http:\\asadipour.kmu.ac.ir 24 slides 1
Typical Applications of Neutralization Titrations
Elemental Analysis
13920208
http:\\asadipour.kmu.ac.ir 24 slides 2
Keldahl Nitrogen Analysis
1)With HCl2)With H3BO313920208
http:\\asadipour.kmu.ac.ir 24 slides 3
Apparatus used in Kjeldahl
I. Digestion apparatus
II. Distillation & absorption apparatus
(I) (II)13920208
http:\\asadipour.kmu.ac.ir 24 slides13920208
Keldahl Nitrogen Analysis (1)
Step 1: Kjeldahl digestion (decomposing and dissolving)
OHCONHN Organic 224Seor Cu or Hg
)OS(KSOH 82242
Step 2: Neutralization by adding base
OHNHOH+NH 23(g)4
Step 3: Distillation NH3 into excess HCl standardNH + H O NH H O3(g) 3
+4 2
Step 4: Titrating unreacted HCl with NaOH standard
OH2OHOH 2+
3
http:\\asadipour.kmu.ac.ir 24 slides13920208 TMHsiung@2007 5/42
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:
) 1
1
1
0198.0 26.3
1
02140.0 00.10(
OHmmol
Hmmol
NaOHml
OHmmolNaOHml
HClml
HmmolHClml
mlproteinmgsamplemLNmg
proteinmg
Nmmol
Nmg
Hmmol
Ndigestedmmol/ 8.25
5.0
1
2.16
100
1
00674.14
1
1
http:\\asadipour.kmu.ac.ir 24 slides 6
催化剂
煮沸
1. Digestion
NCOC + H2SO4 ( NH4) 2SO4 + CO2 + SO2 + H2O
2. Neutralization &distillation
2NaOH +( NH4) 2SO4 2NH3↑+Na2SO4 + 2H2O
3. Absorption by boric acid :
2NH3 + 4H3BO3 ( NH4 ) 2B4O7 + 5H2O
4. Titration by strong acid
( NH4) 2B4O7 + 5H2O + 2HCl 2NH4Cl + 4H3BO3
(* NCOC, N containing organic compounds, N:HCl = 1:1)
Keldahl Nitrogen Analysis (2)
13920208
http:\\asadipour.kmu.ac.ir 24 slides13920208 TMHsiung@2007 7/42
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:
) 1
0198.0 26.3(
HClml
HmmolHClml
mlproteinmgsamplemLNmg
proteinmg
Nmmol
Nmg
Hmmol
Ndigestedmmol/ 39.36
5.0
1
2.16
100
1
00674.14
1
1
http:\\asadipour.kmu.ac.ir 24 slides 8
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.
13920208
http:\\asadipour.kmu.ac.ir 24 slides 9
CO2 & CO32- chemistry
13920208
432
3
1332 105.1][
]][[
COH
HCOHKHCOHCOH a
11
3
23
22
33 1069.4][
]][[
HCO
COHKCOHHCO a
3
)(2
323222(aq) 108.2
][ OH CO
aqhyd CO
COHKCOH
743
)(2
3
1322(aq) 102.4)105.1()108.2(][
]][[ OH CO
aqa CO
HCOHKHCOH
pKa1=6.38
pKa2=10.33
http:\\asadipour.kmu.ac.ir 24 slides 10
CO2 & CO32- chemistry
13920208
• Na2CO3 →2Na+ + CO32-
• CO32- HCO3
- H2CO3pKb2=7.62pKb1=3.64
http:\\asadipour.kmu.ac.ir 24 slides 11
0 1 2 30
2
4
6
8
10
12
14
Phenolphthalein
Titration of Na2CO3 with HCl
Step 1:
Step 2:
Na2CO3 + HCl NaHCO3 + NaCl
NaHCO3 + HCl H2CO3 + NaCl
Overall:Na2CO3 + 2HCl H2CO3 + 2NaCl
Equivalents of HCl added
pH
n=1 Ve1 n=2 Ve2
BCG
13920208
http:\\asadipour.kmu.ac.ir 24 slides 12
Determination of Carbonate in a Sample
0 1 2 30
2
4
6
8
10
12
14
A 1.2040-g sample containing sodium carbonate and inert material was dissolved in water 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 isNa2CO3 + 2HCl H2CO3 + 2NaCl
13920208
BCG
%59.141204
100
.1
.106
.2
.1
.
.1020.0.50.32
32
3232 CONammol
CONamg
HClmmol
CONammol
HClml
HClmmolHClml
http:\\asadipour.kmu.ac.ir 24 slides 14
Effect of Absorbed CO2 on Titration of Na2CO3 with HCl
0 10 20 300
2
4
6
8
10
12
14
Volume of HCl added, mL
pH
0 10 20 300
2
4
6
8
10
12
14
Volume of HCl added, mL
pH
CO2(g) + H2O(l) + CO32(aq) 2HCO3
(aq)
Ve1 Ve2
After absorption
Before absorption
• Ve1 decreases
• Ve2 is not affected
• Use Ve2 for calculations
13920208
http:\\asadipour.kmu.ac.ir 24 slides 15
Effect of CO2 Absorption on OH- concentration
OH- standard solutions can react with CO2
13920208
In presence of Phenol phetalein------- Carbonate Error
In presence of BCG--------no error
CO2(g) + 2 OH- → CO32
(aq)
This effect may be avoided by excluding air from the titration system.
http:\\asadipour.kmu.ac.ir 24 slides 16
Boiling to Enhance Visual Endpoint in Titration of Na2CO3 with HCl
Na2CO3 +HCl HCO3-
NaHCO3+
H2CO3
NaHCO3
Boiling removes H2CO3 as gaseous CO2
H2CO3(aq) CO2(g) + H2O
13920208
HCO3- +HCl H2CO3
http:\\asadipour.kmu.ac.ir 24 slides 17
Titration curves and indicator transition ranges for the analysis of mixtures containing :
13920208
OH- CO32-
HCO3- OH- CO3
2-
CO32- HCO3
-
Winkler method Mixture of CO3
2- +OH-
1)Titrate the mixture with H+
In presence of BCGOH-≈ 1H+ --CO3
2-≈2H+
2)Excess BaCl2 is addedCO3
2- + Ba2+→BaCO3
--Titrate the mixture with H+
Titrant-≈ OH-
3) Titrant1-titrant2 ≈ CO32-
http:\\asadipour.kmu.ac.ir 24 slides 18
Titration curves and indicator transition ranges for the analysis of mixtures containing :
13920208
OH- CO32-
HCO3- OH- CO3
2-
CO32- HCO3
-
Winkler method Mixture of CO3
2- + HCO3-
1)Titrate the mixture with H+
In presence of BCGHCO3
-≈ 1H+ --CO32-≈2H+
2)Excess BaCl2 is addedCO3
2- + Ba2+→BaCO3
--Titrate the mixture with H+
Titrant-≈ HCO3-
3) Titrant1-titrant2 ≈ CO32-
http:\\asadipour.kmu.ac.ir 24 slides 19
Titration curves and indicator transition ranges for the analysis of mixtures containing :
13920208
OH- CO32-
HCO3- OH- CO3
2-
CO32- HCO3
-
Mixture of 2 weak acids Mixture of CO3
2- + HCO3-
1)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≈ CO3
2-
3) (A-2C)=Bml ≈ HCO3-
http:\\asadipour.kmu.ac.ir 24 slides 20
Carbonate and Carbonate Mixtures
13920208
http:\\asadipour.kmu.ac.ir 24 slides 21
The Determination of Organic functional Groups
Carboxylic acid groupMost 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 groupSulfonic acids are generally strong acids and readily dissolve in water. Their titration with a base is therefore straightforward.
13920208
http:\\asadipour.kmu.ac.ir 24 slides 22
Amine groupA) 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 amines, such as pyridine and its derivatives. 13920208
http:\\asadipour.kmu.ac.ir 24 slides 23
Ester groupsEsters 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 groupsHydroxyl 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 original amount of anhydride.13920208
http:\\asadipour.kmu.ac.ir 24 slides 24
Carbonyl groupsMany aldehydes and ketones can be determined with a solution of hydroxylamine hydrochloride. The reaction, which produces an oxime, is
R1 R1
C=O + NH2OHHCl C=NOH + HCl + H2O
R2 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.
13920208