Transcript Titration curves

Titration curves
Titration of a strong acid
• When a strong acid is titrated with a strong
base the pH at any point is determined solely
by the concentration of un titrated acid or
excess base.
• The conjugated base that is formed has no
effect on pH.
Titration of a weak acid
• Weak acid dissociates in aqueous solution
partially to give a small amount of H+ ions.
HA
H + + A• When OH- ions are added during titration it is
neutralized by H+ ions to produce H2O.
• The removal of the H+ ions disturbs the
equilibrium thus more HA molecules will ionize to
produce H+ ions to reestablish the equilibrium.
• This process will continue until all the HA
molecules are ionized.
Titration curve of a monoprotic weak
acid
e
9
PH
8
d
7
6
c
5
b
4
3
a
2
1
ml of 0.1M KOH added
NOTICE:
– When the acid is less than half titrated the pH is
less than pKa
– When the acid is half titrated the pH = pKa
– When the acid is more than half titrated the pH is
greater than pKa
Titration of amino acids
• Dissociation of the α-amino group;
When the α-carboxyl group is fully titrated the additional
base added will start titrating the α-amino group
which is a weaker acidic group pka = 9.1 compared to
the α-carboxylic group which which explains it being
titrated after the α-carboxylic group , thus
dissociating and donating a proton to the enviroment
leading to the formation of –NH2 or form III, which
carries a net negative charge.
Titration of amino acids
Titration curve of glycine ;
a) At point(a) predominant form
is the fully protonated form.
b) At point (b) 50% of the a.a is in
the fully protonated form and
50% in the zwitterions' form .
At this point the concentration
of the conjugated acid is equal
to the concentration of the
conjugated base and pH= pKa1
(pKa of α-carboxylic group)
Point( b) represents the midpoint
Where half of the α-carboxylic
group is titrated. At the midpoint
buffering behavior is maximum.
Titration of amino acids
c) At point (c) the predominating form is zwitterion form
which has
a net charge of zero . At point (c) the pH = pI isoelectric
point of the amino acid. The isoelectric point pI for
neutral amino acids
Can be calculated from ; pI = pKa1 + pKa2 / 2
d) At point(d) 50% of the amino acid is in the zwitterion
form and 50% in the anionic form .Point (d) represents
the second midpoint where half the α-amino group is
titrated , and pH = pKa2 , the pka of the α-amino group,
with maximum buffer capacity.
e) At point (e) the predominating form is the anionic fully
deprotonated form.
Titration of amino acids
The titration of acidic and basic amino acids involves
three stages of titration one for the titration of the αcarboxylic group , a second for the titration of the αamino group and the third for the additional acidic or
basic group in the side chain.
Thus showing three midpoints , three pka”s and three
buffering zones.
The isoelectric point pI is obtained by calculating the
average of the two closest pKa values.
Reactions of amino acids
Ninhydrin Reaction; (2,2-Dihydroxyindane-1,3-dione) is a
chemical used to detect ammonia or primary and secondary
amines . When reacting with these free amines a deep blue
or purple color is produced.
Reactions of amino acids
The ninhydrin reaction targets the free α-amino group
in amino acids , thus all the standard α-amino acids
will produce a blue purple complex when reacted
with ninhydrin except proline which will produce a
yellow color since it does not contain a free αamino, its α-amino group is involved in a ring
structure.
This reaction is an important reaction used to detect
amino acids.