Estimations Based On
Kinetic and Acid-Base
Equilibria Studies
UNIT 9 NEUTRALIZATION TITRATIONS-II
Structure
9.1
Introduction
Objectives
9.2
9.3
9.4
9.5
9.6
9.7
9.8
9.9
9.10
9.11
9.1
Non-aqueous Titrations
Role of Solvents in Acid-Base Reactions
Solvent Systems
Importance of Dielectric Constant
Hammett’s Acidity Functions
Titrants and End Point Detection
Some Applications
Summary
Terminal Questions
Answers
INTRODUCTION
In the last unit you have learnt about the neutralization titrations in aqueous medium. I
hope you know that water is poorly dissociated. But that does not prevent water to
dissolve many of the electrolytes. Quantitative methods of analysis have been
developed by these sort of rapid ionic reactions. It is economical and easier to handle
aqueous solutions leading to the wide use of aqueous solution for analysis. But in
many instances it is seen that non-aqueous ionizing solvents are advantageous in case
of acidimetry and alkalimetry. This is specially true for cases where compounds
cannot be titrated in an aqueous medium.
In this unit I am going to introduce you to the non-aqueous titrations, the purpose of a
particular solvent in specific reactions and also the various solvent systems. Have you
heard about the term “dielectric constant”? Well in this unit you will also learn about
its importance. After discussing Hammett’s acidity functions, titrants, end point
detection, I will also bring to your notice certain applications for such titrations. This
unit will help you to learn about the different aspects of nonaqueous neutralisation
titrations which are utilised in analytical chemistry.
Objectives
After studying this unit you would be able to:
70
•
understand nonaqueous titrations
•
understand the difficulties encountered in the titration of a dilute solution of a
weak acid with alkali solution
•
discuss the role of solvents in acid-base reactions with the help of acid-base
concept of Bronsted and Lowry
•
discuss the conjugate acid-base pairs in a given Bronsted acid-base reaction
•
understand the three groups of nonaqueous solvents, i.e. amphiprotic, aprotic
and basic
•
discuss the importance of dielectric constant
•
understand Hammett’s acidity function
•
discuss the different titrants and end-point detection
•
discuss some applications of nonaqueous titrants
9.2
Neutralization
Titrations-II
NONAQUEOUS TITRATION
There are limitations of using water as a solvent for acid-base titrations and often the
usage of non-aqueous solvents is advantageous. Nonaqueous titration is the titration of
substances dissolved in nonaqueous solvents. Why do you need to know about this
sort of titrimetric procedure? Because it is suitable for the titration of very weak acids
and very weak bases and it provides a solvent in which organic compounds are
soluble. Titration of weak acid is very difficult and this difficulty may be overcome by
using a basic solvent. In many cases, like many organic acids will dissolve in
methanol.
There are many problems in the alkalimetric determination of weak acid in aqueous
medium. Acids and bases with ionization constants less than about 10 − 7 to 10 − 8 are
too weak to be titrated accurately in aqueous solutions by conventional methods as
discussed in the previous unit. If you choose a solvent less basic than water, it is
possible to titrate much weaker bases. Same principle can be applied for titrating weak
acids. So, if you choose a solvent less acidic than water, you can titrate a very weak
acid.
You can see by doing experiments too that, whenever a strong acid (like 0.1N HCl) is
titrated with a strong base (0.1N NaOH), then the inflection or the jump in pH at the
equivalence point is by almost 5.4 units of pH. So it is much easy to calculate the
equivalence point in this case. But the problem arises when it is a weak acid, like
CH3COOH and the hump in pH is only by 2.3 pH units. So if you refer to Fig. 9.1,
you can easily follow that the pH jump decreases as the strength of the acid (which is
directly measured by dissociation constant) decreases. In other words you can say that
as the dissociation constant decreases (for weak acids), the pH jump at the equivalence
point also decreases.
You have to carry out non aqueous titration because weak acid & weak bases are not
completely ionised when dissolved in water at reasonable concentrations (~ 0.1 M),
but when we use non aqueous solvent it is strongly acidic in nature or basic in nature.
Due to this nature, it ionises the given organic or inorganic substance into it. So if you
have to have complete ionisation of weak acid and bases, then you must use
nonaqueous solvent. The problem occurs when the pKA or pKB of the material
concerned is ≥ 7.
The end point break of the titration curve of a weak acid or a weak base (Fig. 9.1) is
not sharp enough when pKA is about 7. Can you find an answer as to why there are
such limitations in these type of titrations? Now, consider the reaction of the salt
formed at the end point with water:
A − + H 2 O HA + OH −
… (9.1)
When the pKA of HA is nearly 7, the basicity of A − is considerably high so that it can
readily accept a proton and form HA. So the neutralization is not 100%. In fact it is
even less than 99% in these cases. But the limiting value for quantitative
neutralization is 99.9%.
This situation is further worsened with the increase in the dilution of the acid/alkali
solutions when the inflection becomes still smaller. It is very difficult to make out the
end point of titration when the inflection is very small, that is even less than 2 pH
units. So there will be enormous error in such cases.
71
Estimations Based On
Kinetic and Acid-Base
Equilibria Studies
Fig. 9.1: Titration curve of a weak acid or a weak base
By this time you know that phenolphthalein is a suitable indicator in the titration of a
weak acid with strong base. When a small amount of a weak acid is given we have to
use a dilute solution of NaOH for the titration. It has been observed that if 0·01N or
more dilute solution of NaOH is used, the pink colour of the indicator fades away
rapidly at the end point. This causes difficulty in the recognition of the end point. (A
faint pink colour first appears. Now, it is necessary to shake the solution so that there
is a thorough mixing of the added titrant. When this is done the pink colour practically
disappears. This creates a doubt that the end point has not been reached. In order to
confirm the end point, a drop or two of the titrant are then added when solution again
appears to be faint pink but on shaking it fades away rapidly. This is known as fleeting
end point. Due to this problem, more titrant has to be added to locate the end point
than required stoichiometrically).
Another problem in the titration of a dilute solution of a weak acid is the interference
due to atmospheric CO2. Hence the titration is done at a higher temperature to drive
out the dissolved CO2. But if the acid under question is volatile, a part of the acid will
be lost.
The difficulties encountered in the titration of a dilute solution of a weak acid with
alkali solution can be summarised as:
i)
the inflection on the pH-neutralisation curve is small,
ii)
the phenolphthalein colour fades away at the end point, and
iii)
interference due to atmospheric CO2.
The above difficulties can be overcome if by some means the dissociation constant of
the weak acid can be increased so that it behaves like a strong acid. This can be done
by using a suitable non-aqueous solvent in place of water. In order to understand the
increase in the dissociation of a weak acid in a suitable solvent we must study the
concept of acids and bases suggested by Bronsted.
SAQ 1
a)
Why the end-point of the titration curve of a weak acid or a weak base is not
sharp enough, when pKA is about 7 ?
…………………………………………………………………………………………...
…………………………………………………………………………………………...
72
b)
How can the difficulties encountered in the titration of a dilute solution of a
weak acid with alkali solution be overcomed?
Neutralization
Titrations-II
…………………………………………………………………………………………...
…………………………………………………………………………………………...
…………………………………………………………………………………………...
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9.3
ROLE OF SOLVENTS IN ACID-BASE REACTIONS
Acid-Base Concept of Bronsted and Lowry
The Arrhenius theory successfully explains acid-base reactions in aqueous medium.
But it cannot be extended to acid-base neutralization reactions in non-aqueous
medium. Bronsted and Lowry in 1923 came up with a new concept defining acids and
bases in terms of proton transfer; an acid being a substance which donates proton
whereas a base acts as an acceptor of proton. This concept can be extended to many
substances which could not be included as acids and bases in the original Arrhenius
theory.
For example,
NH +4 + H 2 O → NH 3 + H 3 O +
… (9.2)
In the above reaction, a proton is being donated from ammonium to a water molecule,
hence according to Bronsted-Lowry theory, NH4+ is an acid and H2O is a base. Here
the concept of base is also different. In the Bronsted model, any substance in any
medium is a base if it can accept a proton. Thus in the following reactions,
NH 3 + H 2 O → NH 4+ + OH −
… (9.3)
NH 3 + HCl → NH +4 Cl −
… (9.4)
ammonia will be a base because it can accept proton although it does not contain
hydroxyl group.The Bronsted theory can be applied to nonaqueous solvents.
Some acid-base reactions in different solvents are illustrated in Table 9.1.
Table 9.1: Bronsted Acid-Base Reaction
Base 2
Acid 2
HOAc
NH3
NH+4
OAc −
H2 O
HCl
H2O
H3O+
Cl −
H2 O
NH4+
H2O
H3O+
NH3
H2O
H2 O
OAc −
HOAc
OH −
H2 O
HCO 3−
OH −
H2O
CO 32−
C2H5OH
NH4+
C2H5O −
C2H5OH
NH3
C6H6
HPicrate
C6H5NH2
C6 H5NH3+
Picrate-
Solvent
Acid 1
NH3 (liq)
+
+
Base 1
73
Estimations Based On
Kinetic and Acid-Base
Equilibria Studies
The basic solvent in that case facilitates the loss of proton from a very weak acid. As a
result the weak acid becomes much stronger in effect. This can be similarly extended
to cases where the solute is weakly basic and thereupon acidic solvents can be used to
make the solute effectively stronger base. But do not forget to be careful about
choosing the reagents for these sort of non-aqueous titrations. In Table 9.1 and 9.2 are
the data for non-aqueous titration media.
The most commonly used procedure is the titration of organic bases with perchloric
acid in anhydrous acetic acid.
Glacial acetic acid, an amphiprotic solvent is widely used for the titration of weak
bases such as amines. (Ionized as follows: RNH 2 + HOAc RNH 3+ + OAc− ; the
OAc − is then titrated, as OH − is in water.) Perchloric acid is intrinsically the
strongest mineral acid, so it is usually used as the titrant, dissolved in glacial acetic
acid. Even this acid is only partially dissociated in glacial acetic acid – about the same
extent as acetic acid in water.
A compound when dissolved in a suitable nonaqueous solvent is then titrated with a
standard solution of a strong acid or base which is again dissolved in nonaqueous
solvent. But how can you detect the end point in these cases? Well that can be done
with a visual indicator or with a pH meter.
Table 9.2: Acidic solvent. Reagents: Perchloric acid(acid) and Potassium
hydrogen phthalate(base)
Solvent
iso-propyl alcohol /
ethylene glycol
acetic acid
Analyte
sodium carboxylates
Indicator
phenol red
amines, heterocyclic bases,
amides, urea
crystal violet
neutral red
nitromethane/
acetic anhydride
very weak bases
methyl violet
neutral red
The convention of acid or base number is used to overcome any ambiguity which may
be in the expression of acid or base content.
The Bronsted theory defines acid as any species having a tendency to lose a proton
and can do so only when another substance is present to accept protons; which in turn
then acts as base. I will explain this to you by taking any acid say HB. Then I dissolve
this acid in a solvent S which is capable of accepting proton. Then you can write:
HB
+
S
HS +
+
B… (9.5)
Acid I
Base II
Acid II
Base I
Can you find out the two conjugate acid-base pairs in the above reaction? Well they
are HB---B- and HS+----S.
SAQ 2
a)
What will be the Bronsted acid-base reaction in ethanol solvent for ethanol and
ammonia?
…………………………………………………………………………………………...
…………………………………………………………………………………………...
74
b)
What will be the conjugate acid-base pair for reaction of hydrochloric acid and
water ?
…………………………………………………………………………………………...
Neutralization
Titrations-II
…………………………………………………………………………………………...
…………………………………………………………………………………………...
9.4
SOLVENT SYSTEMS
The Nonaqueous solvents used may in principle be classified into three groups:
•
Amphiprotic
•
Aprotic
•
Basic but not acidic – nonionizable
1.
Amphiprotic, those which possess both acidic and basic properties, such as
water, ethanol, and methanol. These are ionizable solvents. Amphiprotic
solvents undergo self-ionization, or autoprotolysis:
2Hsolv = H 2Solv + + Solv-
… (9.6)
where Hsolv represents the (hydrogen-containing) solvent. The autoprotolysis
constant is
Ks = [H2 Solv+][Solv-]
… (9.7)
These have both acidic and basic properties. Examples are water, acetic acid and
the alcohols (like methanol, ethanol). They are dissociated to a slight extent.
The dissociation of acetic acid, which is frequently used as a solvent for titration
of basic substances, is shown in the equation below:
… (9.8)
CH 3COOH = H + + CH 3COO In the above reaction, the acetic acid functions as an acid. Whenever a very
strong acid (like perchloric acid) is dissolved in acetic acid, the latter can
function as a base and combine with protons donated by the perchloric acid to
form protonated acetic acid, an onium ion:
HClO 4 = H + + ClO −4
… (9.9)
CH 3COOH + H + = CH 3COOH +2 (onium ion)
… (9.10)
Now this onium ion donates its proton to a base readily. So, a solution of
perchloric acid in glacial acetic acid functions as a strongly acidic solution.
Pyridine, a weak base has its basic property enhanced when dissolved in acetic
acid. It is possible, therefore, to titrate a solution of a weak base in acetic acid
with perchloric acid in acetic acid, and obtain a sharp endpoint when attempts to
carry out the titration in aqueous solution are unsuccessful.
HClO4 + CH3COOH CH3COOH2+ + ClO4-
… (9.11)
C5H5N + CH3COOH C5H5NH+ + CH3COO −
….(9.12)
CH3COOH2+ + CH3COO − 2CH3COOH
… (9.13)
Adding HClO4 + C5H5N C5H5NH+ + ClO4-
… (9.14)
75
Estimations Based On
Kinetic and Acid-Base
Equilibria Studies
2.
Aprotic, those that are neither appreciably acidic nor basic, the “inert” solvents,
such as benzene, toluene, petroleum ether and carbon tetrachloride.
Aprotic solvents are neutral, chemically inert substances such as benzene and
chloroform. These are not much acidic or basic. Low dielectric constant of
these solvents and non-reactivity with acids or bases do not make them favor
ionization. If you add picric acid to benzene you will get a colourless solution.
But when you add aniline to picric acid the solution becomes yellow. What
does this indicate? It shows that picric acid is not dissociated in benzene
solution but in the presence of the base aniline it functions as an acid. Due to
formation of the picrate ion, the yellow colour develops.
Since dissociation is not an essential preliminary to neutralization, aprotic
solvents are often added to 'ionizing' solvents to depress solvolysis (which is
comparable to hydrolysis) of the neutralization product and so sharpen the
endpoint.
Protophilic solvents are basic in character and react with acids to form solvated
protons.
HB + Sol. Sol.H+ + B-
… (9.15)
Acid + Basic solvent Solvated proton + Conjugate base of acid
… (9.16)
A weakly basic solvent has less tendency than a strongly basic one to accept a
proton. Similarly a weak acid has less tendency to donate protons than a strong
acid. As a result a strong acid such as perchloric acid exhibits more strongly
acidic properties than a weak acid such as acetic acid when dissolved in a
weakly basic solvent. But the situation becomes different when you dissolve
acids of different strengths in a strongly basic solvent. Then, all acids tend to
become indistinguishable in strength owing to the greater affinity of strong
bases for protons. This is called the leveling effect. Strong bases are leveling
solvents for acids, weak bases are differentiating solvents for acids.
Protogenic solvents are acidic substances, e.g. sulphuric acid. They exert a
leveling effect on bases.
3.
Basic but not acidic- nonionizable- for example, ether, dioxane, ketones, and
pyridine. Now, how can these react with an acid if they are nonionizable? Well,
ethers have their weakly basic oxygen through which they can react with an acid
and pyridine can do the same through its basic nitrogen. Generally it is seen that
bases do not react with such solvents but may be just solvated. Most of these
are extremely weak bases. There are no known examples of solvents that are
acidic but not basic.
SAQ 3
a)
What is the autoprotolysis constant?
…………………………………………………………………………………………...
…………………………………………………………………………………………...
b)
What are aprotic solvents?
…………………………………………………………………………………………...
…………………………………………………………………………………………...
76
c)
Neutralization
Titrations-II
Which type of solvent exert a leveling effect on bases?
…………………………………………………………………………………………...
…………………………………………………………………………………………...
…………………………………………………………………………………………...
…………………………………………………………………………………………...
…………………………………………………………………………………………...
9.5
IMPORTANCE OF DIELECTRIC CONSTANT
For a particular solvent, the dielectric constant is a measure of the ease of separation of
two oppositely charged ions or the ease of dissociation of an ion-pair in that solvent.
Let me explain this to you with the following illustration. Consider the acidic
behaviour of acetic acid in water and in ethanol. The dielectric constant of water at
25° C is 78.5 but only 24.3 for ethanol. Then what can you conclude about the
dissociation of acetic acid in these solvents? Well, it will be greater in water than in
ethanol. So whenever you have to select a particular solvent for a particular titration,
you also have to consider its dielectric constant.
9.6
HAMMETT’S ACIDITY FUNCTIONS
Commonly used acidity functions refer to concentrated acidic or basic solutions.
Acidity functions are usually established over a range of composition of such a system
by UV/VIS spectrophotometric or NMR measurements of the degree of hydronation
(protonation or Lewis adduct formation) for the members of a series of structurally
similar indicator bases (or acids) of different strength: the best known of these
functions is the Hammett acidity function H0 (for uncharged indicator bases that are
primary aromatic amines. It is a measure of acidity that is used for very concentrated
solutions of strong acids, even the superacid. It is particularly useful for physical
organic chemistry where you may come across acid-catalyzed reactions which uses
acids in very high concentrations.
Hammett’s acidity function,

γ
H 0 = − logh0 = − log  aH O + B
 3 γ +
BH





… (9.17)
a = activity
γ = activity coefficients of a base B and its conjugate acid BH+.
Do note that the Hammett’s acidity function do not include water in its equation.
Lastly you must note that this is the best known acidity function.
Table 9.3: H0 for some concentrated acids
Fluoroantimonic acid
-31.3
Magic acid
-19.2
Carborane superacid
-18.0
Fluorosulfuric acid
-15.1
77
Estimations Based On
Kinetic and Acid-Base
Equilibria Studies
SAQ 4
What is Hammett’s acidity function?
…………………………………………………………………………………………...
…………………………………………………………………………………………...
…………………………………………………………………………………………...
…………………………………………………………………………………………...
9.7
TITRANTS AND END POINT DETECTION
Perchloric acid: Titrations in presence of acetic acid/non basic solvents are carried out
by using perchloric acid as a titrant. It is a strong acid and is easily available. Many
other strong acids may also be used. Then what is so advantageous about using
perchloric acid? The reason is primarily that in acetic acid, perchloric acid gives a
longer potentiometric break than hydrochloric acid. It gives a much longer break than
nitric acid also. You have to choose the solvent in which perchloric acid will be
dissolved depending on the type of titration. Usually perchloric acid in acetic acid is
used. Titrant is prepared by dissolving required amount of 70-72% perchloric acid
(HClO4.2H2O) in acetic acid. Remember to use acetic anhydride to remove water
from perchloric acid. This precaution you must specially follow whenever you have to
titrate a very weak base. But thereafter you have to also be careful so that you do not
add excess acetic anhydride. You have to remove any excess acetic anhydride if it is
present in the titrant. Why do you have to remove excess acetic anhydride from the
titrant? Well, it can undergo acid-catalysed reaction with water in a fast rate and also
may react with primary or secondary amines while titrating them. Now you must be
thinking that why you have to take all these precautions while preparing titrants? Well
the aim is that they should be stable for long periods of time. If you add calculated
amount of 70-72% perchloric acid to reagent-grade or purified dioxane, then you will
get a very good titrant for mixtures of certain bases where acetic acid may have a
leveling effect. For basic titrants in aqueous solution, the primary standard acid KHP
(potassium acid phthalate) is much used. In glacial acetic acid, KHP is used as a
primary standard base for standardizing perchloric acid titrants. You have to apply
heat in order to dissolve KHP in acetic acid.
Alkali Metal Bases: Whenever you have to titrate weak acids in nonaqueous solvent
then you can use solutions of different sodium or potassium alcoholates in a suitable
alcohol. In this category you can obtain the best titrants from a solution of sodium or
potassium methoxide in methanol or benzene-methanol. Can you tell why methanol is
not suitable as a nonaqueous solvent for titrating weak acids? Well this is because
methanol has a certain amount of acidity (comparable to that of water). So such
titrants are not much in use presently. You should not replace methanol with benzene
(an inert diluent) as benzene is toxic.
Quaternary Ammonium Hydroxides. As a titrant for acids tetrabutylammonium
hydroxide in 2-propanol is often used. The advantage of using them is that the
tetraalkylammonium salt of the titrated acid is soluble in the usual solvents. If you use
the alkalies then you will see that the alkali metal salts of titrated acids often form
gelatinous precipitates. Also the potentiometric curves obtained for
tetraalkylammonium hydroxides are very good with ordinary glass and calomel
electrodes.
You must be careful so that there is no carbonate (a moderately weak base) impurity
while preparing a strongly basic titrant. If any carbonate would be present then it
would give error in detecting the end point of a titration. Also solutions of quaternary
78
ammonium hydroxides gradually decompose to a weaker base tributylamine. But if
you prepare carefully and especially if you keep in refrigerator, solution of
Bu4N+OH − in 2-propanol is stable for more than a month.
Neutralization
Titrations-II
Basic titrants can be standardized by the primary standard benzoic acid. Do remember
to measure any acidic impurities in the solvent when you will have to calculate the
molarity of the titrant.
Titration of halogen acid salts of bases
The halide ions (chloride, bromide and iodide) are quite weakly basic. So they cannot
react quantitatively with acetous perchloric acid. If you add mercuric acetate (which is
undissociated in acetic acid solution) to a halide salt then the halide ion will be
replaced by an equivalent quantity of acetate ion, which is a strong base in acetic acid.
2R.NH2.HCl 2RNH3+ + 2Cl −
… (9.18)
(CH 3COO) 2 Hg (undissociated) + 2Cl - → HgCl 2 (undissociated) + 2CH 3COO … (9.19)
CH3COOH2+ + 2CH3COO − 4CH3COOH
… (9.20)
Endpoint detection for titration of bases:
The relative basicities of organic amines when titrated as bases in nonaqueous solution
is the same as in water.
Fig. 9.2: Titration of amines in acetic acid with 0.1 M perchloric acid measured with
glass-calomel electrodes:
a)
Aniline, pKb in H2O = 9.4
b)
p-bromoaniline, pKb in H2O = 10.1
c)
o-chloroaniline, pKb in H2O = 11.2
d)
p-nitroaniline, pKb in H2O = 12.1
e)
quinoxaline, pKb in H2O = 13.2
79
Estimations Based On
Kinetic and Acid-Base
Equilibria Studies
The titration of amine halide salts needs some modifications as they are very much
weakly basic and so their titrations cannot be carried out directly in acetic acid. If you
add mercuric acetate to the amine halide, it will convert the halide to undissociated
mercuric halide and then you can carry out such titrations. The advantage of using
mercuric acetate is that it remains undissociated in acetic acid. Such modifications can
be extended to amine salts of hydrochloric, hydrobromic, and hydroiodic acid. If the
titrant is very much diluted, say to the extent of 0.01 M, then you can mix some
dioxane with acetic acid to increase the sharpness of the potentiometric end point.
These are the reasons why I would recommend you to use the titrant perchloric acid in
dioxane in these cases.
Fig. 9.3: Potentiometric titration curve of a mixture of butylamine and pyridine titrated
in acetonitrile with perchloric acid in dioxane
Amines in mixtures can often be titrated separately whenever they have large
difference in their basicities. So if you see the titration curve for a mixture of
butylamine and pyridine in acetonitrile solution then you will see that there are two
potentiometric breaks when it is titrated with perchloric acid in dioxane. What can be
the advantage of using the solvent acetonitrile? Well, it is not acidic and also it exerts
no leveling effect on the two amines. But then what may be the reason that when the
same mixture is titrated in acetic acid solution you can observe only one end point? If
you take a close look you will see that this end point corresponds to the sum of the
amines. This is because butylamine reacts with acetic acid to form acetate ion, which
has about the same basicity as pyridine.
Titration of Acids
Sulfonic acids, carboxylic acids, phenols, enols, imides, some nitro compounds and
different sulfur-containing compounds are acidic to some extent. This acidity enables
them to be titrated in nonaqueous solvents. But certain conditions are to be maintained
for these sort of titrations. One thing you must remember for these cases is that the
solvents should not have any acidic properties itself but should be able to dissolve the
acidic compounds. Also, in the case of mixtures of acids the solvents should not be a
strong base so that the mixture can be titrated appropriately.
A strong base like sodium methoxide or tetrabutylammonium hydroxide dissolved in
benzene-methyl alcohol/ isopropyl alcohol are usually used as a titrant. But I will
suggest you not to use alcohols as solvents for titrating weak acids as they are often
quite acidic.
SAQ 5
a)
Why should precautions be taken while preparing titrants?
…………………………………………………………………………………………...
…………………………………………………………………………………………...
80
b)
What are the advantages of using Quaternary Ammonium Hydroxides as a
titrant for acids?
Neutralization
Titrations-II
…………………………………………………………………………………………...
…………………………………………………………………………………………...
…………………………………………………………………………………………...
c)
When can mixtures of amines be titrated separately?
…………………………………………………………………………………………...
…………………………………………………………………………………………...
…………………………………………………………………………………………...
9.8
SOME APPLICATIONS
Correct pH conditions are to be maintained in manufacturing process industry. You
will also have to check the acids and bases in the reactants as well as the products if
you have to work in the food, petroleum as well as the beverage industries. Acid-base
titrations is also used in certain analytical methods. This sort of application can be
seen in the case of Kjeldahl method for determination of nitrogen. In the
pharmaceutical industry it is required to find out whether the amine present in the drug
is present as the free amine or as the salt. So in these cases, direct titration of amine
salts is very much important.
The end point of most titrations is detected by the use of visual indicator. But when
you will be dealing with very dilute or colored solutions then this method can be
inaccurate. In such cases you can get accurate results by the potentiometric method
for the detection of the equivalence point. This you can do without much difficulty
too. What you need is an electrical apparatus (potentiometer or pH meter) with a
suitable indicator and reference electrode. Along with this you will require a burette,
beaker and stirrer. The reference electrode should have a constant potential
throughout the titration. According to a particular type of titration you have to choose
the indicator electrode. Take a glass electrode for acid-base reactions and a platinum
electrode for redox titrations. Also the equilibrium should be reached rapidly. You
have to immerse the electrodes in the solution to be titrated and then measure the
potential difference between the electrodes. You have to add measured volumes of
titrant with thorough (magnetic) stirring. Record the corresponding values of emf
(electromotive force) or pH. You should note the burette reading corresponding to the
maximum change of emf or pH per unit change of volume. This you have to do
graphically. Then you must add small increments in volume of titrant near the
equivalence point. But you will face problem to locate the equivalent point by this
method when the slope of the curve is more gradual. Instead you can add small
increments (0.1 cm³ or less) of titrant near the end point of the titration. Next you plot
a curve of change of emf or pH per unit volume against volume of titrant and you will
get a differential curve in which the peak is the equivalence point.
9.9
SUMMARY
In this unit you have studied the different types of nonaqueous titrations. You have
learnt about the difficulties encountered in the titration of a dilute solution of a weak
acid with alkali solution and how to overcome it. You have also learnt the role of
solvents in acid-base reactions with the help of acid-base concept of Bronsted and
81
Estimations Based On
Kinetic and Acid-Base
Equilibria Studies
Lowry. You have also learnt how to find out the conjugate acid-base pairs in a given
Bronsted acid-base reaction. The three groups of nonaqueous solvents, i.e.
amphiprotic, aprotic and basic have also been discussed. The importance of dielectric
constant and Hammett’s acidity function have also been illustrated. Then we have
discussed the different titrants and end-point detection. Some applications of
nonaqueous titrants have also been illustrated.
9.10 TERMINAL QUESTIONS
1.
2.
a)
Amphiprotic
b)
Nonionizable
c)
Aprotic (inert)
Describe how the following titrants may be prepared for nonaqueous titrations.
For each, list a suitable primary standard.
a)
Perchloric acid
b)
Sodium methoxide
c)
Tetrabutylammonium hydroxide
3.
List two weakly basic impurities that could be in a tetrabutylammonium
hydroxide titrant. Suggest an experimental method for determining whether this
titrant contains any weakly basic impurities.
4.
Perchloric acid is used as titrant in glacial acetic acid solvent. Explain why
perchloric acid, a strong acid in water, is not strongly ionized in acetic acid.
5.
Sodium hydroxide is a strong base in water and is commonly used as a titrant for
acid-base titrations. Suggest a sodium-containing base for acid-base titrations in
acetic acid.
6.
Explain why the autoprotolysis constant is an important factor in choosing a
solvent for an acid base titration. Suggest a simple experimental way to estimate
the autoprotolysis constant of an amphiprotic solvent.
7.
Ammonium acetate ( NH +4 Ac − ) cannot be titrated as an acid or a base in
aqueous solution, but it can be titrated as either an acid or a base in a
nonaqueous solvent. Give the chemical reaction and conditions for titrating
ammonium acetate as an acid in a nonaqueous solvent.
8.
Explain how an amine hydrochloride ( RNH 3+ Cl − ) can be titrated as a base in
noaqueous solution. Suggest a scheme for determining separately the amount of
each component in the following mixtures:
9.
82
Define the following types of solvents.
a)
Amine and amine hydrochloride
b)
Amine hydrochloride and hydrochloric acid
When a mixture of two bases of different basicity is to be titrated in acetonitrile
to give two potentiometric breaks, explain why the perchloric acid titrant should
be made up in dioxane rather than in acetic acid.
Neutralization
Titrations-II
9.11 ANSWERS
Self Assessment Questions
1.
2.
a)
When the pKA of HA is nearly 7, the basicity of A- is considerably high
so that it can readily accept a proton and form HA. So the neutralization is
not 100%. In fact it is even less than 99% in these cases. But the limiting
value for quantitative neutralization is 99.9%. When there is increase in
the dilution of the acid/alkali solutions then the inflection becomes still
smaller. It is very difficult to make out the end point of titration when the
inflection is very small, that is even less than 2 pH units. So there will be
enormous error in such cases.
b)
The difficulties encountered in the titration of a dilute solution of a weak
acid with alkali solution can be removed, if by some means the
dissociation constant of the weak acid can be increased so that it behaves
like a strong acid. This can be done by using a suitable non-aqueous
solvent in place of water.
a)
Bronsted Acid-Base Reaction
Solvent
C2H5OH
3.
Acid 1
NH
+
4
+ Base 2
C2H5O2–
Acid 2
+
C2H5OH
Base 1
NH3
b)
The conjugate acid-base pairs will be HCl-Cl − and H3O+-H2O.
a)
The autoprotolysis constant is Ks = [H2 Solv+][Solv − ]
b)
Aprotic, those that are neither appreciably acidic nor basic, are the “inert”
solvents, such as benzene, toluene, petroleum ether and carbon
tetrachloride.
c)
The protogenic solvents are acidic substances, e.g. sulphuric acid which
exert a leveling effect on bases.
4.
Hammett acidity function H0 (for uncharged indicator bases that are primary
aromatic amines is a measure of acidity that is used for very concentrated
solutions of strong acids, even the superacid.
5.
a)
Precautions have to be taken while preparing titrants as they should be
stable for long periods of time.
b)
The advantage of using Quaternary Ammonium Hydroxides as a titrant
for acids tetrabutylammonium hydroxide in 2-propanol is that the
tetraalkylammonium salt of the titrated acid is soluble in the usual
solvents.
c)
Amines in mixtures can often be titrated separately whenever they have
large difference in their basicities. So if you see the titration curve for a
mixture of butylamine and pyridine in acetonitrile solution then you will
see that there are two potentiometric breaks when it is titrated with
perchloric acid in dioxane.
Terminal Questions
1.
a)
Amphiprotic, those which possess both acidic and basic properties, such
as water, ethanol, and methanol. These are ionizable solvents.
Amphiprotic solvents undergo self-ionization, or autoprotolysis:
2Hsolv = H 2 Solv + + Solv −
83
Estimations Based On
Kinetic and Acid-Base
Equilibria Studies
2.
3.
84
b)
Nonionizable are basic but not acidic, for example, ether, dioxane,
ketones, and pyridine. Ethers have their weakly basic oxygen through
which they can react with an acid and pyridine can do the same through its
basic nitrogen.
c)
Aprotic, those that are neither appreciably acidic nor basic, the “inert”
solvents, such as benzene, toluene, petroleum ether and carbon
tetrachloride.
a)
Titrant is prepared by dissolving required amount of 70-72% perchloric
acid (HClO4.2H2O) in acetic acid.
b)
A strong base like sodium methoxide or tetrabutylammonium hydroxide
dissolved in benzene-methyl alcohol/ isopropyl alcohol are usually used as
a titrant. But I will suggest you not to use alcohols as solvents for titrating
weak acids as they are often quite acidic.
c)
tetrabutylammonium hydroxide in 2-propanol
i)
carbonate (a moderately weak base)
ii)
weaker base tributylamine
It gives error in detecting the end point of a titration
4.
The reason is primarily that in acetic acid, perchloric acid gives a longer
potentiometric break than hydrochloric acid. It gives a much longer break than
nitric acid also.
5.
sodium methoxide
6.
The autoprotolysis constant (refer to Eq. (9.17)), if high will indicate
amphiprotic solvent, if low will indicate aprotic.
7.
Bronsted Acid-Base Reaction
Solvent
Acid 1
NH3 (liq)
HOAc
+
Base 2
Acid 2 +
Base 1
NH3
NH +4
OAc −
8.
Refer to the Eqs. (9.18) to Eq. (9.20)
9.
If you add calculated amount of 70-72% perchloric acid to reagent-grade or
purified dioxane, then you will get a very good titrant for mixtures of certain
bases where acetic acid may have a leveling effect.
Neutralization
Titrations-II
Further Reading
Websites
S.No.
1.
Topic
Kinetic Methods of
Analysis
Web Sites/Books
http://ull.chemistry.uakron.edu/analytical/Kinetic/
2.
Method of Initial rate
http://www.saskschools.ca/curr_content/chem30/mo
dules/module4/lesson3/methodofinitialrates.htm
Books
1.
Vogel’s Textbook of Quantitative Chemical Analysis by J. Menham, R.C.
Denney, J.D. Barnes and M.J.K. Thomas, 6th Edn, Low Price Edition, Pearson
Education Ltd, New Delhi (2000).
2.
Instrumental Analysis, Editors, H.H. Bauer, G.D. Christian and J.E.O’ Reilly,
2nd Edn, Allyn and Bacon, Inc., Boston (1991).
3.
Analytical Chemistry by G.D. Christian, 6th Edn, Wiley-India.
4.
Principles and Practice of Analytical Chemistry by F.W. Fifield and D. Kealey,
5th Edn, Blackwell Science Ltd, New Delhi (2004).
5.
Instrumental Methods of Chemical Analysis by G.W. Ewing, 5th Edn, Mc-Graw
Hill, Singapore (1985).
6.
Instrumental Methods of Analysis by Willard Merritt, Dean Sattle, 7e, Cbs
Publishers & Distributors.
7.
Fundamentals of Analytical Chemistry by Skoog, West, Holler and Crouch,
Thomson Brooks/Cole.
8.
Quantitative Analytical Chemistry, 5th Edn (James S. Fritz, George H. Schenk
85