Non-Aqueous Titration

In any chemistry lab, titration is a coming-of-age experiment—a tried-and-true experiment in which colour changes will get you the concentration of the solution. But not every compound gets along with water. Some are too weak, recalcitrant, or insoluble for regular titration, where water is the hero of the experiment. That's where non-aqueous titration comes in, using organic solvents to provide reliable results. Here, we will explore this resourceful alternative to traditional titration used in various industries. 

1.0Non-Aqueous Titration Meaning

Non-Aqueous Titration is an acid-base titration which doesn’t use water but other organic media, such as glacial acetic acid, dimethylformamide, and pyridine, as the solvent. The main aim of this type of titration is to find the concentration of the substances that are either insoluble, unstable, or weakly reactive in water but are good reactants in a suitable organic medium. This ultimately leads to more accurate and precise measurements of the concentration of such substances. 

2.0Non-Aqueous Titration Principle 

The Principle of Non-Aqueous Titration is based on the fact that acid-base reactions are more complete in solvents than in water. In simple words, weak acids and bases do not fully dissociate in an aqueous solution, which leads to incorrect titration results. However, in a non-aqueous medium, this situation completely reverses, that is:  

• Weak acids behave more acidic, and that too, a strong one. 
• Similarly, weak bases behave more strongly basic. 

The reason behind this change of behaviour is that non-aqueous solvents can efficiently accept or donate protons, eventually helping the reaction to completion. 

The Need for Non-Aqueous Titration

The need for non-aqueous titration arises due to an important yet annoying property, in some cases, of water — the property to act both as a weak acid and a weak base. This double character makes it compete with weak acids or bases in accepting or donating protons. Which ultimately disrupts the titration and prevents the determination of distinct end points.

However, Non-aqueous titration sidesteps this problem by utilising organic solvents that do not engage in proton transfer, permitting weak acids or bases to react more fully. It also facilitates the dissolution of organic compounds that are insoluble in water. For example, weak acid in an organic solvent reacts like this: 

$R-\mathrm{ch}NH2+\mathrm{ch}H+\rightleftharpoons R-\mathrm{ch}NH3+$

On the other hand, this reaction competes with water in standard titration: 

$\mathrm{ch}H2O+\mathrm{ch}H+\rightleftharpoons \mathrm{ch}H3O+$

3.0Solvents Used in Non-Aqueous Titration

The solvents used in non-aqueous titration are chosen on the basis of the chemical nature of the chemical to be analysed. These solvents are divided into several categories depending on their potential to either donate or accept protons. These categories are: 

Types of Non-Aqueous Solvents

1. Aprotic Solvents: This type is also known as neutral solvents, meaning they neither donate nor accept protons and do not interfere in acid-base reactions. These are more commonly used when analytes are required to dissolve in a solvent without participating in the titration itself. Examples: Acetone, benzene, chloroform, ethyl acetate. 
2. Protogenic Solvents: Protogenic (Proton + genic) refers to the solvents which donate protons and are acidic. These are used to enhance the strength of weak bases by giving them protons. Examples: Glacial acetic acid, sulfuric acid, formic acid.
3. Protophilic Solvents: These are proton-loving or proton-accepting solvents, meaning they are basic in nature. Protophillic solvents are ideal for weak acids to enhance their strength by accepting protons. Examples: Ammonia, ethylenediamine, pyridine. 
4. Amphiprotic Solvents: These solvents are both proton donors and proton acceptors, and act as the requirement of the reaction. Although these are versatile solvents, they still can interfere with titration just like water. Example: Water, ethanol, methanol. 

4.0Working of Non-Aqueous Titration

The working of non-aqueous titration strictly follows the following steps: 

• Dissolution of the Analyte: The required analyte is dissolved in the selected non-aqueous solvent to ensure proper reaction with the titrant. 
• Addition of Indicator: Now add a suitable non-aqueous indicator to detect the endpoint. Here are the examples and ph range of certain non-aqueous indicators: 

• Titration Process: Now, just like traditional titration, slowly add the titrant from a burette until a visible colour appears, indicating the endpoint of the titration. 
• Calculation of Result: Note the amount of titrant used and use the standard formula for titration to calculate the concentration: 

$N_1\times V_1=N_2\times V_2$

To better understand the above-mentioned concepts, see the following non-aqueous titration examples:

1. Non-Aqueous Titration of a Weak Base: Take Ephedrine Hydrochloride, in this reaction, the: 

• Solvent: Glacial acetic acid
• Titrant: Perchloric acid (HClO₄)
• Reaction: $\mathrm{B}+{\mathrm{HClO}}_4\to {\mathrm{BH}}^{+}+{\mathrm{ClO}}_4^{-}$

2. Non-Aqueous Titration of Weak Acid: Take Benzoic Acid, in this reaction, the: 

• Solvent: Methanol or DMF
• Titrant: Sodium methoxide $\left({\mathrm{CH}}_3\mathrm{ONa}\right)$
• Reaction: $\mathrm{R}-\mathrm{COOH}+{\mathrm{CH}}_3\mathrm{ONa}\to \mathrm{R}-\mathrm{COONa}+{\mathrm{CH}}_3\mathrm{OH}$

5.0Applications of Non-Aqueous Titration 

The Applications of Non-Aqueous Titration are widespread in different industries for different works, such as: 

• Assessing drugs like ephedrine, chlorpromazine, antihistamines, etc.
• Inspection of quality in dyes, cosmetics, and fatty acids
• Determination of the values of saponification in oils and fats. 
• Used in academic research to study the properties of weak acids or bases. 
• And lastly, to determine the purity of weak acids and bases in pharmaceuticals. 

6.0Aqueous and Non-Aqueous Titration