Unit 9 Neutralization Titrations-Ii

Home , Leveling effect

Estimations Based On Kinetic and Acid-Base UNIT 9 NEUTRALIZATION TITRATIONS-II Equilibria Studies Structure 9.1 Introduction Objectives 9.2 Non-aqueous Titrations 9.3 Role of Solvents in Acid-Base Reactions 9.4 Solvent Systems 9.5 Importance of Dielectric Constant 9.6 Hammett’s Acidity Functions 9.7 Titrants and End Point Detection 9.8 Some Applications 9.9 Summary 9.10 Terminal Questions 9.11 Answers

9.1 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: • 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

Neutralization 9.2 NONAQUEOUS TITRATION Titrations-II 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 CH 3COOH 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 pK A or pK B 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 pK A 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 O HA + OH 2 … (9.1)

− When the pK A 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.

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 CO 2. Hence the titration is done at a higher temperature to drive out the dissolved CO 2. 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 CO 2. 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 pK A is about 7 ?

…………………………………………………………………………………………...

b) How can the difficulties encountered in the titration of a dilute solution of a Neutralization weak acid with alkali solution be overcomed? Titrations-II

…………………………………………………………………………………………...

…………………………………………………………………………………………... …………………………………………………………………………………………... …………………………………………………………………………………………... …………………………………………………………………………………………...

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 + + H O → NH + H O+ 4 2 3 3 … (9.2) In the above reaction, a proton is being donated from ammonium to a water molecule, + hence according to Bronsted-Lowry theory, NH 4 is an acid and H 2O 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 2O NH 4 OH … (9.3) NH + HCl → NH + Cl − 3 4 … (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

Solvent Acid 1 + Base 2 Acid 2 + Base 1

+ − NH 3 (liq) HOAc NH 3 NH 4 OAc

+ − H2O HCl H2O H3O Cl + + H2O NH 4 H2O H3O NH 3 − − H2O H2O OAc HOAc OH − − 2− H2O HCO 3 OH H2O CO 3

+ − C2H5OH NH 4 C2H5O C2H5OH NH 3 + - C6H6 HPicrate C6H5NH 2 C6 H5NH 3 Picrate

Estimations Based On The basic solvent in that case facilitates the loss of proton from a very weak acid. As a Kinetic and Acid-Base result the weak acid becomes much stronger in effect. This can be similarly extended Equilibria Studies 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 Analyte Indicator iso-propyl alcohol / sodium carboxylates phenol red ethylene glycol acetic acid amines, heterocyclic bases, crystal violet amides, urea neutral red

nitromethane/ very weak bases methyl violet acetic anhydride 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?

…………………………………………………………………………………………...

b) What will be the conjugate acid-base pair for reaction of hydrochloric acid and Neutralization water ? 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 = H2Solv + Solv … (9.6) where Hsolv represents the (hydrogen-containing) solvent. The autoprotolysis constant is + - Ks = [H 2 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:

+ - CH 3COOH = H + CH 3COO … (9.8) 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: = + + − HClO4 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.

+ - HClO 4 + CH 3COOH CH 3COOH 2 + ClO 4 … (9.11)

+ − C5H5N + CH 3COOH C 5H5NH + CH 3COO ….(9.12)

+ − CH 3COOH 2 + CH 3COO 2CH 3COOH … (9.13)

+ - Adding HClO 4 + C 5H5N C 5H5NH + ClO 4 … (9.14)

Estimations Based On 2. Aprotic, those that are neither appreciably acidic nor basic, the “inert” solvents, Kinetic and Acid-Base such as benzene, toluene, petroleum ether and carbon tetrachloride. Equilibria Studies 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?

c) Which type of solvent exert a leveling effect on bases? Neutralization Titrations-II …………………………………………………………………………………………... …………………………………………………………………………………………... …………………………………………………………………………………………... …………………………………………………………………………………………... …………………………………………………………………………………………...

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  = − = −  B  H 0 log h0 log a + … (9.17)  H3O  γ +  BH  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

Estimations Based On SAQ 4 Kinetic and Acid-Base Equilibria Studies 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 (HClO 4.2H 2O) 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

ammonium hydroxides gradually decompose to a weaker base tributylamine. But if Neutralization you prepare carefully and especially if you keep in refrigerator, solution of Titrations-II + − Bu 4N OH in 2-propanol is stable for more than a month.

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.NH 2.HCl 2RNH 3 + 2Cl … (9.18) - → - (CH 3COO) 2 Hg (undissoci ated) + 2Cl HgCl 2 (undissoci ated) + 2CH 3COO … (9.19)

+ − CH 3COOH 2 + 2CH 3COO 4CH 3COOH … (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, p Kb in H 2O = 9.4 b) p-bromoaniline, p Kb in H 2O = 10.1 c) o-chloroaniline, p Kb in H 2O = 11.2 d) p-nitroaniline, p Kb in H 2O = 12.1 e) quinoxaline, p Kb in H 2O = 13.2

Estimations Based On The titration of amine halide salts needs some modifications as they are very much Kinetic and Acid-Base weakly basic and so their titrations cannot be carried out directly in acetic acid. If you Equilibria Studies 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?

…………………………………………………………………………………………...

b) What are the advantages of using Quaternary Ammonium Hydroxides as a Neutralization titrant for acids? 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

Estimations Based On Lowry. You have also learnt how to find out the conjugate acid-base pairs in a given Kinetic and Acid-Base Bronsted acid-base reaction. The three groups of nonaqueous solvents, i.e. Equilibria Studies 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. Define the following types of solvents. a) Amphiprotic b) Nonionizable c) Aprotic (inert)

2. 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: a) Amine and amine hydrochloride b) Amine hydrochloride and hydrochloric acid

9. 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 9.11 ANSWERS Titrations-II

Self Assessment Questions - 1. a) When the pK A 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.

2. a) Bronsted Acid-Base Reaction Solvent Acid 1 + Base 2 Acid 2 + Base 1 + 2– C2H5OH NH 4 C2H5O C2H5OH NH 3 − + b) The conjugate acid-base pairs will be HCl-Cl and H 3O -H2O.

+ − 3. a) The autoprotolysis constant is Ks = [H 2 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 2Solv + Solv

Estimations Based On b) Nonionizable are basic but not acidic, for example, ether, dioxane, Kinetic and Acid-Base ketones, and pyridine. Ethers have their weakly basic oxygen through Equilibria Studies 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.

2. a) Titrant is prepared by dissolving required amount of 70-72% perchloric acid (HClO 4.2H 2O) 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

3. 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 + Base 2 Acid 2 + Base 1 + − NH 3 (liq) HOAc NH 3 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.

Further Reading Neutralization Titrations-II Websites S.No. Topic Web Sites/Books 1. Kinetic Methods of http://ull.chemistry.uakron.edu/analytical/Kinetic/ Analysis

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, 6 th 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, 6 th 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, 5 th 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, 5 th Edn (James S. Fritz, George H. Schenk