ACID BASE TITRATION

1. ASSIGNMENT
CHEMISTRY
TOPIC:-ACID BASE TITRATION
SUBMITTED TO:-TEJINDER SIR
SUBMITTED BY:-MOIN KHAN HUSSAIN
UID:-14BBT1129

2. INTRODUCTION:-
An acid–basetitration is the
determination of the concentrationof an acid or base by
exactly neutralizingthe acid or base with an acid or base
of known concentration. This allows for quantitative
analysis of the concentrationof an unknown acid or base
solution. It makes use of the neutralizationreaction that
occurs between acids and bases.
Acid–base titrations can also be used to find percent
purity of chemicals.
Alkalimetryand Acidimetry
Alkalimetry and acidimetry are a kind of volumetric
analysis in which the fundamental reaction is a
neutralization reaction. Alkalimetry is the specialized
analytic use of acid-base titration to determine the
concentrationof a basic (synonymous to alkaline)
substance. Acidimetry, sometimes spelled acidometry, is
the same concept of specialized analytic acid-base
titration, but for an acidic substance.

3. Method
Before starting the titration a suitable pH indicator must
be chosen. The equivalence point of the reaction, the
point at which equivalent amounts of the reactants have
reacted, will have a pH dependenton the relative
strengths of the acid and base used. The pH of the
equivalence point can be estimated using the following
rules:
 A strong acid will react with a strong base to form a
neutral (pH = 7) solution.
 A strong acid will react with a weak base to form an
acidic (pH < 7) solution.
 A weak acid will react with a strong base to form a
basic (pH > 7) solution.
When a weak acid reacts with a weak base, the
equivalence point solution will be basic if the base is
stronger and acidic if the acid is stronger. If both are of
equal strength, then the equivalence pH will be neutral.
However, weak acids are not often titrated against weak
bases because the colourchange shown with the indicator
is often quick, and therefore very difficult for the observer
to see the change of colour.
The point at which the indicatorchanges colouris called
the end point. A suitableindicator should be chosen,

4. preferably one that will experience a change in colour(an
end point) close to the equivalence point of the reaction.
First, the buretteshouldbe rinsed with the standard
solution, the pipettewith the unknown solution, and the
conical flask with distilled water.
Secondly, a known volume of the unknown concentration
solution should be taken with the pipetteand placed into
the conical flask, along with a small amount of the
indicatorchosen.
The known solution should then be allowed out of the
burette, into the conical flask. At this stage we want a
rough estimate of the amount of this solution it took to
neutralizethe unknown solution. The solution shouldbe
let out of the buretteuntil the indicatorchanges colour
and the value on the buretteshouldbe recorded. This is
the first (or rough) titration volume and should be
excluded from any calculation.
At least three more titrations should be performed, this
time more accurately, taking into account roughly where
the end point will occur. The initial and final readings on
the burette(prior to starting the titration and at the end
point, respectively) should be recorded. Subtractingthe
initial volume from the final volume will yield the amount
of titrant used to reach the end point. The end point is

5. reached when the indicator just changes colour
permanently.
Acid–base titration is performed with a bromthymol blue
indicator, when it is a strong acid – strong base titration, a
phenolphthalein indicatorin weak acid – strong base
reactions, and a methyl orange indicator for strong acid –
weak base reactions. If the base is off the scale, i.e. a pH
of >13.5, and the acid has a pH >5.5, then an Alizarine
yellow indicator may be used. On the other hand, if the
acid is off the scale, i.e. a pH of <0.5, and the base has a
pH <8.5, then a Thymol Blue indicator may be used.
Titrationof weak acid
The pH of a weak acid solution being titrated with a
strong base solution can be found at different points along
the way. These points fall into one of four categories:[2]

6. 1.initial pH
2.pH before the equivalence point
3.pH at the equivalence point
4.pH after the equivalence point
1. The initial pH is approximated for a weak acid
solution in water using the equation
where Ka is the dissociation constant and F is the
concentrationof the acid.
2. The pH before the equivalencepoint dependson the
amount of weak acid remaining and the amount of
conjugate base formed. The pH can be calculatedby the
following formula (which is a variation of the Henderson-
Hasselbalch equation):
where:
 pKa is the negative log of the acid dissociation
constant of the weak acid.
 nOH- added is the number of moles of added strong base
in the solution.
 nHA initial is the number of moles the weak acid
initially present.

7. When the numeratorof the log term equals the
denominator( ), then the ratio
goes to 1 and the log term goes to zero. Thus the pH will
equal the pKa which occurs half-way to the equivalence
point.
Animation of titration with base titrant
3. At the equivalencepoint, the weak acid is consumed
and converted to its conjugate base. The pH will be
greater than 7 and can be calculatedfrom an equation
derived from the following relationships:
1.pH + pOH = 14
2.KaKb = 10−14
3.at equivalence CaVa = CbVb
The previous 3 relationshipsare used to generate the
equivalence point pH formula below:

8.  Ca = concentrationof acid and Cb = concentrationof
base
 Kw = dissociation constant for water and Ka = for the
acid
Note that when an acid neutralizesa base, the pH may or
may not be neutral (pH = 7). The pH dependson the
strengths of the acid and base.
4. After the equivalencepoint, the solution will contain
two bases: the conjugate base of the acid and the strong
base of the titrant. However, the base of the titrant is
stronger than the conjugate base of the acid. Therefore,
the pH in this region is controlled by the strong base. As
such the pH can be found using the following:
Single formula. More accurately, a single formula[3]
that
describes the titration of a weak acid with a strong base
from start to finish is given below:

9.  φ = fraction of completion of the titration (φ < 1 is
before the equivalence point, φ = 1 is the equivalence
point, and φ > 1 is after the equivalence point)
 Ca, Cb = the concentrationsof the acid and base
respectively
 Va, Vb = the volumes of the acid and base
respectively
 αA- = the fraction of the weak acid that is ionized
 Ka = the dissociation constant for the acid
 [H+
], [OH−
] = concentrationsof the H+
and OH−
ions
respectively
This formula is somewhat cumbersome, but does describe
the titration curve as a single equation.