Lecture 38 : Titrations : Acid-Base, Redox and Complexometric
38.1 Acid-Base Titrations:
Acid-base reactions are of great practical importance in analysis, not only because of their use in titrating a large number of inorganic and organic substances, but also because the hydrogen ion concentration of a solution often is of great importance in controlling reactions.
Titration :
The process of determining the volume of a given solution of a reagent equivalent to the amount of another reactant present in a standard solution is known as titration.
Equivalent Weight of Acids and Bases :
The equivalent weight of an acid is that weight which yields one mole of hydrogen ions in the reaction employed whereas the equivalent weight of a base is that weight which reacts with one mole of hydrogen ions in the reaction.
Normal solution :
A solution containing one equivalent weight of solute per litre of solution.
Equivalence Point :
When the number of equivalents of acid (respectively base) added is equal to the number of equivalents of base (respectively acid) taken initially, we have reached the equivalence point.
Acid-Base Indicators :
Weak organic acids or bases having different colours for their dissociated or undissociated forms e.g., phenol red which is yellow in colour in its undissociated form in acidic solution and red in its dissociated form in basic solution constitute these indicators.
Name of indicator
Colour in acid solution
Colour in basic solution
pH range
Methyl Orange
Red
Orange – yellow
3.1 – 4.6
Bromophenol Blue
Yellow
Blue – violet
3.0 – 4.6
Methyl Red
Red
Yellow
4.2 – 6.3
Bromothymol Blue
Yellow
Blue
6.0 – 7.6
Thymol Blue
Yellow
Blue
8.0 – 9.6
Phenolphthalein
Colourless
Red
8.0 – 9.8
Thymolphthalein
Colourless
Blue
9.4 – 10.6
Table 38.1 Some acid-base indicators, their colour changes and pH range:
