THEORY OF INDICATOR'S:
It
is method to determine concentration of any Solution with the help of a
solution of known concentration.
EQUIVALENT POINT:
When the complete reaction occurs between the solutions, it is called equivalent point of titration.
END POINT:
When sudden change in colour of solution occurs, it is called end point of titration.
INDICATORS:
(1) They are very weak organic acids or bases that show different colours at different pH value.
(2) They exist in ionized and unionized form which have different colours .
(3) The point at which Number of equivalent of acid and base become equal is known as equivalent point / Stoichiometric point / neutralization point.
(4) We assure that indicators do affect the pH of Solution.
(5) Around the equivalent point pH change the drastic change.
(6) For the successful titration end point should be as close as possible to the equivalent point.
TYPE OF INDICATOR'S:
(1) ACIDIC INDICATOR'S
(2) BASIC INDICATOR'S
EQUIVALENT POINT:
When the complete reaction occurs between the solutions, it is called equivalent point of titration.
END POINT:
When sudden change in colour of solution occurs, it is called end point of titration.
INDICATORS:
(1) They are very weak organic acids or bases that show different colours at different pH value.
(2) They exist in ionized and unionized form which have different colours .
(3) The point at which Number of equivalent of acid and base become equal is known as equivalent point / Stoichiometric point / neutralization point.
(4) We assure that indicators do affect the pH of Solution.
(5) Around the equivalent point pH change the drastic change.
(6) For the successful titration end point should be as close as possible to the equivalent point.
TYPE OF INDICATOR'S:
(1) ACIDIC INDICATOR'S
(2) BASIC INDICATOR'S
(1) ACIDIC INDICATOR'S:
Phenophthalene (HPh): It is weak acid and can represented as [HPh]
If it ionised gives [H+ ] and [Ph- ]
Addition of [H+]: In addition of an acid the ionization of HPh is practically
negligible and as the equilibrium shift left hand side due to high
concentration of [H+] ion thus solution would remain colourless.
Addition of [OH- ]: In the presence of a base H+][
ions are removed by [OH-] ions in the in the form of water
molecules and the above equilibrium shift to right hand side .Thus the
concentration of Ph- ions increases in solution and they impart a pink colours
to the solution.
INDICATOR THEORY: Let consider acidic indicators [HPh] Phenophthalene.
An indicator has two colouring parts.
(1) Unionized part of indicators
(2) Ionized part of indicators
The relative concentration of these species will
depend upon PH of medium .
Take negative log both sides
CASE (1): It is observed that
Then colour of indicator
decided by concentration of [Ph-)]
CASE (2): It is observed that
Then colour of indicator
decided by concentration of [HPh]
PH RANGE OF INDICATORS:
The given indicators work
between a pH range i.e.
Then working of indicators is
best;
ILLUSTRATIVE EXAMPLE (1): For an acidic indicator, dissociation constant, Kin
is 2×10-6 Calculate pH range of indicator.
SOLUTION:
ILLUSTRATIVE EXAMPLE (2): The pH range of a basic indicator is 4 to 6.5
Calculate the dissociation constant of indicator?.
SOLUTION:
pKIn must be
midpoint of pH range for acidic indicators and pOH range for basic indicators
The pH range = 4 to 6.5 so pOH
range is 10 to 7.5
Hence PkIn = (10+7.5) /2 = 8.75
ILLUSTRATIVE EXAMPLE (3): For an indicator pKa is 6 Calculate pH of Solution
having this indicator such that 40% indicator molecules remain in ionised form.
SOLUTION: We know that
(2) BASIC INDICATOR'S
Methyl orange (MeOH): It is weak base and can represented an MeOH
If it ionised gives [ Me+] and[ OH- ] and methyl orange is
an intensely coloured indicator that is red below pH 3.1 and
orange-yellow above pH 4.4
The red (acid) form has an
[H+] attached to one of the N atoms and the yellow (basic) form
has lost the [H+]
Addition of [H+ ] : In
the presence of an acid, OH- ions are removed in the form
of water molecules and the above equilibrium shift to right hand side .This
effect Me+ ions are produced which impart red coloure to the
Solution.
Addition of [OH- ]: In addition of alkali the concentration of OH-
increases in the solution and equilibrium shift left hand side i.e. the
ionization of MeOH is practically negligible .thus solution acquired the colour
of unionised methyl orange molecules i.e. yellow
DEGREE OF DISSOCISTION OF INDICATOR'S (DOD):
Consider a general indicator dissociation;
ILLUSTRATIVE EXAMPLE (4): For acidic indicator, pH range is 3 to 4.6 calculate
the ratio of[ In- ] and H+ for the appearance of solution
in a single colour.
SOLUTION:
Given pH range 3.0 to 4.6 so pKIn=
(3.0+ 4.6)/2 =3.6
ILLUSTRATIVE EXAMPLE (5): An indicator with Ka = 10-5 is solution
with pH = 6 Calculate % of indicator in ionised form?
SOLUTION:
ILLUSTRATIVE EXAMPLE (6): The pH of at which an acid indicator with Ka is 10-15
changes colour when indicator concentration 1×10-5 M is?
SOLUTION:
TYPE OF TITRATION:
(A) ACID-BASE TITRATION:
(A-1): Strong acid Vs Strong base:
(A-2): Weak acid Vs Strong base:
(A-3): Weak base Vs Strong base:
(A-4): Weak acid Vs Weak base: weak acid and weak base titration can not be carried out because due to very low PH change , their is no suitable indicator for this titration.
(A-5); Salt of SB and WA Vs Strong acid:
(A-3): Weak base Vs Strong base:
(A-4): Weak acid Vs Weak base: weak acid and weak base titration can not be carried out because due to very low PH change , their is no suitable indicator for this titration.
(A-5); Salt of SB and WA Vs Strong acid:
(A-6); Double Indicators Titration:
(A-7): Back Titration:
(B):TITRATION OF POLYPROTIC ACIDS:
(1) Titration of diprotic acid with strong base:
(2) Titration of triprotic acid with strong base:
(B):TITRATION OF POLYPROTIC ACIDS:
(1) Titration of diprotic acid with strong base:
(2) Titration of triprotic acid with strong base:
No comments:
Post a Comment