Quantitative Analytical Chemistry

7/24/2019 Quantitative Analytical Chemistry

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Quantitative analytical chemistry

I- Acid-Base:

Aqueous solutions of substances differ in their behavior when submitted to an electric

current. Some of them allow the current to pass, i.e. they conduct the electric current,

these are termed "electrolytes"; while other do not allow the current to pass, i.e. they

yield nonconducting solutions, and are called "nonelectrolytes". The first class

includes mineral acid, caustic alkalies and salts, while the second class is eemplified by

cane sugar, glycerin and ethyl acetate.

!ure water is a bad conductor of electricity, but when acid like "#l, a base such as $%"

or a salt like &a'S%(is dissolved in water its conductivity is greatly improved. At the

same time, it is noticed that the solute decomposes by the passage of the electric current

into its components at the cathode and the anode. These components of the electrolyte are

called "ions".

Acids and bases:

According to the classical theory of Arrhenius all acids when dissolved in water dissociate

giving rise to hydrogen ions as positive ions. )ases, on the other hands, undergo dissociation

with the formation of hydroyl ions *%"+ as the only negative ions. The old definition of

both acids and bases was laid, therefore, on that observation. The acidity of a solution or its

basicity, can also be determined by measuring the amount of either hydrogen ions or hydroyl

ions it contains, respectively. The degree of dissociation of the dissolved acid or base can be

used to calculate the concentration of the ions present in the solution.

According to the degree of dissociation acids can be divided into two groups

A- Strong acids, having a high degree of dissociation and

)- eak acids, which are feebly dissociated.

Similarly strong bases have a high degree of ionisation. hile weak bases dissociate feebly.

Apart from monobasic acids, which dissociate in one stage, polybasic acids dissociate in

consecutive stages. Sulphuric acid, for eample, dissociate in two stages, in the first stage one

hydrogen is almost completely ionised, thus

"'S%( "/ / "S%(

0n the second stage, the other hydrogen is only partially ionised.

!hosphoric acid dissociates in three stages"1!%( "

/ / "'!%(

2


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"'!%( "/ / "!%(

'

"!%(' "/ / !%(

1

These stages are called the primary, secondary and tertiary dissociations, respectively, the

first stage is the most complete while the others are smaller and smaller. The equilibrium, which eists in a dilute solution of an acid like acetic acid 3"Ac- at

constant temperature, is

"Ac "/ / Ac

Applying the law of mass action

here $ is called 4dissociation4 4ionisation4 or 4acidity constant4

The stronger the acid, the larger the acidity constant. 5or a completely ionised acid, the

acidity constant is assumed to be 2, and the mass action law does not help much in this case.

)uffers are mitures of compounds which by their presence in solution, resist changes in pH

caused by addition of small amounts of acid or base; or upon dilution.The resistance to a

change in p" is known as buffer action.

The end point in neutrali6ation reactions can be detected by including in the reaction medium

a third material which shows a definite change in some physical character when a given p"

value is reached. This third material is the indicator.

Two theories have been advanced to account for the change of the colour of neutrali6ation

indicators according to the acidity or alkalinity of the medium.

1) Ostwald Theory or Ionisation Theory

&eutrali6ation indicators are either weak acids or weak bases. when put in a solution, will

ionise to a limited etent and a state of equilibrium will eist between the undissociated

molecule and its ions as follows

" 0n "/ / 0n 3acid indicators-

0n %" 0n/ / %" 3basic indicators-

The colour of the undissociated molecule is different from that the ions for e!ample;

phenolphthalein is an acid indicator which beha"es in solution as follows:

" 0n "/ / 0n

(colourless) (red)

'

*"Ac+

+*Ac+*" +

=K


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#ethyl orange is a basic indicator which can be represented by the following

e$uilibrium in solution:

0n %" 0n/ / %"

(yellow) (red)

%enerally the net colour of the indicator will depend upon the ratio of the

concentrations of the ionised and unionised indicator which is in turn decided by the

hydrogen ion concentration in the medium.

#ertain ob7ections have been raised against this simple theory of %stwald. 5or eample,

when a small quantity of alkali is added to phenolphthalein solution it turns red, but addition

of more alkali yields a colourless solution while, as epected from the theory; the colour

should increase.

Another ob&ection to the ionisation theory is the obser"ed slow colour change in certain

instances while ionic reactions are usually instantaneous. A number of acid ' base

indicators show their characteristic colour changes in non'a$ueous media where

ionisation is mar(edly depressed.

) The chromophoric theory:

Acid base indicators are dyes which contain one or more groups of atoms loosely bound to

each other forming an unsaturated group fundamentally responsible for the colour, called

8chromophoric group9. :amples of such colour confirming groups are the a6o 3 &&-,

nitro 3&%'- or nitroso 3&%- groups or con7ugated double bonds. These groups confer

on the molecule the ability to absorb certain radiations from those composing the visible light.

The colour of the compound depends upon the type of radiations it absorbs. %n the otherhand, if the substance absorbs radiations in the ultraviolet region and does not absorb any of

the coloured radiations composing the visible light it will appear colourless.

0n the most dyes there are also other groups, usually of the phenolic or amino types, which

increase the effect of the chromophores, and are called au!ochromes. 0n eplaining the effect

of such auochromes, it was suggested that they are sufficiently polar to set the molecule in

resonance between two or more hybrids. This resonance confers greater mobility on the

electrons of the chromophoric system with consequent increase in the intensity of light

absorption and displacement of the absorption maima to longer wavelengths.

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)ernthsen and 5riedlander eplained the behavior of phenolphthalein in acid and in basic

solutions as follows

The compound had different structures in each medium. 0n acid solutions phenolphthalein

possessed a lactone structure which is colourless, while the red salt formed by alkaliescontained a chromophore quinone group. "ence the colour transformation was accompanied

by change in structure.

"ant6sch proposed that indicators being either acids or bases would either donate or accept

protons, respectively. This is naturally accompanied by rearrangement of the bonding in the

molecule leading to some different structure having a different absorption maimum and

consequently a different colour.

The theory is really a completion to %stwald theory. "ant6ch regarded the colour change not

to be due to the ionisation alone, but also some tautomericchanges in the structure

participate in the process. "e showed that an indicator is able to eist in two forms one of

them is a non electrolyte 3pseudo acid and pseudo base- the other form is a true acid or a

true base which has a high conductivity and ionisation constant. "e called then 8aci and

baso forms9, respectively, and suggested that these true forms are alone able to ionise and

that the colour of the aci forms 3or baso forms- is different from that of the pseudo ones.

The whole system can be represented by the following equilibrium

*seudo ' form True ' form H+ + In'

The following eamples may be given to illustrate the above concepts

The colour changes of some common indicators:

*henolphthaleinis an eample of all phthalein indicators. This compound is prepared by

fusing phenol with phthalic anhydride in presence of a dehydrating agent.

(


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HO

C

OH

O

CO

!seudo < 2?

O

C

O

COO

O

C

COO

Quinonoidph.ph, resonance hybride of the twotautomeric forms.

Redin colour, p" > < 2?

O

C

O

COO

OH

Tribasic salt of ph.ph. devoid of quinonoid chromophore.Colourless p" above 2'

O

%""

O

C

OH

COO

HO

C

COO

O

/ H2O/ H2O 300-

Aci


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0n the free state, the indicator is obtained in the form of reddish violet scales, soluble in

considerable amount of water.

The colour change which takes place between p" 1.2 (.( can be eplained as follows

& N

SO3H N

CH3H3C

Benzenoidstructure, yellow,due to a6o group < &;&; in the

middle of methyl orange colour change interval.

Therefore, sodium carbonate may be titrated with standard acid;

a- To the bicarbonate stage 3half neutralisation-, thus consuming one equivalent of

acid, using phenolphthalein as indicator.

&a'#%1 / "#l &a"#%1 / &a#l or

b- Gntil all the carbonic acid displaced 3complete neutralisation-, thus consuming two

equivalents of acid, using methyl orange as indicator.

&a'#%1 / '"#l '&a#l / "'#%1

Suitable indicators for complete neutralisation are also congo red, methyl orange and

bromophenol blue.

!ractical application of the above displacement titration using double indicator technique

are in the analysis of a mitures containing sodium carbonate together with sodium

hydroide, sodium bicarbonate and bora.

a) Mixture of sodium carbonate and sodium hydroxide:-

Titrations with standard acid, using phenolphthalein, would give a colour change of pink to

colourless only. hen sodium hydroide has been neutralised and the sodium carbonate hasbeen half neutralised 3i.e. converted to &a"#%1-. The use of creasol red plus thymol blue in

place of phenolphthalein also gives better accuracy.

Bethyl orange, and better bromophenol blue, would give a yellow colour to the solution, and

the additional volume of acid required to change the colour to red would be that required to

complete the reaction with &a"#%1.in this case, the aditiononal volume of acids requried for

the methyl the methyl orange end point is less than the volume required for the

phenolphthalein end point.

II 2edo!:

#hemical reactions which involve oidation reduction are more widely used in quantitative

analysis than precipitation, compleformation, acid base reactions. This attributed to fact

that a large number of elements can eist in more than one oidation state. %idation

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reduction 3redo- reactions involve the loss and gain of electrons and frequently many

hydrogen ions are also involved.

%idation is the loss of electrons by an atom, molecule or ion; while reduction is the gain of

electrons by such particles. 5or a redo reaction to take place, there must be an oidising

agent to accept the electrons given by the reducers. The oidised form and the reduced form

of a substance 3the redo con7ugated pair-, may be regarded as inter convertible is called a

half reduction

Ceduction form electrons oidi6ed form

The sum of two half reactions constitutes the redo reaction, e.g. the oidation of ferrous

ion by ceric ion

5e'/ e 5e1/ 3oidation half reaction-

#e(/ / e #e1/ 3reduction half reaction-

Adding

5e'/ / #e(/ 5e1/ / #e1/ 3redo reaction-

0n this redo reaction 3left to right-, ferrous ion is the reducer 3donor of electrons-, it loses

an electron and becomes oidised to ferric ion, while ceric ion is the oidiser 3acceptor of

electrons-, it accepts an electrons and becomes reduced to cerous ion. This transfer of

electrons can be epected to continue until the concentrations of the ions involved achieves

corresponding to equilibrium for the redo reaction.

1.1 The o!idation number:

%idation numbers are arbitrary numbers assigned to atoms to indicate their states of

oidation. Any change in these numbers can be attributed to a loss or gain of electrons.

Although it frequently happens that the oidation number assigned to an atom corresponds to

its valence number, yet in several cases the oidation numbers turn out to the entirely different

from known valences, they have plus or minus signs associated to them and are sometimes

fractional. The following rules are to be followed in assigning oidation numbers.

2. Any uncombined atom 3e.g. &a- or any atom in a molecule of an element 3e.g. "'- is

assigned an oidation number of 6ero.

2'


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'. The oidation number of a simple, monoatomic ion is the same as the charge on

the in 3the electrovalence number-. Thus the oidation number of Ln'/is '/ and that

of #lis 2.

1. The oidation number of hydrogen ion is 2/ in all its compounds; ecept the metallic

hydrides 3e.g., &a%"-, in which hydrogen has an oidation number of 2.

(. The oidation number of oygen is always ', ecept in peroides 3e.g., &a'%'-, in

which oygen has an oidation number of 2

@. 0n a comple ion, the algebraic sum of the oidation numbers of the constituent

atoms must equal the charge on the ion. 5or eample, ferrocyanide ion, 5e3#&-D(

has a charge of ( ; and since the oidation number of ferrous ion is '/, the

oidation number of each cyanide radical is 2 3total of ( for the four cyanide

radicals-

D. 0n a neutral molecule, the algebraic sum of oidation number of the constituent

atoms must add up to 6ero 3because the molecule is electrically neutral-. Thus in a

molecule of &a#l, the oidation number of sodium is 2/ and that of chloride is 2.

=. %idation numbers are sometimes fractionals. Thus, the oidation number of sulphur

in sodium tetrathionate, &a'S(%D. is 'K / ; and the oidation number of crbon in

butane , #("2?is 'K .

3lectrode potentials:

0f a metal plate is dipped into water, processes occur at the metal surface resulting in the

formation of what is known as an electric double layer.

Gnder the action of the polar water molecules the metal ions are torn out of its crystal lattice

into the water. As a result, the metallic surface becomes negatively owing to the electrons

remaining in the metal, while the layer of water becomes positively charged due to the ions

which have passed into solution the metallic ions do not spread into the bulk of the liquid, but

concentrate around the surface of the metal, being hold there by its charge. An electric double

layer forms on the surface of the metal and a dynamic equilibrium is set up between the metal

and the solution, corresponding to the definite difference in potentials. As metals differ in

ability to lose ions to the solution, the difference of potentials thus caused will evidently vary

from metal to metal.

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A similar phenomenon is observed when a metal plate is dipped into a solution of a salt of

that metal. The increased concentration of metallic ions in the solution reduces the ability of

metallic ions to pass from the plate into solution, equilibrium is reached at a smaller potential

difference between the metal and the solution. Betals having a low ability to lose their ions to

the solution for e.g. gold, silver, platinum Metc. may take up ions from the solution thus

acquiring a /ve charge.

The equilibrium potential difference established at the boundary between a metal and

solution of its readily soluble salt is called electrode potential. There are two ma7or factors

that determine the electrode potential. The first is the electrolytic solution pressure of the

element, which is the tendency of an active element to send its ions into solution. At a given

temperature and pressure this is a characteristic constant for an element. The second is the

ionic pressure, which is the activity of the dissolved ions of the element, ions which in turn

varies with their concentration at constant temperature.

The dependence of the equilibrium potential of an electrode on these two factors is epresses

by the &ernst equation.

The potential established between a metal and its ions can be calculated from the equation

formulated by &ernst in 2>>F as follows

B

nB

eo

tA

Alog

n5

CT::

+

+=

This is called 8&ernst equation9 where

the equation can be simplified by introducing the known values of C and 5 and converting the

natural logarithm to common logarithm it then becomes

B

nBo

tA

Alog

n

T?.???2F>'::

+

+=

since the activity of massive metal or any solid phase is taken as unity, and the activity of

the metal ions equals its molar concentration being a dilute solution.

-3Blogn

T?.???2F>':: not

++=

5or a temperature of '@o# 3T 'F>o-;

-3Blogn

?.?@F2::

no

#o

'@

++=

5or a non metal, which yields negative ions. The equation becomes

2(


15/26

=

=

=

=

==

=+==

==

=

=

+

metal.theofactivityA

solution.inionsmetaltheofactivityA

ions.theofvalencyn

'.1?1bygmultiplyinby2?,base

thetoisthat,logarithmscommontoeconvertabl

isand'.=2>,basethetoisthatlogarithm,naturallog

coulombsFD@??5arady5

'F>'=1'@

#'@atwhichsystemtheofretemperatuabsoluteT

>.12(constantgasC

potential4electrodestandard4called

systemredoeveryforsticcharacterialue,constant va:

Tatelectroderedotheofpotentialoidationthe:

B

nB

e

o

o

t

Co

-3Blogn

?.?@F2::

no

#o

'@

=

if 3Bn/- and 3Bn- are equal to one molar then its logarithm will be 6ero and the second part

of the equation will be equal to 6ero and : is equal to :othat is equal to the standard 3or

normal- electrode potential of the metal system.

2< &o indicator

0n some cases, it is possible to do the titration without the use of any indicator, if the

colour of the titrating solution undergoes a sharp enough change at the equivalence point.

Such titration are possible when various reducing agents are oidised by permanganate in

acid solution. e known that the purple violet colour of permanganate disappears owing

to reduction to the almost colourless Bn'/. hen all the reducing agent has been oidised

single ecess drop of permanganate colourless the whole solution a distinct pink.

Similarly, titrations with iodine solution may be performed without the use of indicators

as a result of its reduction to iodine ions. "owever, since the colour of iodine solutions is

not very deep, titrations with iodine solution are beast done in presence of an indicator

starch that gives an intense blue colour even with very small amounts of free iodine. The

use of starch is based on its ability to form a blue adsorption compound with iodine

3unrelated to the oidising properties of iodine-

2@

4etection of end point in redo! titrations4etection of end point in redo! titrations


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'. :ternal indicator

The best known eample is the spot test method for the titration of ferrous iron with

potassium dichromate. &ear the equivalence point, drops of solution are removed and

brought into contact with dilute freshly prepared potassium ferricyanide solution on a

spot plate. The end point is reached when the drop first fails to give a blue coloration.

Another eample is provided by the titration of 6inc ions with standard potassium

ferrocyanide solution; here a solution of urnayl acetate or nitrate is the eternal indicator,

and titration is continued until a drop of the solution 7ust imparts a brown colour to the

indicator. :ternal indicators are gradually being superseded by the more satisfactory

internal oidation reduction indicators.

1. 0netrnal redo indicator

An oidation reduction indicator should mark the sudden change in the oidation potential

that occurs in the neighbor hood of the equivalence point in an oidation reduction

titration 3see titration curves-. Cedo indicator change colour when the oidation potential

of the titrated solution reaches adefinite value. Cedo indicator is a compound which has

different colours in the oidised and reduced forms.

0no / n e 0nred.

The oidation and reduction of the indicator should be reversible, the leucocompounds

obtained on reduction are usually colourless, and are converting by oidation into the

coloured dyes

0no / n e 0nred.

Applying &ernst equation

+*0n

+*0nlog

n

?.?@F2::

red

oo+=

"ere, :ois the standard oidation potential of the indicator system i.e. potential where

*0no+ *0nred+

Gpon addition of 2 ' drops of a redo indicator to a solution of reducing or oidising

agent, the concentration of the oidised and reduced forms of the indicator will be in a

ratio corresponding to the oidation potential of the solution. The solution acquire the

colour corresponding to the that ratio. 0f the solution is now titrated with an oidi6ing or

reducing agent, the oidation potential : changes. The *0no+ N *0nred+ ratio alters

2D


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accordingly. "owever, not every change in this ratio corresponds to a colour change

perceived by the eye. As in acid base indicators colour change may be detected when

2

2?or

2?

2

+*0n

+*0n

red

o=

hence;

2

2?or

2?

2log

n

?.?@F2:: o +=

n

?.?@F2::

o=

1.2 0odine iodide system

The redo system 0'N '0 has a standard oidation potential of / ?.@1@ volt. This

intermediate position makes the system open to oidation by stronger oidi6ing agents at the

top of list such as Bn%( 3:o 2.@-. #r'%=

' 3:o / 2.1-, #l%1 3:o/ 2.(@-, 0%1

(3:o /

2.'- etc; the iodide ion being converted into iodine element

Thus

' Bn%(

/ 2D "/

/ 2? 0

' Bn'/

/ > "'% / @ 0'

#r'%=' / 2("/ / D 0 ' #r1/ / = "'% / 1 0'

#l%1 / D"/ / D0 #l / 1"'% / 1 0'

"'%' / ' "/ / ' 0 ' "'% / 0'

&%' / '"/ / ' 0 &% / "'% / 0'

#l' / '0 ' #l / 0'

And so on,

%n the other hand, systems with low oidation potentials like Sn

(/

NSn

'/

, S%('

N S%1'

,S(%D

'N'S'%1' are easily oidi6ed with iodine

S%1' / 0'/"'% S%(

' / ' "// ' 0

'S'%1' / 0' S(%D

' / ' 0

Sn'/ / 0' Sn(// ' 0

0odine is a weak oidi6ing agent, and iodide ion a fairly strong reducing against since the

half cell e.m.f. 5or the reaction

' 0 0' / ' e :o ?.@(

2=

2edo! reaction in"ol"ing iodine2edo! reaction in"ol"ing iodine


18/26

0n the development of analytical chemistry several factors led to an early utili6ation of

iodine methods. They are

2< a sensitive indicator, starch, has long been known, and

'< a reagent, sodium thiosulphate, is available for quantitative titration of free iodine,because of these conditions a large number of analytical methods has been developed

about the iodine iodide ion equilibrium.

There are two types of iodine methods, direct and indirect. 0n the first, known as the

iodimetric method, iodine solutions are used for titration of reducing agents, which can be

quantitatively oidi6ed at the equivalence point. The number of such reactions is limited

because iodine is itself a weak oidising agent; further, most of these determinations can

today be better done by some other method than the use of iodine. Thiosulphate alone, of the

common reducing agents must be determined by oidation with iodine. 0t happens that the

stronger oidi6ing agents give side reactions with thiosulphate, whereas iodine oidises it

quantitatively to tetrathionate ion S(%D'. Some of the oidations which can be well done by

iodine are represented in the following equations

"'S / 0' ' "/ / S / ' 0

"'S%1 / 0' / "'% S%(' / ' 0 / ( "/

"'As%1 / 0' / "'% "'As%(

/ ' 0 / '"/

'S'%1' / 0' S(%D

' / ' 0

Sn'/ / 0' Sn(// ' 0

0n indirect or iodometric methods the oidising agent which is to be analyte is treated with

an ecess of iodide ion under suitable conditions. 0odine is liberated quantitatively, and it is

titrated by a standard solution of sodium thiosulphate or arsenious acid. These methods can be

used for the analysis of almost any strong oidising agent, consequently, there are many more

applications than there are few iodimetric methods.

' Bn%( / 2D "/ / 2? 0 ' Bn'/ / > "'% / @ 0'

#r'%=' / 2("/ / D 0 ' #r1/ / = "'% / 1 0'

)r%1 / D"/ / D0 )r / 1"'% / 1 0'

l%1 / D"/ / @0 1"'% / 1 0'

#l%1 / D"/ / D0 #l / 1"'% / 1 0'

'"&%' / '"

/

/ ' 0

'&% / '"'% / 0'

'#u'/ / (0 #u0 / 0

2>


19/26

Indicators:

0n both iodimetric and iodometric methods the end point is based on the presence of iodine.

This can be detected by starch or by etraction of iodine from water by immiscible solvent.

1. 5tarch:

Starch does not dissolve as a true solution, but it is readily dispersed as a colloid. ith iodine

it give a deep blue colour which is formed on the surface of the colloid particles when iodine

is adsorbed and is discharged when iodine is reduced to iodide ion. The colour change is

reversible and the presence of iodide ion is necessary for development of a good colour.

Two milliliters of a 2O solution is added to 2?? ml of the solution to be titrated, taking the

following precaution into consideration

*a + The sensitivity of the colour decreases with increasing temperature of the solution;

thus at @?o# it is about 2? times less sensitive than at '@ o#.

*b + 0n the titration of iodine, starch must not be added until 7ust before the end point. 0f

added when the iodine in high concentration some iodine may remain adsorbed and it

becomes difficult to discharge the deep blue colour of the iodine starch adsorbate.

*c + 0t cannot be employed in alcoholic solution; the alcohol hinders the formation of

adsorbate, nor it can be used in strongly acid medium; strong acids hydrolyse the starch

and destroy the adsorption compound.

,hloroform or carbon tertrachloride:

0n alcoholic or strongly acidic solutions the end point is detected by the use of either

chloroform or carbon tetrachloride. The solubility of iodine in chloroform is about F? times as

in water. So, if chloroform is added to iodine solution and well shaken, the great part of iodine

will dissolve in the organic layer, which settles down in the bottom and is coloured deep

violet. Titration with, for eample, sodium thiosulphate will gradually remove iodine that is inthe aqueous layer and on gentle shaking another part of the iodine in the organic layer passes

to the aqueous layer, and so on. &ear the end point, when the organic layer contains only a

faint violet colour, vigorous shaking is necessary to bring the aqueous thiosulphate solution

and the organic layer in contact with each other. The end point is reached when the violet

colour in the organic layer 7ust disappears.

III ,omple!ometry:

2F


20/26

#omple formation titration are those titrations involving the formation of substances

known as complees which are only slightly ioni6ing in solution, releasing ions to the solution

in minute amounts as does an insoluble electrolyte.

Bost metal ions react with electron pair donors to form coordination compounds orcomple ions. The donor species, or ligand, must have at least one pair of unshared electrons

available for bond formation.

The comple formation reaction is a type of acid base reaction according to Pewis concept

where the metal ion is the Pewis acid 3electron acceptor-, while the ligand is the Pewis base

3electron donor-.

Chelation

Pigands, which possess two or more donor groups, combine with metal to form ring structure

called chelate. The ligands in this case, are called chelating agents, and the products are called

chelates.

The chelate forming ligand must possess at least an acidic group or more and a

corresponding group or more as shown in the following tables

Acidic groups #oordinating groups

#%%" #arboylic # % #arbonyl

OH!henolic %" "ydroyl

S%1" Sulphonic % :thers

& " 0mino &"' Amino

C S " Bercaptyl & #yclic nitrogen

&% &itroso

The presence of acidic or coorddination groups in an organic compound is not alone

sufficient to ensure capability of chelate formation. The groups must be located in the

molecule in such position that will involve the metal ion in stable ring.

:thylenediamine tetraacetic acid disodium salt dihydrate 3:ETA- is widely used as ligand

3titrant-.

'?


21/26

N

N

CH2COONa

CH2COOH

CH2COOH

CH2COONa

. ' "'%

4etection of the end point:

The end point can be detected by may methods include potentiometric, conductometric,

spectrophotometric techniques in addition to the use of metal indicators.

#etal indicators: 6#etallochromic or p# indicators)

These are dyestuffs, which form coloured comple with specific metal cations. They are

cheleting agent i.e. the dye stuff molecule possesses several ligand atoms suitably disposed

for coordination with a metal atom. They are considered as lewis base and the metal as lewis

acid. Betal indicators are at the same time acid base indicators, i.e. they change their colour

according to the p" of the medium. So, in order to restrict the colour change to the metal

reaction, these indicator are usually used in buffered solutions. Betallochromic indicators

produce one colour in the free form and a different colour in the metalli6ed form.

eneral requirements for a good metal indicators!

2- 0ts colour reaction with the metal ion should be specific or at least selective.

'- The metal indicator comple must be weaker than the metal :ETA comple.

1- The metal indicator comple must posses sufficient stability, otherwise, because of

dissociation, a sharp colour change is not obtained.

(- The colour of the metalli6ed indicator and free indicator must be sufficiently different and

intense enough to allow visual detection of the end point with a minimum amount of

indicator.

The sample containing the metal ion is buffered to the suitable p" and titrated directly with

:ETA using suitable indicator. 0f the p" is high, metal hydroides 3or basic salts- are

sometimes precipitated and their reaction with :ETA becomes slow. The addition of auiliary

compleing agents helps retain the metal ions in solution e.g. tartaric acid is used for direct

titration of lead. The buffer itself may act sometimes as auiliary compleing agent as in the

case of ammonia buffer,.

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0f direct titration is not sufficiently rapid, a catalyst may be included or the titration liquid is

heated to accelerate the reaction.

#etermination of some metal ions by direct titration with $#%

Betal ion Bedium 0ndicator #olour change

)i1/ p" ' 1 nitric Qylenol orange !ink yellow

#d'/ p" @ heamine Qylenol orange Ced yellow

#a'/ p" 2' &a%" Bureide !ink purple

#u'/ :nough &"1R Bureide ellow purple

Ln'/ or Bg'/ Ammonia buffer :)T ine red blue

RR#a'/ Bg'/ p" 2? ammonia

buffer

:)T Ced blue

R to give blue #u3&"1-('/

**Total hardness of water

The presence of traces of other metals as impurities, may compete with ion to be estimated

and block the indicator as in the case of direct titration of #a and Bg with :ETA, traces of

#u, #o, &i, 5e are frequently present and produce similar difficulties. 0n this case the

impurities is mas(edbefore adding the indicator by reducing it 3using ascorbic acid-, then

compleing with cyanide ion forming more stable complees than their edetates.

)ack titration

)ack titrations are useful for compleometric determination of metal ions in the following

cases

2. 0f the metal ion precipitates at the p" suitable for titration.

'. 0f the medium contains precipitating agent that precipitates the metal ion at the p"

suitable for titration e.g. #a//in the presence of oalate.

1. 0f the reaction between the metal ion and :ETA is too slow as in the case of Al and

#r.

(. 0f a suitable metal indicator is not available.

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)ack titration is also suitable for determining insoluble substance such as )a and !bS%(,

magnesium phosphates, etc.

The procedure involves addition of known ecess of standard :ETA to the metal being

determined. The medium is then suitably buffered and the ecess :ETA is back titrated with

standard metal solution 3e.g. Ln#l'orBg#l'- in presence of suitable indicator.

Aluminum can be determined by adding known ecess of :ETA and then ad7usting the p"

of the solution to p" =.> using ammonia solution. the solution is then boiled for few minutes

to insure complete compleation. The ecess :ETA is back titrated with standard Ln solution

using eriochrome back T as indicator, the change of colour is from blue to win red.

#hromium can be also determine by back titration at p" D using acetate buffer and pb3&% 1-'

as atitrant for the ecess :ETA. Qylenol orange is used for detecting the end point. )oiling

for 2? 2@ minutes is also necessary.

0J

Quantitative Analytical Chemistry

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