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Typical Applications of Neutralization Titrations
Elemental Analysis
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Keldahl Nitrogen Analysis
1)With HCl
2)With H3BO3
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3. Apparatus used in Kjeldahl
II. Distillation & absorption apparatus
I. Digestion apparatus
(I)
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(II)

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Keldahl Nitrogen Analysis (1)
Step 1: Kjeldahl digestion (decomposing and dissolving)
Step 2: Neutralization by adding base
Step 3: Distillation NH3 into excess HCl standard
Step 4: Titrating unreacted HCl with NaOH standard
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Example: A typical meat protein contains 16.2% (w/w) nitrogen. A mL aliquot of protein solution was digested, and the liberated NH3 was distilled into mL of M HCl. The unreacted HCl required 3.26 mL of M NaOH for complete titration. Find the concentration of protein (mg protein/ml) in the original sample.
Solution:
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6. Keldahl Nitrogen Analysis (2)
1. Digestion
NCOC + H2SO （NH4）2SO4 + CO2 + SO2 + H2O
2. Neutralization &distillation
2NaOH +（NH4）2SO NH3↑+Na2SO4 + 2H2O
3. Absorption by boric acid :
    
2NH3 + 4H3BO （NH4）2B4O7 + 5H2O
4. Titration by strong acid
（NH4）2B4O7 + 5H2O + 2HCl NH4Cl + 4H3BO3
(* NCOC, N containing organic compounds, N:HCl = 1:1)
催化剂 煮沸
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Example: A typical meat protein contains 16.2% (w/w) nitrogen. A mL aliquot of protein solution was digested, and the liberated NH3 was distilled into mL of M H3BO3. The mixture required 3.26 mL of M HCl for complete titration. Find the concentration of protein (mg protein/ml) in the original sample.
Solution:
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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
pKa1=6.38
pKa2=10.33
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CO2 & CO32- chemistry
Na2CO3 →2Na+ + CO32-
CO32- HCO3-  H2CO3
pKb1=3.64
pKb2=7.62
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11. Titration of Na2CO3 with HCl
Step 1:
Na2CO3 + HCl  NaHCO3 + NaCl
Step 2:
NaHCO3 + HCl  H2CO3 + NaCl
Overall:
Na2CO3 + 2HCl  H2CO3 + 2NaCl
Phenolphthalein pH
BCG
n=1 Ve1
n=2 Ve2
Equivalents of HCl added
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12. Determination of Carbonate in a Sample
A g sample containing sodium carbonate and inert material was dissolved in water and titrated to the bromcresol green end point, requiring mL of M HCl.
Calculate the % Na2CO3 in the sample.
BCG
For the BCG endpoint, the reaction is
Na2CO3 + 2HCl  H2CO3 + 2NaCl
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13. Effect of CO2 Absorption on Phenophthalein Endpoint
Reaction of CO2 with carbonate solutions
CO2(g) + H2O(l) + CO32(aq)  2HCO3(aq)
pH of carbonate solutions may be unstable due to exchange of CO2 with atmosphere.
This effect may be avoided by excluding air from the titration system.
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14. Effect of Absorbed CO2 on Titration of Na2CO3 with HCl
Ve1
Ve2
Before absorption
After absorption
Ve1 decreases
Ve2 is not affected
Use Ve2 for calculations
CO2(g) + H2O(l) + CO32(aq) 2HCO3(aq)
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15. 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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16. Boiling to Enhance Visual Endpoint in Titration of Na2CO3 with HCl
Na2CO3 +HCl  HCO3-
NaHCO3
HCO3- +HCl  H2CO3
Boiling removes H2CO3 as gaseous CO2
NaHCO3 +
H2CO3
H2CO3(aq)  CO2(g) + H2O
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Titration curves and indicator transition ranges for the analysis of mixtures containing :
OH-
CO32-
Winkler method
Mixture of CO32- +OH-
1)Titrate the mixture with H+
In presence of BCG
OH-≈ 1H+ --CO32-≈2H+
2)Excess BaCl2 is added
CO32- + Ba2+→BaCO3
--Titrate the mixture with H+
Titrant-≈ OH-
3) Titrant1-titrant2 ≈ CO32-
HCO3-
OH-
CO32-
CO32-
HCO3-
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Titration curves and indicator transition ranges for the analysis of mixtures containing :
OH-
CO32-
Winkler method
Mixture of CO32- + HCO3-
1)Titrate the mixture with H+
In presence of BCG
HCO3-≈ 1H+ --CO32-≈2H+
2)Excess BaCl2 is added
CO32- + Ba2+→BaCO3
--Titrate the mixture with H+
Titrant-≈ HCO3-
3) Titrant1-titrant2 ≈ CO32-
HCO3-
OH-
CO32-
CO32-
HCO3-
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Titration curves and indicator transition ranges for the analysis of mixtures containing :
OH-
CO32-
Mixture of 2 weak acids
Mixture of CO32- + 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≈ CO32-
3) (A-2C)=Bml ≈ HCO3-
HCO3-
OH-
CO32-
CO32-
HCO3-
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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 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 original amount of anhydride.
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Carbonyl groups
Many aldehydes and ketones can be determined with a solution of hydroxylamine hydrochloride. The reaction, which produces an oxime, is
R R1
C=O + NH2OHHCl  C=NOH + HCl + H2O
R 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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