Welcome to Chem Zipper.com......: ACID-BASE TITRATION:

Search This Blog

Wednesday, December 5, 2018


It is method to determine concentration of any Solution with the help of a solution of known concentration.
When the complete reaction occurs between the solutions, it is called equivalent point of titration.
When sudden change in colour of solution occurs, it is called end point of titration.
(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.
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]

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.
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  
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.
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?
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? 
(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-6); Double Indicators Titration:
(A-7): Back Titration:
(1) Titration of diprotic acid with strong base:
(2) Titration of triprotic acid with strong base:

No comments:

Post a Comment