Transcript Analytical Chemistry Lecture Note

Typical Applications of Neutralization Titrations
Elemental Analysis
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Keldahl Nitrogen Analysis
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1)With HCl
2)With
H3BO3
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Apparatus used in Kjeldahl
I. Digestion
apparatus
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(I)
II. Distillation &
absorption
apparatus
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(II)
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Keldahl Nitrogen Analysis (1)
Step 1: Kjeldahl digestion (decomposing and dissolving)
H 2SO 4 (K 2S 2 O 8 )
Hg or Cu or Se
Organic N 
 NH4  CO2  H2O
Step 2: Neutralization by adding base
NH4 + OH  NH3(g)  H2O
Step 3: Distillation NH3 into excess HCl standard
NH3(g) + H 3O+  NH4  H 2 O
Step 4: Titrating unreacted HCl with NaOH standard
H 3O +  OH   2H 2O
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Example: A typical meat protein contains 16.2% (w/w) nitrogen.
A 0.500 mL aliquot of protein solution was digested, and the
liberated NH3 was distilled into 10.00 mL of 0.02140 M HCl.
The unreacted HCl required 3.26 mL of 0.0198 M NaOH for
complete titration. Find the concentration of protein (mg
protein/ml) in the original sample.
Solution:
0.02140m m olH 
0.0198m m olOH  1 m m olH 
 (10.00 m l HCl 
 3.26 m l NaOH 

)
1 m l HCl
1 m l NaOH
1 m m olOH 

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1 m m oldigested N 14.00674m g N 100 m g protein
1



 25.8 m g protein/ m l
1 m m olH 
1 m m olN
16.2 m g N
0.5 m L sam ple
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TMHsiung@2007 5/42
Keldahl Nitrogen Analysis (2)
1. Digestion
催化剂
NCOC + H2SO4
煮沸
（NH4）2SO4 + CO2 + SO2 + H2O
2. Neutralization &distillation
2NH3↑+Na2SO4 + 2H2O
2NaOH +（NH4）2SO4
3. Absorption by boric acid :
2NH3 + 4H3BO3
（NH4）2B4O7 + 5H2O
4. Titration by strong acid
（NH4）2B4O7 + 5H2O + 2HCl
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2NH4Cl + 4H3BO3
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(* NCOC, N containing organic compounds, N:HCl = 1:1)
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Example: A typical meat protein contains 16.2% (w/w) nitrogen.
A 0.500 mL aliquot of protein solution was digested, and the
liberated NH3 was distilled into 100.00 mL of 0.0200 M H3BO3.
The mixture required 3.26 mL of 0.0198 M HCl for complete
titration. Find the concentration of protein (mg protein/ml) in the
original sample.
Solution:
0.0198m m olH 
 (3.26 m l HCl 
)
1 m l HCl

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1 m m oldigested N 14.00674m g N 100 m g protein
1



 36.39 m g protein/ m l
1 m m olH 
1 m m olN
16.2 m g N
0.5 m L sam ple
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TMHsiung@2007 7/42
The Determination of Inorganic Substances
Ammonium Salts are conveniently determined by conversion to
ammonia with strong base followed by distillation. Ammonia is collected and
titrated as in the Kjedahl method.
Nitrates and Nitrites ions are first reduced to
ammonium ion by Devarda’s alloy (50% Cu, 45% Al, 5% Zn) or Arnd’s alloy
(60% Cu, 40% Mg). Granules of the alloy are introduced into strongly
alkaline solution of the sample in Kjedahl flask. The ammonia is distilled after
reaction is complete.
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CO2 & CO32- chemistry

CO2(aq)  H 2O 
 H 2CO3  K hyd
[ H 2CO3 ]

 2.8  103
CO2( aq )

3
[
H
][
HCO
]


4
H 2CO3 
 H  HCO3  K a1 

1
.
5

10
[ H 2CO3 ]

[ H  ][HCO3 ]
CO2(aq)  H2O 
 H  HCO3  Ka1 
 (2.8  103 )  (1.5  10 4 )  4.2  107
[CO2 ]( aq )

HCO3



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

H  CO3

2

2
pKa1=6.38
[ H ][CO3 ]
11
 Ka 2 

4
.
69

10

[ HCO3 ]
pKa2=10.33
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CO2 & CO32- chemistry
• Na2CO3 →2Na+ + CO32• CO32- HCO3-  H2CO3
pKb1=3.64
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pKb2=7.62
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Titration of Na2CO3 with HCl
Step 1: Na2CO3 + HCl  NaHCO3 + NaCl
Step 2: NaHCO3 + HCl  H2CO3 + NaCl
Overall: Na2CO3 + 2HCl  H2CO3 + 2NaCl
14
12
10
Phenolphthalein
8
pH
6
BCG
4
2
0
n=1 Ve1
0
1
n=2 Ve2
2
3
Equivalents of HCl added
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Determination of Carbonate in a Sample
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12
A 1.2040-g sample containing
sodium carbonate and inert
material was dissolved in water
BCG
and titrated to the bromcresol
green end point, requiring
32.50 mL of 0.1020 M HCl.
Calculate the % Na2CO3 in the
sample.
For the BCG endpoint, the reaction is
10
8
6
4
2
0
0
1
3
2
Na2CO3 + 2HCl  H2CO3 + 2NaCl
32.50m l.HCl 
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0.1020m m ol.HCl 1m m ol.Na2CO3 106m g.Na2CO3 100



 14.59%
m l.HCl
2m m ol.HCl
1m m ol.Na2CO3 1204
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Effect of Absorbed CO2 on Titration of Na2CO3 with HCl
14
Ve1
12
Ve2
10
Before absorption
pH
8
6
4
2
0
0
10
20
30
14
Volum e of H Cl added, m L
After absorption
12
• Ve1 decreases
10
CO2(g) + H2O(l) + CO32(aq) 2HCO3(aq)
• Ve2 is not affected
pH
8
6
• Use Ve2 for calculations
4
2
0
0
10
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Volum e of H Cl added, m L
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Effect of CO2 Absorption on OH- concentration
OH- standard solutions can react with CO2
CO2(g) + 2 OH- → CO32(aq)
This effect may be avoided by excluding air from the titration system.
In presence of BCG--------no error
In presence of Phenol phetalein------- Carbonate Error
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Boiling to Enhance Visual Endpoint in Titration of Na2CO3
with HCl
Na2CO3 +HCl  HCO3-
NaHCO3
NaHCO3
+
H2CO3
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HCO3- +HCl  H2CO3
Boiling removes H2CO3 as
gaseous CO2
H2CO3(aq)  CO2(g) + H2O
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OH-
HCO3-
CO32-
OH-
CO32-
Titration curves and indicator
transition ranges for the analysis
of mixtures containing :
Winkler method
Mixture of CO32- +OH1)Titrate the mixture with H+
In presence of BCG
OH-≈ 1H+ --CO32-≈2H+
2)Excess BaCl2 is added
CO32- + Ba2+→BaCO3
CO32- HCO313920208
--Titrate the mixture with H+
Titrant-≈ OH23)
Titrant1-titrant2
≈
CO
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OH-
HCO3-
CO32-
OH-
CO32-
Titration curves and indicator
transition ranges for the analysis
of mixtures containing :
Winkler method
Mixture of CO32- + HCO31)Titrate the mixture with H+
In presence of BCG
HCO3-≈ 1H+ --CO32-≈2H+
2)Excess BaCl2 is added
CO32- + Ba2+→BaCO3
CO32- HCO313920208
--Titrate the mixture with H+
Titrant-≈ HCO323)
Titrant1-titrant2
≈
CO
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OH-
HCO3-
CO32-
OH-
CO32-
Titration curves and indicator
transition ranges for the analysis
of mixtures containing :
Mixture of 2 weak acids
Mixture of CO32- + HCO31)Titrate the mixture with H+
In presence of BCG (A ml)
HCO3-≈ 1H+ -- CO32-≈2H+
2)Titrate the mixture with H+
In presence of Phenol phetalein
(C ml) Titrant≈ CO32-
CO32- HCO313920208
3) (A-2C)=Bml ≈ HCO3http:\\asadipour.kmu.ac.ir 24 slides
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Carbonate and Carbonate Mixtures
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The Determination of Organic functional Groups
Carboxylic acid group
Most CA have between 10 –4 > Ka > 10–6.
CA are not sufficiently soluble in water for direct titration in this medium.
the acid can be dissolved in ethanol and titrated with aqueous base.
Alternatively, the acid can be dissolved in an excess of standard base followed
by back-titration with standard acid.
Sulfonic acid group
Sulfonic acids are generally strong acids and readily dissolve in water. Their
titration with a base is therefore straightforward.
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Amine group
A) titrated in aqueous media.
1) Aliphatic amines generally have Kb on the order of 10–5 and can thus be titrated
directly with a solution of a strong acid.
2) Many saturated cyclic amines, such as piperidine, tend to resemble aliphatic
amines in their acid-base behavior and thus can be titrated in aqueous media.
B) titrated in non-aqueous solvents
Many amines that are too weak to be titrated as bases in water are readily titrated
in non-aqueous solvents, such as anhydrous acetic acid, which enhance their
basicity.
3) aromatic amines such as aniline and its derivatives are usually too
weak for titration in aqueous medium (Kb 10–10 ).
4) The same is true for cyclic
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amines
, such as pyridine and its derivatives.
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Ester groups
Esters are commonly determined by saponification with a measured quantity of
standard base:
R1COOR2 + OH–  R1COO – + HOR2
The excess base is then titrated with standard acid.
Hydroxyl groups
Hydroxyl groups in organic compounds can be determined by esterification with
various carboxylic acid anhydrides or chlorides; the two most common reagents
are acetic anhydride and phthalic anhydride.
(CH3CO)2O + ROH  CH3COOR + CH3COOH
The acetylation is ordinarily carried out by mixing the sample with a carefully
measured volume of acetic anhydride in pyridine. After heating, water is added to
hydrolyze the unreacted anhydride :
(CH3CO)2O + H2O  2CH3COOH
The acetic acid is then titrated with a standard solution of alcoholic sodium or
potassium hydroxide. A blank is carried through the analysis to establish the
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original
amount of anhydride.
Carbonyl groups
Many aldehydes and ketones can be determined with a solution of hydroxylamine
hydrochloride. The reaction, which produces an oxime, is
R1
R1
C=O + NH2OHHCl 
R2
C=NOH + HCl + H2O
R2
where may be an atom of hydrogen. The liberated HCl is titrated with base.
Here, the conditions necessary for quantitative reaction vary. Typically, 30 min
suffices for aldhydes. Many ketones require refluxing with the reagents for 1 hr or
more.
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