Relation of indicator colour to p h

1. Relation of indicator colour to pH

2.  Acid
Chemical Substance having characteristics:-
(1) pH < 7
(2) Nutralize base/Alkali
(3) Red Litmus change to blue
 Base
 Chemical Substance having characteristics:-
(1) pH > 7
 (2) Nutralize Acid
 (3) Blue Litmus to red

3. •
The Arrhenius Model
Acid – produces hydrogen ions in aqueous solution
Base – produces hydroxide ions in aqueous solution

4. The Bronsted-Lowry Model
 Acid – proton donor
 Base – proton acceptor
 The general reaction for an acid dissolving in water is

5. Lewis Concept:-
Acid:-
 Molecule which accept electron pair.
Base:-
 Molecule which donate electron pair

6.  pH
 pH is a measure of the acidity or basicity of an aqueous
solution.
pH Scale:-

7. Indicator
 Is organic dye or molecule which indicates
the end point of titration by changing colour of
solution under examination.
 Example:-
 Phenolphatalene / Methyle blue are
suitable indicators for titration of strong acid &
base.

8. The indicator solution contain both the yellow
In- and the red HIn molecules. The actual
colour shade of the indicator depends on the
ratio of concentration of In- and Hin present
in solution. From the equlibrium constant
expression (1) we can write
[H+] = Kin [In-]/[Hin] ……(2)
Relation of indicator colour to pH

9. If [H+] is large, the concentration of In- ions is
alos large and the colour is yellow. When [H+]
is small,[HIn] is large and the solution is red .
At the equivalence point, [In] = [Hin] and the
colour is orange (red+yellow).
indicator colour is controlled by
hydrogen ion concentration or pH of the
solution.

10. Taking logarithims and using definnition of pH
and kin,the experession (2) can be converted to
Henderson Hasselbalch equation.
pH = pkin log[In]/[Hin] ………..(3)
At the equation point, [In-] = [Hin] and metyl
orange in solution is orange. Then,
pH = pkin

11. The numerical value of the indicator constant
kin for the indicator constant kin for methyl
orange is 3.6 and pH of the orange solution is ,
therefor, about 4. As the value of kin for the
various indicators are different, the will have
intermediate intense colour (middle tint) at
different pH values.

12. When a base is added to an acid solution in a
titration, the colour change of of the indicator is
gradual. It just becomes visible to the human eye
when [in -]/[Hin] = 10 when pH is4.4.
The range between 3.1 (red) and 4.4 (yellow) is
called the colour change interval of methyl orange
. The visible indicator colour colour change takes
place between these values.

13.  Indicator Action of Phenolphthalein.
 It can be explained as in case of methyl orange. It is a
weak acid exists as the following equilibrium in
solution,
 HIn H+ + In-
Colourless Pink

14. HIn molecules are colourless, while In- ions
are pink.Thus in acid solution, phenolphthalein
is colourless and in basic solution it is pink.
The value of kin = 9.6 and the pH of the
intermediate intense pink tint is also 9.6.The
colour change interval of phenolphathalein is
8.1- 10.0.

15. The Ostwald’s theory takes care of the
quantitative aspect of indicator action
adequately. The quinonoid theory, on the other
hand, tell us the cause of colour change of an
indicator in acid-base solution. It lays down
that :

16. The unionised HIn molecule and the anion In-
are tautomeric form of the indicator which is an
organic dye.
One tautomeric form possesses the quinonoid
structural unit and is called the quinonoid
form.
Quinonoid
structure

17. It has a deep colour. The form has a lesser
colouring group,say, -N = N – and or simply
benzene rings and is called the benzenoid form.
This form has a light colour or no colour.
The colour change of the indicator occurs
when one tautomeric form is transformed into
the other due to the change of pH of the
solution.

18. Let us illustrate the quinonoid theory by taking
example of methyl orange and phenolphthalein.
 Methyl orange.
The red quinonoid form of methyl orange
exists in acid solution. It is converted to yellow
benzenoid form when pH alters to the basic
side.

19. N
Ch3
Ch3
SO3N = N
N
Ch3
Ch3
SO3HN = HN
Red Acid solution
Yellow basic solution

20. Phenolphthalein.
 Phenolphthalein exists in two
tautomeric forms : (i) the benzoid form
which is yellow and present in basic
solution; and (ii) the quinonoid form
which is pink and present in acid solution.

21. =O
C
HO
OH
COOH COOH
C
Colorless acid solution Pink basic solution