Acid - Base Titration

 Chapter 5 

 Acid - Base Titration 

 Introduction to Acid - Base Titration

• Acid-base titrations are also called as neutralization or aqueous acid-base titrations. These titrations involve basically the reaction of H3O in solution against OH. This basic reaction is applied for any acids and bases including strong acids, strong bases, weak acids, weak bases, salts of weak acid and salts of weak bases. For example: In the following molecular form reactions various acids are reacted with bases.

HCI+ NaOH NaCl + H₂O

CH COOH + NaOH CH3COONa+ H₂O HCI NH, OH NHCI + H₂O

HCIO4 + CH3COONa CH3COOH + Nacole

However, the basic reaction involved in the above-mentioned reactions is given below.

Theories of Acids and Bases 

• The theories of acids and bases have been postulated by describing acid-base properties of the substances in water as well as in other solvents. The first theory was postulated by Arrhenius in 1884 as a part of general theory on electrolytic dissociation. As per this theory, acid is defined "as a substance which generates hydrogen ions when dissolved in water and further these hydrogen ions in association with solvent form hydronium ions.

The Law of Mass Action

•  The law of mass action is proposed by Goldberg and Wage in 1867. The basics of this law are about the mass of the substances that react in a reaction. The law states that "The rate of a chemical reaction is proportional to the active masses of the reacting substances".

• In dilute solutions where conditions approach the ideal state, active mass may be represented by the concentration of the reacting substances, I.e., gm-molecules or gm-ions.

• per litter. The constant of proportionality is known as 'velocity constant".

Acid - Base Equilibrium

Buffer Solution

Detection Of End Point

• In an acid-base titration, the end point detection targets at the completion of reaction which is nothing but the neutralization reaction. That means, when the equivalent amount of a titrant is added to titrate to complete reaction or neutralize in case of acid and base reaction, the end point is achieved.

• However, the detection of end point cannot be done directly which needs an indicator. Thus, indicator is described as a substance which is holoful in detecting the end point during a titration without interfering with t reaction. Most of the indicators used in volumetric titrations produce co. end point.

Neutralization Indicators

• These are the substances used in acid-base titrations that are helpful in detection of end point at the end of reaction. They generally exhibit different colours at the end point at various values of pH. These Indicators exhibit some colour in acidic pH whereas when the pH changes, it produces different colours.

• So, after the neutralization point, they produce colour change as per the pH of the titrant or titrate, and thus denotes the end point. They are weak acids or weak bases, which have different colours in their conjugate base and acid forms.

• Most indicators are used in dilute solution form. For an acid-base titration, we select an indicator which will show a distinct colour change.

Mixed Indicators 

• Though the acid-base indicators are widely used as per the above-mentioned theories, the pH range is very narrow and the colour change over this range must be very sharp. This is not easily possible with ordinary acid-base indicators. 

• The result may be achieved by the use of the suitable mixture of indicators. These are generally selected so that their pKI, values are close together and overlapping colours are complementary at an intermediate pH value.

• A mixture of phenolphthalein (03 parts of 0.1% and a-naphthol phthalein 01-part of0.1%) passes from pale rose to violet at pH 08.9 (titration of phosphoric acid to dibasic stage).

• For example: Additions of xylene cyanol to methyl orange. Here, a colour change from alkaline to acid side is green grey → magenta.

Universal or Multiple Range Indicators 

The limitation of narrow pH range for the indicators can be suitably tackled by mixing certain indicators so that the colour change may be made to extend over a considerable portion of the pH range. Such mixtures are usually called "Universal indicators". 

They are not suitable for quantitative titrations but may be employed for the determination of the approximate pH of a solution by colourimetric method.

Classification of Acid-Base Titrations: 

The acid base titrations are classified as follows

1. Strong acid-strong base titrations
2. Strong acid-weak base titrations 
3. Weak acid-Strong base titrations
4. Weak acid - Weak base titrations.

Neutralization Curves 

• Neutralization reaction can be well understood by observing closely the process of changes in the hydrogen ion concentration during the titration.

• The pH is an important parameter in neutralization reaction as it decides the selection of suitable indicator to detect the end point. As we are aware from the previous sections that the indicators exist in different forms in acidic and alkaline media thus pH plays an important role.

Strong Acid - Strong Base Titration 

In this titration, a strong acid such as HCI, H₂SO4 is reacted with strong base such as NaOH using a suitable indicator. This reaction allows each acid molecules to react with each base to produce salt and water. A titration curve is plotted by taking the pH of pH 8 the solution against the volume of alkali added. Initially, the curve shows low pH due to the presence of strong acid which slowly.

Weak Acid - Strong Base Titration

In this titration, a weak acid such as acetic acid is titrated with strong base such as NaOH using suitable indicator to detect the end point. The titration curve of this type of titration will have the shape similar to the curve obtained from strong acid Vs strong. base with slight changes.

A weak acid dissociates its salt partially. Initially, the pH rises normally and when it reaches a zone where reactions proceeds, the curve will not rise much.

Weak Base - Strong Acid Titration

In this titration, initially the curve shows a high pH due to the weak base. Once titration starts, the reaction of weak base with strong acid results in slow decrease in pH value.

  At equivalence point, once all weak base molecules are completely reacted with strong acid, the curve shows a sharp decrease in pH. After equivalence point, the curve will show pH to the strong acid.

Weak Acid - Weak Base Titration 

Titration Curves of Polybasic Acids 

When polybasic or polyprotic acids are titrated, the protons get released in different stages and thus the curve gets different shapes. For example, let us consider sulfuric acid (H₂SO4) which is a diprotic acid. It has two. H * ions which can be given up during the reaction. The first ion will break off in water by the dissociation which is shown below.