Water can behave as weak acid and a weak base. If we are titrating very weak acid or weak base in aqueous solution, then it is possible that it can compete with weak acid or base with regard to proton donation and acceptance. For example –
H3O+ can compete with RNH3+. If it happens then we don’t get correct end point by titration. A general rule is that acids with pKa>7 and bases with pKa<7 cannot be determined accurately in aqueous solution. So, for titration of weak acids or bases we use non-aqueous solvents. Many organic solvents such as acetone, alcohol etc. can easily replace water as they compete less effectively with analyte for proton donation or acceptance. Reactions which occurred in non- aqueous titrations can be explained by Bronsted -Lowry theory and its definition of acids and bases.
Following four types of solvents are used to dissolve analytes in non-aqueous titrations –
Aprotic Solvents – These are chemically inert substances such as benzene, chloroform etc. these are added to ionizing substances to depress solvolysis of the neutralization product which in result sharpen the end point. Examples – benzene, toluene, carbon tetrachloride etc.
Protophilic Solvents – These are basic in nature and react with acids to form solvated protons. A strong protophilic solvent changes the weak acids to strong acids. Examples – liquid ammonia, amines, ether etc.
Protogenic Solvents – These are acidic substances and readily donate the proton. These are used to increase basicity of weak acid. They show a levelling effect on bases. Examples- sulfuric acid, formic acid etc.
Amphiprotic Solvents – These solvents have properties of both protophilic and protogenic solvents. Examples – alcohol, acetic acid etc.
Non- aqueous titration has following advantages -
It is useful for the titrations of very weak acids or bases.
Many organic acids which are not soluble in water, can be dissolved in non-aqueous solvents. Thus, titration of these organic acid is very easy.
It can be used for titration of mixture of acids as well.
These titrations show sharp end point with internal indicator.
It is simple, qualitative and selective method.
It is a highly accurate method.
Preferably non-aqueous titration is used for biological matters.
It is very important in pharmacopoeial assays.
Non-aqueous titration has various uses in numerous fields. Specially, in medicinal field non-aqueous titration is very useful. We have listed here few applications of non-aqueous titration -
Non – aqueous titration is used to know the purity of assays.
It is used for determination of concentration expressions.
It is used in determination of hydrophobic compounds, phenobarbitone, diuretics, steroids.
It is used in the determination of composition of antitubercular drugs and adrenergic drugs.
Non – aqueous titration has following disadvantages -
Solvents used in non-aqueous titration are not stable compared to aqueous solvents.
In non-aqueous titration, non-aqueous solvents are required calibration after every use.
In non-aqueous titrations temperature corrections are necessary.
If you want to know more about titration then go through the articles Mohr salt titration with KMnO4 precipitation titration, oxalic acid titration with KMN04 etc. available on Vedantu. You can register yourself on Vedantu or download Vedantu learning app for more such articles, NCERT Solutions, study material and mock tests etc.