Non-Aqueous Titration - Types of Non-Aqueous Titrations, Advantages, Disadvantages, Practice Problems and FAQ

Do you know what nicotine is? 

Nicotine is the primary drug present in cigarettes, which is toxic and extremely addictive. Nicotine intake may result in an increase in heart rate, blood flow to the heart, blood pressure and artery constriction. As a result, excessive nicotine usage might result in heart attacks. Therefore, only a certain amount of nicotine should be present in cigarettes. The precise amount of nicotine to be added in a cigarette is regulated by a volumetric analysis technique called non-aqueous titration. 

In this concept page, we will discuss non-aqueous titration in detail.

TABLE OF CONTENTS

• Non-Aqueous Titration
• Types of Non-Aqueous Solvents
• Advantages of Non-Aqueous Titrations
• Disadvantages of Non-Aqueous Titrations
• Applications of Non-Aqueous Titrations
• Practice Problems
• Frequently Asked Questions - FAQ

Non-Aqueous Titration

Non-aqueous titration is a volumetric analysis performed solely in solvents with no water molecules. Titrations against acids or bases are performed in a non-aqueous solvent with dissolved analytes or samples. The method used in active pharmaceutical assays.

The non-interference of water molecules in titration is what makes non-aqueous titrations significant. Water is both a weak acid and a weak base. Water molecules compete for proton donation with other bases and accept protons from other acids dissolved in them. This makes it difficult to determine the endpoint in titration. As a result, avoiding the interference of water molecules in the titration procedure is critical. Due to the absence of water molecules in non-aqueous titrations, it gives sharp endpoints and accurate results.

Non-aqueous titration is a highly helpful process because it satisfies two requirements: 

1. It is appropriate for titrating very weak acids or bases

2. It provides a solvent that may dissolve organic molecules.

The following reaction is an illustration of a reaction in which water is an inappropriate solvent:

RNH₂+H⁺ ⇌ RNH₃⁺

The above reaction in an aqueous solvent is competed with by the reaction given by:

H₂O+H⁺⇌ H₃⁺O

It is challenging to determine the titration's end point because of the type of competition that water provides to weak bases or weak acids. Due to their slightly basic or weakly acidic nature, these compounds typically need to be titrated in non-aqueous solvents since they have very sharp end points when titrated in aqueous solutions.

The Bronsted-Lowry Theory and its definition of acids and bases can be used to explain many of the reactions that take place during non-aqueous titration methods. In general, bases are considered proton acceptors while acids are considered proton donors.

Types of Non-Aqueous Solvents

There are four types of solvents that are commonly used in the non-aqueous titration of a given analyte. They are as follows:

1. Aprotic Solvents: Aprotic solvents are chemically inert. These solvents are non-reactive and do not react with acids or bases. They have a low dielectric constant and do not cause solute ionisation. Aprotic solvents' primary function is to dilute the reaction mixture. 

Examples: Toluene, acetonitrile, tetrachloride, benzene, chlorinated hydrocarbons, etc. 

2. Protophilic Solvents: Protophilic solvents are those that have higher basicity than water. These solvents have a strong attraction to positively charged protons. In the presence of an acidic solution, protophilic solvents form a conjugate base of the acid and a solvated proton. A protophilic solvent's primary function is to increase the acidic strength of very weak acids. They increase the strength of weak acids by readily accepting protons. Because of their high affinity for protons, they have a levelling effect on weak acids. 

Examples: Liquid ammonia, ethers, amines and ketones.

3. Protogenic Solvents: Protogenic solvents are those that generate protons (Hydrogen ions). They are acidic in nature. These solvents are typically more acidic than water. The primary function of protogenic solvents is to increase the basic strength of weak bases. They make weak bases stronger by donating protons. Because of their high proton donating capacity, they have a levelling effect on weak bases. 

Examples: Sulphuric acid, anhydrous hydrogen fluoride, formic acid, etc. 

4. Amphiprotic Solvents: Amphiprotic solvents are those that have both protophilic and protogenic properties. Amphiprotic solvents are chemically similar to water molecules in that they have both acidic and basic properties. Depending on the type of solute used, they can readily accept or donate protons. They easily donate protons in the presence of a weak base, increasing the basicity strength of the base used. They readily accept protons in the presence of a weak acid, increasing the acidic strength of the acid used. 

Examples: Weak organic acid such as acetic acid, and alcohols, such as ethanol and methanol.

Potentiometric titration procedures can also be used to precisely measure the endpoints of these titrations.

Advantages of Non-Aqueous Titrations

Non-aqueous titrations have the following advantages.

1. Non-aqueous titrations make it easier to titrate weak acids and bases. Weak acids and bases are difficult to titrate with aqueous solvents.

2. Non-aqueous titrations are used to perform volumetric analysis on organic acids that are insoluble in water.

3. Non-aqueous titrations are also used to analyse an acid mixture.

4. Non-aqueous titrations produce precise results with sharp endpoints.

5. Non-aqueous titrations are simple and straightforward.

6. Non-aqueous titrations are extremely important in pharmaceutical product assays.

7. Non-aqueous titrations have a high degree of selectivity.

8. Non-aqueous titrations include the analysis of drugs such as tetracycline, antihistamines, codeine phosphate, and many others.

Disadvantages of Non-Aqueous Titrations 

The following are the drawbacks of non-aqueous titrations.

1. When compared to aqueous solvents, non-aqueous solvents are less stable.

2. Temperature corrections are required from time to time in non-aqueous titrations.

3. Calibration is required after each and every use.

Applications of Non-Aqueous Titrations

Non-aqueous titrations have the following applications

1. The purity of assays are determined by non-aqueous titrations.

2. To ascertain the concentration of the specified analyte, non-aqueous titrations are employed.

3. Hydrophobic medications including phenobarbitone, steroids, diuretics, tetracyclines, etc, are identified via non-aqueous titrations.

4. Adrenergic and anti-tubercular medications' drug compositions are evaluated using non-aqueous titrations.

Practice Problems

Q1. Which of the following solvents is protophilic in nature?

A. Sulphuric acid
B. Formic acid
C. Ammonia
D. Methanol

Solution: Protophilic solvents possess higher basicity than water. Positively charged protons are strongly attracted to these solvents. They make weak acids stronger by readily accepting protons. 

Sulphuric acid and formic acid are acidic in nature, ethanol is amphoteric in nature, and ammonia is basic in nature. Therefore, ammonia is protophilic in nature. 

So, option C) is the correct answer. 

Q2. Which of the following solvents is protogenic in nature?

A. Ethanol
B. Acetic acid
C. Chloroform
D. Hydrogen Flouride

Solution: Protogenic solvents are ones that produce protons (Hydrogen ions). These solvents are typically acidic in comparison to water. They strengthen weak bases by donating protons. 

Chloroform is aprotic, ethanol and acetic acid are amphoteric, and hydrogen fluoride is acidic in nature. Therefore, hydrogen fluoride is protogenic in nature. 

So, option D) is the correct answer.

Q3. Which of the following class of solvents is amphiprotic in nature?

A. Acids
B. Ketones
C. Ethers
D. Alcohols

Solution: Amphiprotic solvents possess both protophilic and protogenic properties. Amphiprotic solvents have acidic and basic properties, making them chemically similar to water molecules. They can accept or donate protons depending on the type of solute used. 

The examples of amphiprotic solvents include weak organic acids like acetic acid, and alcohols like ethanol and methanol. Acids (excluding weak acids) belong to the category of protogenic solvents, ethers and ketones belong to the category of protophilic solvents, and alcohols belong to the category of amphiprotic solvents. 

So, option D) is the correct answer. 

Q4. Which of the following solvents is aprotic in nature?

A. Toluene
B. Acetonitrile
C. Tetrachloromethane
D. All of these

Solution: Aprotic solvents are chemically inert. These solvents are non-reactive and do not react with acids or bases. They have a low dielectric constant and do not cause solute ionisation. Aprotic solvents' primary function is to dilute the reaction mixture. All the given solvents are aprotic in nature. 

So, option D) is the correct answer.

Q5. Which of the following theories is used to define Non-aqueous titrations?

A. Arrhenius Acid Base theory
B. Lewis Acid Base theory
C. Bronsted-Lowry Theory
D. None of these

Solution: Many of the reactions that occur during non-aqueous titration methods can be explained using the Bronsted-Lowry Theory and its definition of acids and bases.

So, option C) is the correct answer.

Frequently Asked Questions - FAQ

Question 1. What exactly is titration?
Answer: Titration is defined as the slow process of adding one solution of any known concentration (titrator or titrant) to a specified volume of another solution of unknown concentration (titrand or analyte). The reaction continues until it reaches neutralisation, which is indicated by a change in colour. The reaction can be investigated until an equivalence point is reached.

Question 2. Which indicators are used in non-aqueous titrations? 
Answer: The following are some of the indicators used in non-aqueous titrations.

• Crystal Violet: In a basic media, it produces a violet colour, while in an acidic one, a yellowish-green one. It is frequently utilised for pyridine titration.

• Oracet Blue B: Pink in acidic medium and blue in basic medium.

• Other indicators include quinaldine red and alpha naphtholbenzein

Question 3. What is Levelling effect in non-aqueous titration?
Answer: The acidity of weak acids can be increased by using basic solvents because basic solvents have a higher affinity for taking up protons from acid. In ammonia solution, acetic acid behaves as a strong acid. The basicity of weak bases can also be increased in the presence of an acidic solvent. This is known as the solvent's levelling effect.

Question 4. In a non-aqueous titration, what should be avoided?
Answer: In a non-aqueous titration, moisture should be avoided in order to neutralise the solution and reach the endpoint.

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