Titration is a method to estimate the strength of any given substance in analytical chemistry.
Titrations are carried out in labs on a routine basis by methods like acid-base titration, redox titration, complexometry and precipitation.
An acid-base titration is a most simple and widely applicable technique. It is used to estimate the strength of acids, bases, and even salts.
Here an acid reacts with a base to form salt and water. This reaction is called neutralization reaction.
The pH at the neutralization point is 7.
In an acid-base titration, a known quantity of acid is used to estimate an unknown amount of a base and vice-versa. One of the reactants is taken in a burette and other in a beaker. The reactant from burette is added drop by drop while the beaker or flask is swirled till the endpoint is reached.
The reactant taken in burette is called titrant while that taken in flask is called as titer
In between an indicator is used to indicate the end point of reaction in between acid and base. In modern labs instead of indicators, pH meters are used to indicate endpoint.
Even the substance which is not acidic or basic can be converted to by use of some reaction and then estimated employing back titration or reverse titrations.
Acid-Base Titration: 5 Types
A. Aqueous acid-base titration.
These are normal titration between acids and base dissolved in water. Hence the name aqueous titration. They are prominently used in academic labs and for standardization.
1) Strong acid V/s strong base: Here are strong acid reacts with a strong alkali to form salt and water. The reaction of this type is swift and also complete. The reaction happens in stoichiometric means, i.e., each molecule of acid reacts with the corresponding molecule of the base. At the end of the reaction, no molecule of acid or base exists as every molecule in the reaction has completely reacted to form a salt. Hence the endpoint or equivalence point is precise and sharp.
Example of these types of acids are HCl, H2So4, HNO3, HBr, HClO4 (perchloric acid), H3PO4, etc. The examples of strong bases are NaOH, MgOH2, Al2OH3, etc.
Reaction example: HCl+NaOH—————-> NaCl + H2O
Either the known quantity of acid is taken in burrete to react with an unknown quantity of strong base in the flask (beaker) or vice-versa.
The pH at the end point is neutral, i.e., 7. So indicator changing their color around pH seven are used here.
2) Strong Acid v/s Weak Base: Here a strong base reacts with weak acid to form salt and water. But since the reaction uses a strong acid, the pH at the endpoint will be towards acidic, i.e., below 7.
Reaction example: HCl+NH4OH—————–>NH4Cl + H2O.
Here the salt formed NH4Cl is slightly acidic. So indicators changing color at lower pH’s are employed.
During the reaction, a known concentration of strong acid is taken in a burette and allowed to react drop by drop with the base in a beaker.
3) Weak Acid V/s Strong Base: Here the reaction happens between a weak acid and strong base. The weak acid is taken in a beaker and known quantity of strong base is dropped from a burrete till the endpoint.
Reaction example: H2CO3+ NaOH————–>Na2Co3+H2O
The salt formed is slightly basic, so the pH at the end point is above 7. The indicator used is one with a change in color at higher pHs.
4) Weak Acid V/s Weak Base: Here both acid and base are weak. So mostly they are avoided due to imprecise endpoints. At the endpoint, the pH will be seven theoretically. But cannot be measured precisely like that in strong acid and strong base case. An extra amount of titrant is needed to reach the endpoint due to the imprecise reaction.
Reaction example: H2CO3+NH4OH————–>NH4OH+H2O
The endpoint is neutral as the salt is neutral but due t excess titrant added the pH could be in favor of it.
Check the video below for method
B. Non-Aqueous titration:
These are conventional methods of non-aqueous titration. Here instead of water as solvent glacial acetic acid is used to make the reactants. They are similar to above types of acid-base reactions. Since many drugs are water-insoluble and slightly acidic or basic, they are analyzed by non-aqueous titrations.They are extensively used for quality control and analysis of drugs.
Reaction example: psuedoephidrin+HCl