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

1

2

3

4

5

6

7

8

9

PH

ml of 0.1M KOH added

bc

a

d

e

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 midpointbuffering 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 α-carboxylic group , a second for the titration of the α-amino 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 acidsNinhydrin 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 α-amino, its α-amino group is involved in a ring structure.

This reaction is an important reaction used to detect amino acids.