Lecture 8

Titrations -II

Kjeldahl digestion

Conc. H2SO4 + catalyst

All N NH4+

CCO2

HH2O

>300o C

NaOH+NH3

NH3+H2O

HCl or H3BO3

NH3+ H+NH4+

Excess of H+ can be titrated withNaOH

If HCl:

If boric acid:

B(OH)4- + H+ H3BO3 + H2O

Precipitation Titration

AgNO3 + NaCl AgCl + NaNO3

Ag+ + Cl- AgCl

AgCl Ag+ + Cl-

KSP = [Ag+][Cl-]

pX = - log10[X]

pAg = - log10[Ag+]

n-

AgCl

Cl

Cl

Cl

Ag

Ag

Ag

Ag

Cl

Cl

Cl

Cl

Cl

Ag

Cl

Cl

Cl

soluble

AgCl

Cl-

Cl-

Cl-

Cl-

Cl-

Cl-

Cl-

Cl-

Na+

Na+

Na+

Na+

Na+

AgNO3

NO3-

AgCl

Na+

Na+Na+

NO3-

NO3-

NO3-

AgNO3

Colloid

equivalence point:coagulation

no chloride ion left in solution, no further precipitation possible

@ first drops In the middle

Equivalence point[Ag+]=[Cl-]

100.1% Ag+

99.9% Ag+

[Cl-]=10-4 M [Ag+]=1.810-6 M

[Ag+]=10-4 M

Acceptable end point

Titration of NaCl with AgNO3C=0.1 M

I- + Ag+ AgI

pAg= -log[Ag+]

Ksp(AgI)=[Ag+][I-]=8.310-17

][

108.3][

17

I

Ag

99.9% Ag+

100.1% Ag+

Acceptable end point

Precipitation titration with silver electrode

The change of V is proportional to pAg

V(mV)

Ag+

Ag+

Ag+ ++

+

++

+

+

+

+

Ind2-

Ind2-

Ag+

I-

I-

I-I-

_

_

__

_

__

_

Ind2-

Ind2-

Precipitation Titrations: