the precipitant is added
in excess, determining the exact volume of titrant needed to
reach the equivalence point is essential. The product of the
equivalence point volume, Veq, and the titrant’s concentration,
CT, gives the moles of titrant reacting with the analyte.
• Moles titrant = Veq . CT
• Knowing the stoichiometry of the titration reaction, we can
calculate the moles of analyte. Unfortunately, in most titrations
we usually have no obvious indication that the equivalence point
has been reached. Instead, we stop adding titrant when we reach
an end point of our choosing. Often this end point is indicated
by a change in the color of a substance added to the solution
containing the analyte. Such substances are known as
indicators.
Equipment for Measuring Volume
• Analytical chemists use a variety of
glassware to measure volume: beaker;
graduated cylinder;volumetric flask; pipet;
dropping pipet.
• Beakers, dropping pipets, and graduated
cylinders are used to measure volumes
approximately, typically with errors of several
percent.
• Pipets and volumetric flasks provide a more
accurate means for measuring volume.
• Volumetric flask contains a solution, it is useful
in preparing solutions with exact concentrations.
The reagent is transferred to the volumetric flask,
and enough solvent is added to dissolve the
reagent. After the reagent is dissolved, additional
solvent is added in several portions, mixing the
solution after each addition. The final adjustment
of volume to the flask’s calibration mark is made
using a dropping pipet.
Pipets
• A pipet is used to deliver a specified
volume of solution. Several different
• styles of pipets are available. Transfer
pipets provide the most accurate
• means for delivering a known volume of
solution; their volume error is similar to
• that from an equivalent volumetric flask
(a)
(b)
(c)
(d)
Common types of pipets and syringes: (a) transfer pipet; (b) measuring pipet;
(c) digital pipet; (d) syringe.
Three important precautions are needed when working with
pipets and volumetric flasks.
First, the volume delivered by a pipet or contained by a
volumetric flask assumes that the glassware is clean.
Second, when filling a pipet or volumetric flask, set the liquid’s
level exactly at the calibration mark. The liquid’s top surface is
curved into a meniscus, the bottom of which should be exactly
even with the glassware’s calibration mark.
Before using a pipet or volumetric flask you should rinse it with
several small portions of the solution whose volume is being
measured.
Acid-base titrations
• Based on acid-base reactions
• The earliest acid–base titrations involved the
determination of the acidity or alkalinity of solutions,
and the purity of carbonates and alkaline earth oxides.
Before 1800, acid–base titrations were conducted using
H2SO4, HCl, and HNO3 as acidic titrants, and K2CO3
and Na2CO3 as basic titrants. End points were
determined using visual indicators such as litmus, which
is red in acidic solutions and blue in basic solutions, or
by observing the cessation of CO2 effervescence when
neutralizing CO32–. The accuracy of an acid-base
titration was limited by the usefulness of the indicator
and by the lack of a strong base titrant for the analysis of
weak acids.
Titrations Based on Complexation Reactions
• The earliest titrimetric applications involving metal-ligand
complexation The use of a monodentate ligand, such as Cl– and
CN–, however, limited the utility of complexation titrations to
those metals that formed only a single stable complex.
• The utility of complexation titrations improved following the
introduction by Schwarzenbach, in 1945, of aminocarboxylic
acids as multidentate ligands capable of forming stable 1:1
complexes with metal ions. The most widely used of these new
ligands was ethylenediaminetetraacetic acid, EDTA, which
forms strong 1:1 complexes with many metal ions.
• Ethylenediaminetetraacetic acid, or EDTA, is an
aminocarboxylic acid. EDTA, which is a Lewis acid, has six
binding sites (the four carboxylate groups and the two amino
groups), providing six pairs of electrons. The resulting metal–
ligand complex, in which EDTA forms a cage-like structure
around the metal ion, is very stable. The actual number of
coordination sites depends on the size of the metal ion; however,
all metal-EDTA complexes have a 1:1 stoichiometry.
Precipitation Titrations
• A reaction in which the analyte and titrant form an insoluble
precipitate also can form the basis for a titration. One of the earliest
precipitation titrations, developed at the end of the eighteenth
century, was for the analysis of K2CO3 and K2SO4 in potash.
Calcium nitrate, Ca(NO3)2, was used as a titrant, forming a
precipitate of CaCO3 and CaSO4. The end point was signaled by
noting when the addition of titrant ceased to generate additional
precipitate. The importance of precipitation titrimetry as an
analytical method reached its zenith in the nineteenth century when
several methods were developed for determining Ag+ and halide
ions.
• Pb2+(aq) + 2Cl–(aq) =PbCl2(s)
• In the equilibrium treatment of precipitation, however, the reverse
reaction describing the dissolution of the precipitate is more
frequently encountered.
• PbCl2(s) = Pb2+(aq) + 2Cl–(aq)
• The equilibrium constant for this reaction is called the solubility
product, Ksp, and is given as
• K = [Pb2+][Cl–]2 = 1.7.10–5
Titrations Based on Redox Reactions
• Redox titrations were introduced shortly after the
development of acid–base
• titrimetry.
• Since titrants in a reduced state are susceptible to air
oxidation, most redox titrations are carried out using an
oxidizing agent as the titrant. The choice of which of
several common oxidizing titrants is best for a particular
analysis depends on the ease with which the analyte can be
oxidized. Analytes that are strong reducing agents can be
successfully titrated with a relatively weak oxidizing
titrant, whereas a strong oxidizing titrant is required for
the analysis of analytes that are weak reducing agents.
Thank you for attention