Acid Base titrations:-
Acid base titrations are generally a neutralization reaction between acid and base to form a salt and water.
As:- HCl + NaOH → NaCl + H₂O
✓ In acidimetry, the solution of alkali is taken in conical flax and is titrated against the acid solution till an equivalent amount of acid is added to the alkali.
✓ the point at which alkali is completely neutralized by acid is called end point.
✓ this end point is observed when the indicator changes its colour from alkali medium to neutral medium.
Acid base indicator:-
✓ In order to detect the endpoint in acid base titrations indicator are used. They are known as acid base indicator or neutralization indicators.
✓ acid base indicators are organic dye that detects the end point by a visual change in color’s.
✓ these indicators change color according to the hydrogen ion concentration(ph) of the solution.
✓ important characteristics of these indicator is that they have one color in acidic medium and develop another color in alkaline medium.
Theory of acid base titrations:-
There are two important theories that can explain the behaviour of the indicators in acid and base solution.
1. Ostwald’s theory:-
According to this theory, the indicators are weak acid and weak base which are dissociate as-
Acid form → H₃O⁺ Basic form
✓ Unionized acid form has a color different form that of the basic form produced by the ionized of the indicator in aqueous solution.
HIn + H₂O → H₃O⁺ + In⁻
Therefore, in acidic solution phenolphthalein is colourless and in basic solution it is pink.
2. Quinonoid theory:-
Ostwald theory gives the quantitative description of colour change by the indicator. But to know about the structural change, Quinonoid theory explain the cause of the colour change on the basic of structural change of the indicator.
✓ according to the Quinonoid theory, the compounds which have benzene ring(benzenoid compounds) are the colourless while those have Quinonoid structure are coloured. (Or high intense colour)
In states that:- Indicator exits as an equilibrium mixture of two tautomeric forms.
✓ concentration of a particular form change which pH so that one tautomeric form in the transformed into another form.
✓ in acidic solution, proton attached to nitrogen atom an electron involved in dilocolization inside benzene ring is distributed leads to transformed of Quinonoid form and shows red color.