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Typical Applications of Neutralization Titrations

Elemental Analysis

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Keldahl Nitrogen Analysis

1)With HCl2)With H3BO313920208

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Apparatus used in Kjeldahl

I. Digestion apparatus

II. Distillation & absorption apparatus

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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

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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:

) 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

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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)

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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:

) 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

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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

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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

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CO2 & CO32- chemistry

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• Na2CO3 →2Na+ + CO32-

• CO32- HCO3

- H2CO3pKb2=7.62pKb1=3.64

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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

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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

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BCG

%59.141204

100

.1

.106

.2

.1

.

.1020.0.50.32

32

3232 CONammol

CONamg

HClmmol

CONammol

HClml

HClmmolHClml

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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

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Effect of CO2 Absorption on OH- concentration

OH- standard solutions can react with CO2

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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.

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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

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HCO3- +HCl H2CO3

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Titration curves and indicator transition ranges for the analysis of mixtures containing :

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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-

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Titration curves and indicator transition ranges for the analysis of mixtures containing :

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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-

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Titration curves and indicator transition ranges for the analysis of mixtures containing :

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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-

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Carbonate and Carbonate Mixtures

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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.

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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

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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

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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.

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