Download TITRATION CURVES

NEUTRALIZATION ANALYSIS
TITRATION CURVES
(A)
analyte
+
(R)
titrant
K
(P)
product
Outline
NEUTRALIZATION
ANALYSIS
Introduction
Titrants
Titration curves
End point
detection
Applications
Important points and regions:
2 points:
before titration
at the end point
2 regions:
before the end point
after the end point
Titration curves:
1. Strong acid with
strong base,
2. Weak acid with
strong base,
3. Polyprotic acid with strong base
(at 0%)
(at 100 %)
(0.00..1 – 99.99…%)
(100.00..1 – ∞)
I. [A]
III. [A] = [R]
II. [A] + [P]
IV. [P] + [R]
Strong base with strong acid
Weak base with strong acid
1
TITRATION CURVES
1. Strong acid with strong base,
Strong base with strong acid
Outline
NEUTRALIZATION
ANALYSIS
Introduction
e.g.
I.
Titrants
Titration curves
II.
End point
detection
Applications
III.
IV.
HCl + NaOH
Cl–
acid1 +
base1
+
acid2
(very weak)
base2
+
Na+(H2O)
At the start:
[H+] = [H3O+]=[HCl]0
[OH–] = [NaOH]0
pH = – lg [HCl]0
pOH = – lg [NaOH]0 pH = 14 – pOH
Before the end point:
[H+] = [H3O+]=[HCl]unreacted
[OH–] = [NaOH]unreacted
pH = – lg [HCl]unreacted
pOH = – lg [NaOH]unreacted
At the end point:
[H+] ≡ [OH–]
KW = 10–14
pH ≡ 7
After the end point:
[OH–] = [NaOH]excess
[H+] = [H3O+]=[HCl]excess
2
pOH = – lg [NaOH]excess
pH = – lg [HCl]excess
TITRATION CURVES
Titration curves:
1. Strong acid with
strong base,
2. Weak acid with
strong base,
3. Polyprotic acid with strong base
Strong base with strong acid
Weak base with strong acid
EFFECTS ON THE TITRATION CURVE:
1. Effect of the temperature:
Outline
NEUTRALIZATION
ANALYSIS
25°C
[H+]·[OH–] = Kw = 10–14
Neutr. point: pH = 7
100°C
[H+]·[OH–] = Kw = 10–12
Neutr. point: pH = 6
Introduction
Titrants
Titration curves
0
100
End point
detection
Applications
3
EFFECTS ON THE TITRATION CURVE
2. Dependence on the initial concentrations (e.g. [HCl]):
Outline
[HCl]0
↓
1N
0%
50%
90%
99%
99.9%
100%
100.1%
101%
110%
0
0,3
1
2
3
7
11
12
13
0,1 N
1
1,3
2
3
4
7
10
11
12
0,01 N
2
2,3
3
4
5
7
9
10
11
0,001 N
3
3,3
4
5
6
7
8
9
10
pH change around the end point
ΔpH
NEUTRALIZATION
ANALYSIS
3 – 11
4 – 10
5– 9
6– 8
Introduction
Titrants
Titration curves
End point
detection
Applications
0
100
4
EFFECTS ON THE TITRATION CURVE
3. Dependence on the acid strength (dissociation constants):
Outline
NEUTRALIZATION
ANALYSIS
Introduction
Titrants
Titration curves
End point
detection
A. Weak acid with strong bases ,
0
100
e.g. 10–1 N CH3COOH is titrated with NaOH (Ka = 2x10–5)
%
0
50
90
99
99.9
100
100.1
101
110
pH
2.9
4.7
5.7
6.7
7.7
8.9
10
11
12
ΔpH pKInd ≈ 9 → PHENOLPHTALEIN
B. Weak base with strong acid
e.g. 10–1 N NH4OH is titrated with HCl (Kb = 2x10–5)
%
0
50
90
99
99.9
100
100.1
101
110
pH
11.1
9.3
8.3
7.3
6.3
5.1
4
3
2
Applications
5
ΔpH pKInd ≈ 5 → METHYL RED
TITRATION CURVES
II. Weak acid with strong base
Weak base with strong acid
e.g. Titration of CH3COOH with NaOH , Titration of NH4OH with HCl:
I. At the start:
Weak acid
Weak base
pH
H  
K a C acid H   

Outline
NEUTRALIZATION
ANALYSIS
Introduction
Titrants
Titration curves
End point
detection
Applications
K a CH 3 COOH
OH  

K b C base
OH  

K b NH 4 OH
II. Before the end point:
Buffer (acid / salt)
Buffer (base / salt)
pH
H   K

a
C acid
C salt
OH   K
CH COOH
H   KCH
COO 

a

3

3
b
C base
C salt
OH   K NH OH

b
NH 
4

4
III. At the end point:
Hydrolysing salt (Brönsted base) pH
Hydrolysing salt (Brönsted acid)
OH  

OH  

Kw
C salt
Ka
K b C salt
H  

OH  


K b CH3 COO

H  
Kw
C salt
Kb

IV. After the end point:
Excess of strong base
pH
[OH–] = Cexcess base
[OH–] = [NaOH]excess
K a C salt
H  


K a NH 4


Excess of strong acid
[H+] = Cexcess acid
[H+] = [HClexcess
6
TITRATION CURVES
III. Polyprotic acid with strong base
e.g. Titration of H3PO4 with NaOH
1.
2.
3.
H3PO4 + OH–
H2PO4– + OH–
HPO42– + OH–
H2PO4– + H2O
HPO42– + H2O
PO43– + H2O
Ka1 = 7x10–3
Ka2 = 6x10–8
Ka3 = 10–12
Outline
NEUTRALIZATION
ANALYSIS
Introduction
Titrants
Titration curves
End point
detection
Applications
7
ACID / BASE INDICATORS
1.
Azo-compounds
Genearal structure:
Outline
NEUTRALIZATION
ANALYSIS
INDICATOR
R1
R2
R3
∆pH
METHYLORANGE
METHYL
-RED
p-ETHOXYCHRISOIDINE
TROPAEOLIN
0
-N(CH3)2
H
SO3Na
-N(CH3)2
COOH
-OC2H5
-SO3Na
3.1 – 4.4
Acidic
color
red
Basic
color
orange
H
4.4 – 6.2
red
yellow
NH2
NH2
3.5 – 5.5
red
yellow
OH
OH
11.1 –
12.7!
yellow
orange
Mechanism:
Introduction
Titrants
Titration curves
End p. detection
- chemical
- instrumental
Applications
Yellow
Yellow
Red
(basic) (intermediate) (acidic)
(aromatic) (protonated) (quinoid)
8
ACID / BASE INDICATORS
2.
PHTHALEIN-derivatives
General structure:
INDICATOR
R
R1
R2
R3
PHTALEINS
PHENOLPHTHALEIN
COOH
COOH
H
H
H
H
CH3
THYMOLPHTHALEIN COOH CH(CH3)2
Outline
NEUTRALIZATION
ANALYSIS
SULFONPHTHALEINS
PHENOL RED
SO3H
SO3H
H
H
H
THYMOL BLUE
SO3H
CH(CH3)2
H
CH3
Mechanism:
∆pH
Acidic
color
colorless
colorless
Basic
color
colored
red
colorless
blue
basic /
acidic
colored
colored
6.4 –
8.0
a)1.2 –
2.8
b)8.0 –
9.6
yellow
red
red
yellow
yellow
blue
basic
8.2 –
10.0
8.3 –
10.5
Thymol blue
Introduction
Titrants
Titration curves
End p. detection
- chemical
- instrumental
Applications
Colorless
(acidic)
Colorless
(intermediate)
Purple
(basic)
9
INSTRUMENTAL DETECTION
(Summary)
Outline
INSTRUMENTAL
DETECTION
Advantages
Types
Potentiometric
end point
detection
The titration process is followed by electrochemical,
photometric or other sensing devices.
Method
Sensing device
POTENTIOMETRY
(Potential vs %)
Different types of
electrodes
Neutralization titr.
Complexometric titr.
Precipitation titr.
Redox titr.
AMPEROMETRY
(Current vs %)
Pt electrode
(dead stop…)
Redox titr.
CONDUCTOMETRY
Conductivity cell
Neutralization titr.
Precipitation titr.
Spectrophotometer
Complexometric
titr.
Neutralization titr.
Complexometric titr.
Precipitation titr.
Redox titr.
(Conductivity vs %)
PHOTOMETRY
(A = ε · c · l vs %)
Conductometric
end point
detection
Type of titration
ENTALPHYMETRY
(Q = f (c, ΔH) vs %
Thermistor
10
POTENTIOMETRY
Electrode potential developed
between:
Outline
Indicator electrode
 Potential (Eind) varies
 Depends on
the analyte concentration
Reference electrode
 Known, constant potential (Eref)
 Independent
of the analyte concentration
Common reference electrodes:
Solid metal / its „unsoluble” salt / saturated conc. of anion
e.g. Ag / AgCl / KCl
Hg / Hg2Cl2 / KCl
Hg / Hg2SO4 / K2SO4
INSTRUMENTAL
DETECTION
Advantages
Nernst equation:
Types
Glass electrode
Potentiometric
end point
detection
Conductometric
end point
detection
Metal
electrode
Ion-selective
electrode
Nobel metal
electrode
E  E0 
0.059
lg c
n
Neutralization titration:
E = E0 + 0.059 lg [H+]
Complexometric titration:
E = E0 + 0.059 lg [Mn+]
n
Precipitation titration:
E = E0 + 0.059 lg [X−]
Redox titration:
E = E0 + 0.059lg [ox]
[red]
n
11
POTENTIOMETRY
Neutralization analysis
External
reference electrode
Indicator electrode:
Outline
Glass electrode
GLASS ELECTRODE
INSTRUMENTAL
DETECTION
H+ conc. to be determined
Electrochemical cell for measurement of pH:
Advantages
Types
Potentiometric
end point
detection
Conductometric
end point
detection
External reference || H+ conc. |pH-sensitive
electrode
|| to be
| glass(Hg/Hg2Cl2/KCl) ||determined | membrane
| Internal
| buffer sol.
| Internal reference
| electrode
| (KCl) (pH = 7) | (Ag/AgCl/KCl)
███████████
External Dry glass Internal
hydrated
hydrated
gel layer
gel layer
12
POTENTIOMETRY
Glass electrode
Composition of glass:
E.g. 22 % Na2O, 6 % CaO, 72 % SiO2.
Outline
INSTRUMENTAL
DETECTION
Advantages
Types
Na+ mobile
membrane
solution
H+
Na++
H
Na+
Ion-exchange reaction:
between
H++
Na
H+ in the solution and
Na+ in the glass:
K
H+ + Na+Gl−  Na+ + H+Gl–
K = LARGE!
solution glass
solution glass
Combination glass electrode:
Potentiometric
end point
detection
Conductometric
end point
detection
13
POTENTIOMETRY
Titration curve
Potentiometric titration curve:
Outline
INSTRUMENTAL
DETECTION
Titration curve
Measuring the potential of a suitable
indicator electrode (pH) as a function
of volume titrant.
1st derivative
Advantages
Determination of the end point:
from the derivatives
Types
Potentiometric
end point
detection
2nd derivative
Conductometric
end point
detection
14
CONDUCTOMETRIC
TITRATION CURVES
I. Titration of strong acid (a) with strong base e.g. HCl with NaOH
(b) with weak base e.g. HCl with NH4OH
Outline
INSTRUMENTAL
DETECTION
Advantages
Types
%
II. Titration of weak acid (c) with strong base e.g. CH3COOH with NaOH
(d) with weak base e.g. CH3COOH with NH4OH
Potentiometric
end point
detection
Conductometric
end point
detection
%
15
APPLICATIONS
TITRATIONS
 Direct
Outline
NEUTRALIZATION
ANALYSIS
Introduction
 Back (indirect):
Analyte
Titrant in excess
to calculate
I.
to measure
Determination of strong acids / bases:
Equivalence point: pH = 7
e.g. NaOH
Titrants
Titration curves
End point
detection
Vphen.
Vmeth.r.
OH−  H2O
Applications
16
APPLICATIONS
II. Determination of weak acids :
Equivalence point: pH > 7 (phenolphtalein indicator)
weak bases :
Equivalence point: pH < 7 (methyl red indicator)
Outline
NEUTRALIZATION
ANALYSIS
Introduction
Titrants
Titration curves
End point
detection
Applications
II. (a) Determination of weak acids : Ka ≥ 10–5. (10–7 - 10–4)
 Direct: e.g.  carboxylic acids of low carbon atoms
e.g. CH3COOH
 fatty acids (e.g. fat, wax, oil)
 Back : if the weak acid is volatile
e.g.  CO2 (as carbonate or hydrogencarbonate)
bubble-free
distillation
CO2
known amount of Ba(OH)2

Distillation apparatus
back titration of excess Ba(OH)2
(Maros- Schulek)
with standard HCl
Application of CO2 determination:

Determination of organic materials

Determination of CO2, HCO3– , CO32–
content of natural waters
 Nonaqueous solvents: Ka < 10–7
17
> 10–12
APPLICATIONS
II. (b) Determination of weak bases : Kb ≥ 10–5 (10–7 - 10–4)
Outline
NEUTRALIZATION
ANALYSIS
Introduction
Titrants
Titration curves
End point
detection
Applications
 Direct:
 e.g. NH4OH
 Back:
 NH4+ -salt
strong base (NaOH)
boiling
NH3
distillation into known excess of acid
Kjeldahl method:

NH3
back titration of excess acid (HCl)
known HCl
with basic titrant (NaOH)
Application of NH3 determination:

N-containing organic compounds (e.g. amino acids, proteins,…)
Decomposition (mineralization) with cc. H2SO4, 300 °C
+ catalyst: Se, or Cu2+
Ox. number: – 3
 (NH4)2SO4
(e.g.. – NH2, –N(CH3)2, =NH, –N<)
Ox. number: + 3, +1
 HNO3 (+5)
(e.g.,azo- (-N=N-), nitro-, nitrozo comp.)
Reduction
with Zn, Na2S2O4,..
NH4+
 Nonaqueous solvents:
Kb < 10–7
> 10–12
18
APPLICATIONS
Outline
NEUTRALIZATION
ANALYSIS
Introduction
Titrants
Titration curves
III. Determination of salts:
(a) Neutral salts: NOT MEASURABLE!
(b) Salt hydrolyzing to acid: Brönsted acid (strong acid + weak base)
MA +
H2O  MOH + A– + H+
if pK > 7! can be TITRATED
with base
E.g. Aniline · HCl; Benzidine ·H2SO4; Papaverine · HCl…
(c) Salt hydrolyzing to base: Brönsted base (strong base + weak acid)
MA +
H2O  HA + M+ + OH–
if pK > 7
can be TITRATED
with acid
2–
–
E.g. Na2B4O7 (B4O7 +7 H2O  4H3BO3 + 2OH ) methyl red
(CO32– + H2O  HCO3– + OH–) phenolpht.
E.g. Na2CO3
(CO32– +2 H2O  H2CO3 +2 OH–) methyl red
–
–
NaHCO (HCO3 + H2O  H2O + CO2 +OH ) methyl red
3
Na2CO3
End point
detection
NaHCO3
Applications
Vphen
Vmeth.r.
CO32− HCO3− H2CO3
Vphen = 0
Vmeth.r.
HCO3−  H2CO3
19
APPLICATIONS
(d)
Specific determinations:
NaOH – Na2CO3
in the presence of each other
NaHCO3 – Na2CO3
in the presence of each other
Outline
NEUTRALIZATION
ANALYSIS
Introduction
Titrants
Titration curves
End point
detection
Applications
OH−, CO32− HCO3− H2CO3
Vphen
Vmeth.r.
Warder’s method :
one sample :
A. OH– + H+ 
CO32– + H+ 
B. HCO3– + H+ 
two samples :
B. OH− + H+ 
CO32– +2H+ 
Winkler’s method :
A. + BaCl2
CO32– +Ba2+
OH– + H+ 
Vphen
Vmeth.r.
H2O
phenolpht.
HCO3–.
H2CO3 methyl red
H2O
H2CO3
methyl red
BaCO3
H2O
phenolpht.
CO32− HCO3− HCO3−  H2CO3
Warder’s method :
two samples :
A. CO32– + H+  HCO3–
phenolpht.
B. HCO. – + H+  H CO
CO3
3
2−
+2H+ 
2
3
H2CO3
methyl red
20