Volumetric analysis
To understand volumetric analysis, we must understand the types of reaction that
happen in it.
Types of reactions used in volumetric analysis :
I- Ionic combination reactions:- The reaction goes to completion due to formation of slightly ionizable or slightly
insoluble products.
a- Neutralization reaction :
In which acid reacts with base to form slightly ionized water.
H+ + OHb- Formation of precipitate :
Ag+ + Cl-
H2O
AgCl 
c- Formation of slightly ionizable complex :
Ag+ + 2 CNCa+2 + H2Y-2 [EDTA]
[Ag(CN)2]2H+ + CaY-2
[Ca-EDTA complex]
II- Electron transfer reactions :
-In which electron transfer from one reactant to another. It is called
(oxidation -reduction reactions) Ce+4 + Fe+2
Ce+3 + Fe+3
i.e.
Fe+2
Ce+4 + e
Fe+3 + e
Ce+3
oxidation (loss of es.)
reduction (gain of es)
Acid-Base
Acid- base theories
1- Arrhenius theory :Acid : Is the substance which ionize to give H+
Base : Is the substance which ionize to give OH-
eg. HCl
eg NaOH
2- Bronsted - Lowry theory :Acid : Is the substance which donate proton.
Base : Is the substance which accept proton.
Every acid has a conjugate base and the base has conjugate acid.
The stronger the acid the weaker its conjugate base and vice versa.
Eg.
HCl +
Acid
H 2O
base
Eg.
NH3 +
base
H 2O
acid
N.B.
Cl+
conj.base
NH4+
+
conj.acid
H 3 O+
conj.acid
OHconj.base
Water behave as acid or base because it is neutral.
3- Lewis theory :Acid : Is substance which accept lone pair of electrons eg. BF3, AlCl3.
Base : Is substance which donate lone pair of electrons eg NH3, amines.
Acid-base titration in aq. medium
Solns. are classified into :Electrolytes : Which desociate (ionize) and conduct electricity.
or Non electrolytes : Which doesn't ionize and doesn't conduct electricity.
Dissociation of water
H2O
H+ + OHDissociation const. Kw = [ H+] [OH-] / [H2O]
- Since H2O is weakly dissociated , therfore H2O is considered unity.
therfore Kw = [H+] [OH-] = 10 -14 at 25oc
Kw : it is called ionic product of water.
At 25oc
[H+] =[OH-] = 10-7
If [H+] = [OH-] , therfore soln. is neutral
If [H+] > 10-7 eg 10-6, 10-5 , therfore soln. is acidic
If [H+] < 10-7 eg 10-8, 10-9 , therfore soln. is alkaline.
Hydrogen exponent : pH
pH = -log [H+]
i.e. If [H+] = 10-7
pH = - log 10-7 = 7
In acidic side i.e. If [H+] = 10-6 pH = - log 10-6 = 6
In basic side i.e. If [H+] = 10-8
pH = - log 10-8 = 8
i.e. as pH value inc.
[H+] conc. decrease.
Therefore acid soln has pH less than 7 , alkaline soln. has pH more than 7 and
neutral soln. has pH = p OH = 7
pH of acid and bases :1- pH of strong acids :Since strong acids are strongly ionized.
Therfore pH = pCa
where Ca ( conc. of acid)
i.e. 0.1 N HCl
pH = - log 0.1= - log 10-1 = 1
2- pH of strong bases :Since strong bases are completely ionized.
Therfore p OH = p Cb
where Cb (conc.of base)
pH = p Kw – p OH
i.e. pH = p Kw – p Cb.
i.e. 0.1 N NaOH
pH = 14 _ 1 = 13
3- pH of weak acids :
pH = 1/2 pCa + 1/2 pKa
4- pH of weak bases :pH =pKw - 1/2 pCb - 1/2 pKb
5- pH of salts :a- Salt of strong acid and strong base eg. NaCl
Always neutral i.e. pH = 7
b- Salt of strong acid and weak base eg.NH4Cl
Always pH is in the acidic side , calculated from eq.
pH = 1/2 pKw - 1/2 pKb + 1/2pCs
where Kb (dissociation constant of weak base) Cs (conc. of salt)
c- Salt of weak acid and strong base eg. Na Ac
Always pH is in the alkaline side, calculated from eq.:
pH = 1/2 pKw + 1/2 pKa - 1/2pCs
where Ka (dissociation constant of weak acid) Cs (conc. of salt)
d- Salt of weak acid and weak base eg. NH4Ac
pH is calculated from eq. :
pH = 1/2 pKw + 1/2 pKa - 1/2pKb
Buffer solutions
-Def : They are solns which resist changes in pH upon addition of small amount of
acid or base.
-They consist of weak acid and its salt or weak base and its salt
Type 1- weak acid and its salt eg. HAc and Na Ac
pH of this buffer is calculated from the eq.:
pH = pKa + log salt / acid
Type 2- weak base and its salt eg. NH4OH and NH4Cl
pH of this buffer is calculated from the eq.:
pH = pKw - pKb - log salt/base
log salt/acid or log salt/base
is called buffer ratio
if [salt] = [acid]
therefore pH =pKa
Examples
1- Calculate the pH of a buffer soln. containing 0.1 M acetic acid and
0.1 M sodium acetate pKa =4.76
soln.
pH = pKa + log salt / acid
pH = 4.76 + log 0.1 / 0.1 = 4.76
Neutralization indicators
Color indicators:
- Substance which change their color with change in pH are used as
neutralization indicators.
eg. phenol phthalein"Ph.Ph" (one color indicator),
methyl orange"M.O" (2 color indicator)
eg. Ph.Ph. =8-10
M.O. =3.3-4.4
M.R. = 4-6
N.B. the indicator is chosen according to pH of the product.
Neutralization titration curves
For neutralization reaction titration . The titration curve is plot of pH versus
the mls of titrant.
Types of neutralization curves:
1- Strong acid -strong base titration :eg. HCl and NaOH
we have sample of 100 ml HCl and titrate against NaOH.
a.Before the titration pH is due to
the sample i.e HCl (strong acid)
therfore pH = pCa
b- At the equivalent point :
HCl +NaOH
NaCl + H2O
pH is due to NaCl i.e. pH= 7 (salt of strong acid and strong base)
c- After the equivalent point :
pH is due to excess titrant i.e. NaOH (strong base) pH = pKw-pCb
N.B. The pH rises slowly till 99.9 % of acid is titrated by adding 0.1 ml NaOH
pH rises from 4 to 7 then another 0.1 ml after end point pH rises from 7 to 10
i.e. at e.p. pH rises from 4 to 10.
So we can use M.O.(3.3 - 4.4)- M.R.(4 - 6)- Ph.Ph.(8 - 10) indicators