Content Curator
Non-Aqueous Titration is a type of titration technique very similar to acid-base titrations. In this type of titration, the analyte substance or sample is dissolved purely in a non-aqueous solvent (free from water molecules). The titration is conducted using the acid or base as titrants. Aqueous solvents readily accept or donate protons when dissolved in a weakly acidic or basic medium. This is why Non-aqueous solvents are needed to generate accurate and sharp endpoints in titration techniques. The most widely used applications of non-aqueous titrations are for pharmacopoeial assays.
| Table of Content |
Key Takeaways: Non-Aqueous Titration, Non-Aqueous Solvents, Bronsted-Lowry theory Weak Acids, Weak Bases, aprotic, Protophilic, Protogenic, Amphiprotic
Non-Aqueous Titration Theory
[Click Here for Sample Questions]
Water is a known weak acid and a weak base. It can accept or donate protons when dissolved in weak acids or bases. This interference of water in the titration reactions makes it extremely difficult to trace the endpoints of titration. Acidic (proton donors) or basic (proton acceptors) substances behave like one only in the presence of a base or acid respectively. Thus the presence of water in the solvent leads to a competitive reaction. This difficulty in volumetric titrations paved the way for the Non-Aqueous Titration Analysis which is based on the Bronsted-Lowry theory and its definition of acids and bases.
Following are the two major reasons that the Non-Aqueous Titration theory satisfies:
- Suitable for weak acid and base titrations
- Organic compounds get a suitable solvent medium to get dissolved
Non-aqueous solvents such as acetones and alcohols can thus replace water as a solvent in titrations as they do not compete with other analytes for donating or accepting the protons.
Read More: Volumetric Analysis
Types of Non-Aqueous Solvents
[Click Here for Sample Questions]
Based on the solvent used, four different solvents are used for non-aqueous titrations:
Aprotic
- Properties: These possess a neutral charge and do not participate in any reaction with acids or bases. There is no ionization of solutes observed when these chemically inert solvents are used for titration. The dielectric constant for these solvents is very low.
- Function: The primary function is to reduce the strength of the reaction mixture.
- Examples: Acetonitrile, Toluene, Benzene, Chlorinated hydrocarbons, chloroforms, etc.
Protophilic
- Properties: These possess a basic charge with basicity much greater than water. These solvents have a great affinity for positively charged protons and hence react readily with acids. This reaction forms solvated protons and a conjugate base of the acid.
- Function: The acidic strength of very weak acids is greatly improved with the application of Protophilic solvents. The proton accepting nature of these solvents results in a leveling action on weak acids.
- Examples: Liquid Ammonia, Pyridine, Ethers, Amines, Ketones, etc.
Protogenic
- Properties: These solvents are highly acidic and hence produce hydrogen ions when in contact with bases. These solvents possess acidity greater than water.
- Function: These solvents improve the basic strength of weak bases by donating protons. This hence provides a leveling effect on the bases.
- Examples: Sulphuric acid, Formic Acid, Anhydrous Hydrogen Fluoride, Acetic acid, etc.
Amphiprotic
- Properties: These solvents are very similar in charge to water molecules. They have a combination of protophilic and protogenic properties. These solvents are capable of readily accepting or donating protons based on the nature of the solute.
- Function: In the presence of a weak base, these solvents readily donate protons thereby increasing the basicity of the base in use. Similarly, in the presence of a weak acid, these solvents readily accept protons, thereby increasing the acidic strength of the acid in use.
- Examples: Acetic acids, Alcohols like Methanol, Ethanol, etc.
Read More:
| Related Articles | ||
|---|---|---|
| Mass Percentage | Normality and Molarity | Molar Mass and Molecular Mass |
| Abnormal Molar Masses | Factors Affecting Solubility | Number of Moles Formula |
Endpoint Detection
[Click Here for Sample Questions]
The endpoint of non-aqueous titrations is determined by using potentiometric titrations. Some of the indicators used for such titrations are:
- Crystal Violet: The endpoint is violet in the basic medium and yellowish-green in the acidic medium. This indicator is predominantly used in the titration of pyridine.
- Oracet Blue B: Th endpoint is blue in basic medium and pink in acidic medium.
- Alpha Naphtholbenzein and quinaldine red (purple-red to pale green)
Read More: Titration Formula
Advantages of Non-Aqueous Solvents
[Click Here for Sample Questions]
- Simple, easy-to-use qualitative titration technique
- Provide accurate results with sharp identifiable endpoints
- Facilitate weak acid and base titrations
- Help in the determination of concentration expressions
- Used in the volumetric analysis of organic acids that are insoluble in water and a mixture of acids
- Offer great selectivity and finds significant use in pharmaceutical products- assay determination of antitubercular drugs and adrenergic drugs
Disadvantages of Non-Aqueous Solvents
[Click Here for Sample Questions]
- Non-specific titration technique with greater chances of impurity interference
- The stability is much lower in comparison to aqueous solvents
- Need to prepare indicators in a non-aqueous medium
- Temperature corrections and mandatory calibration are required after every use
- Solvents are comparatively expensive
- Volatile solvents are potential pollutants for the environment
Applications of Non-Aqueous Titrations
[Click Here for Sample Questions]
- Assay determination
- Determination of analyte concentration
- Determination of hydrophobic drugs like steroids, phenobarbitone, tetracyclines, and diuretics
- Determination of drug composition of adrenergic and antitubercular agents
Read More:
| Related Articles | ||
|---|---|---|
| Conductometric Titration | Precipitation Titration | Redox Titration |
| Concentration of a Solution | Molality and Molarity | Atomic Mass Formula |
Things to Remember
- Non-aqueous titrations use solvents free from water molecules.
- Acids or bases are used as titrants.
- The non-aqueous titration is derived from the Bronsted-Lowry theory which defines the reactivity of acids and bases.
- Non-aqueous titrations can be used for weak acid or base titrations and can dissolve organic compounds.
- Aprotic, Protophilic, Protogenic, and Amphiprotic are the types of non-aqueous solvents used for titration.
- Potentiometric titrations involving indicators like Crystal Violet, Oracet Blue B, quinaldine, etc are used for endpoint detection.
- One of the major applications of non-aqueous titration is the assay and composition determination of pharmaceutical products.
Sample Questions
Ques. Why do non-aqueous titrations use acidic anhydride as the solvent? (2 marks)
Ans. Acidic anhydrides act as co-solvents. This enables the titration of halide salts of organic bases and quaternary ammonium salt type of drugs.
Ques. What is leveling effect? (2 marks)
Ans. Leveling Effect is based on how the properties of acids and bases are affected by solvents. When the strength of a strong acid is limited by the basicity of a solvent, we say that the acid effect is leveled. Similarly, when the strength of a strong base is limited by the acidity of a solvent, we say that the base is leveled.
Ques. Why is water not used in non-aqueous titrations? (3 marks)
Ans. Water behaves like a weak acid or weak base. Based on the medium in which it is dissolved, water can accept or donate protons. Consider the following reaction:
R-NH2 + H+ ⇌ R-NH3+
Water hence is not a suitable solvent for this reaction. The aqueous solvent competes to produce the reaction:
H2O + H+ ⇌ H3O+
This competition makes it difficult to determine the endpoint of the titration. This paves way for substances with sharp endpoints to be titrated against non-aqueous solvents to produce accurate results.
Ques. What are the limitations of aqueous acid-based titrations? (3 marks)
Ans. The limitations of aqueous-based titrations are
- A mixture of acids and bases cannot be titrated.
- A mixture of strong and weak acids (bases) cannot be titrated.
- A mixture of acids (bases) with near-constant of dissociation cannot be titrated.
- Substances insoluble in water cannot be titrated.
Ques. What are the selection criteria for solvents during a titration analysis? (2 marks)
Ans. Following are the selection criteria for solvents during a titration analysis:
- Nature of the analyte- acidic or basic
- Solubility of the analyte- in an aqueous or non-aqueous medium
- Reactivity of the analyte
Ques. What are the ways to determine the endpoint of titration? (2 marks)
Ans. The endpoint of the titration is determined by the following methods:
- Potentiometric titration- This is used for the detection of the equivalence point
- Indicator method- Indicator electrodes made of glass with saturated calomel electrodes as reference are used.
Ques. What is an equivalence point or stoichiometric point? (3 marks)
Ans. An equivalence point or stoichiometric point is when the amount of an acid and a base are just enough to make the solution neutral in nature. In a titration analysis, this refers to a point where the number of moles of the titrant is equal to the moles of the solution with an unknown concentration. This ratio of acid and base needed to neutralize the solution is determined by balancing the chemical equation.
The equivalence point can be determined by noting the change in pH, the color of the solution, precipitate formation, and a change in temperature or conductivity of the solution.
Note: The equivalence point is not the same as the endpoint in the case of titration analysis.
Ques. List down the methods of determining the equivalence point in detail. (5 marks)
Ans. The different techniques to determine the equivalence point of titration are:
- pH Change – A pH indicator (colored) is used. The color is found to change based on pH changes. At the beginning of titration, an indicator dye is added and a change in color is noted that approximated the equivalence point.
- Formation of Precipitate – As part of a reaction, an insoluble precipitate may be formed. A classic example is the formation of silver chloride when the silver cation and chloride anion react. This silver chloride is insoluble in water. The color, precipitate size, or the sedimentation rate of the precipitate can make the determination of equivalence point difficult.
- Color Change – Reactions such as redox titrations that involve transition metals, change color at the equivalence point owing to different oxidation states of the final product.
- Conductance – Conductivity is altered with the formation of ions in the solution. Determination of the equivalence point using electrical conductance is a slightly complex method especially when the ions in the solutions can lead to an increase in conductivity. The predominant use of this method is for acid-base reactions.
- Spectroscopy – When the spectrum of the reactant, product, and titrant is known, the equivalence can be easily determined. This method is used to find the etching of semiconductors.
- Thermometric Titrimetry – When the rate of temperature change in a chemical reaction is used to determine the equivalence point, the method is termed Thermometric Titrimetry.
- Amperometry – When the equivalence point is determined in terms of the change in the measured current value, the method is termed Amperometry.
Ques. How to calculate acidity from titration? (2 marks)
Ans. A standard sodium hydroxide solution is used to determine the titratable acidity or the total quantity of acid in a given solution. A chemical indicator is used at this moment wherein a change in the color determines the extent of completion of the reaction.
The strength of the acid and base used is determined by the equilibrium constant from the titrant curve.
For Latest Updates on Upcoming Board Exams, Click Here: https://t.me/class_10_12_board_updates
Check-Out:
Comments