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Typical Applications of Neutralization Titrations Elemental Analysis 139202081http:\\asadipour.kmu.ac.ir 24 slides.

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Presentation on theme: "Typical Applications of Neutralization Titrations Elemental Analysis 139202081http:\\asadipour.kmu.ac.ir 24 slides."— Presentation transcript:

1 Typical Applications of Neutralization Titrations Elemental Analysis 139202081http:\\asadipour.kmu.ac.ir 24 slides

2 Keldahl Nitrogen Analysis 1)With HCl 2)With H 3 BO 3 139202082http:\\asadipour.kmu.ac.ir 24 slides

3 Apparatus used in Kjeldahl I. Digestion apparatus II. Distillation & absorption apparatus (I) (II) 139202083http:\\asadipour.kmu.ac.ir 24 slides

4 13920208 Keldahl Nitrogen Analysis (1) Step 1: Kjeldahl digestion (decomposing and dissolving) Step 2: Neutralization by adding base Step 3: Distillation NH 3 into excess HCl standard Step 4: Titrating unreacted HCl with NaOH standard http:\\asadipour.kmu.ac.ir 24 slides

5 13920208TMHsiung@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 NH 3 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: http:\\asadipour.kmu.ac.ir 24 slides

6 催化剂 煮沸 1. Digestion NCOC + H 2 SO 4 ( NH 4 ) 2 SO 4 + CO 2 + SO 2 + H 2 O 2. Neutralization &distillation 2NaOH + ( NH 4 ) 2 SO 4 2NH 3 ↑+Na 2 SO 4 + 2H 2 O 3. Absorption by boric acid : 2NH 3 + 4H 3 BO 3 ( NH 4 ) 2 B 4 O 7 + 5H 2 O 4. Titration by strong acid ( NH 4 ) 2 B 4 O 7 + 5H 2 O + 2HCl 2NH 4 Cl + 4H 3 BO 3 (* NCOC, N containing organic compounds, N:HCl = 1:1) Keldahl Nitrogen Analysis (2) 139202086http:\\asadipour.kmu.ac.ir 24 slides

7 13920208TMHsiung@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 NH 3 was distilled into 100.00 mL of 0.0200 M H 3 BO 3. 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: 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. 139202088http:\\asadipour.kmu.ac.ir 24 slides

9 9 CO 2 & CO 3 2- chemistry 13920208http:\\asadipour.kmu.ac.ir 24 slides pK a1 =6.38 pK a2 =10.33

10 10 CO 2 & CO 3 2- chemistry 13920208 Na 2 CO 3 →2Na + + CO 3 2- CO 3 2-  HCO 3 -  H2CO3 pK b2 =7.62pK b1 =3.64 http:\\asadipour.kmu.ac.ir 24 slides

11 11 Phenolphthalein Titration of Na 2 CO 3 with HCl Step 1: Step 2: Na 2 CO 3 + HCl  NaHCO 3 + NaCl NaHCO 3 + HCl  H 2 CO 3 + NaCl Overall: Na 2 CO 3 + 2HCl  H 2 CO 3 + 2NaCl Equivalents of HCl added pH n=1 V e1 n=2 V e2 BCG 13920208http:\\asadipour.kmu.ac.ir 24 slides

12 12 Determination of Carbonate in a Sample 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 % Na 2 CO 3 in the sample. For the BCG endpoint, the reaction is Na 2 CO 3 + 2HCl  H 2 CO 3 + 2NaCl 13920208http:\\asadipour.kmu.ac.ir 24 slides BCG

13 13 Effect of CO 2 Absorption on Phenophthalein Endpoint Reaction of CO 2 with carbonate solutions CO 2 (g) + H 2 O(l) + CO 3 2  (aq)  2HCO 3  (aq) pH of carbonate solutions may be unstable due to exchange of CO 2 with atmosphere. This effect may be avoided by excluding air from the titration system. 13920208http:\\asadipour.kmu.ac.ir 24 slides

14 14 Effect of Absorbed CO 2 on Titration of Na 2 CO 3 with HCl CO 2 (g) + H 2 O(l) + CO 3 2  (aq)  2HCO 3  (aq) V e1 V e2 After absorption Before absorption V e1 decreases V e2 is not affected Use V e2 for calculations 13920208http:\\asadipour.kmu.ac.ir 24 slides

15 15 Effect of CO 2 Absorption on OH - concentration OH - standard solutions can react with CO 2 13920208http:\\asadipour.kmu.ac.ir 24 slides In presence of Phenol phetalein------- Carbonate Error In presence of BCG--------no error CO 2(g) + 2 OH - → CO 3 2  (aq) This effect may be avoided by excluding air from the titration system.

16 16 Boiling to Enhance Visual Endpoint in Titration of Na 2 CO 3 with HCl Na 2 CO 3 +HCl  HCO 3 - NaHCO 3 + H 2 CO 3 NaHCO 3 Boiling removes H 2 CO 3 as gaseous CO 2 H 2 CO 3 (aq)  CO2(g) + H 2 O 13920208http:\\asadipour.kmu.ac.ir 24 slides HCO 3 - +HCl  H 2 CO 3

17 Titration curves and indicator transition ranges for the analysis of mixtures containing : 1392020817http:\\asadipour.kmu.ac.ir 24 slides OH - CO 3 2- HCO 3 - OH - CO 3 2- HCO 3 - Winkler method Mixture of CO 3 2- +OH - 1)Titrate the mixture with H + In presence of BCG OH - ≈ 1H + --CO 3 2- ≈2H + 2)Excess BaCl 2 is added CO 3 2- + Ba 2+ →BaCO 3  --Titrate the mixture with H + Titrant - ≈ OH - 3) Titrant1-titrant2 ≈ CO 3 2-

18 Titration curves and indicator transition ranges for the analysis of mixtures containing : 1392020818http:\\asadipour.kmu.ac.ir 24 slides OH - CO 3 2- HCO 3 - OH - CO 3 2- HCO 3 - Winkler method Mixture of CO 3 2- + HCO 3 - 1)Titrate the mixture with H + In presence of BCG HCO 3 - ≈ 1H + --CO 3 2- ≈2H + 2)Excess BaCl 2 is added CO 3 2- + Ba 2+ →BaCO 3  --Titrate the mixture with H + Titrant - ≈ HCO 3 - 3) Titrant1-titrant2 ≈ CO 3 2-

19 Titration curves and indicator transition ranges for the analysis of mixtures containing : 1392020819http:\\asadipour.kmu.ac.ir 24 slides OH - CO 3 2- HCO 3 - OH - CO 3 2- HCO 3 - Mixture of 2 weak acids Mixture of CO 3 2- + HCO 3 - 1)Titrate the mixture with H + In presence of BCG (A ml) HCO 3 - ≈ 1H + -- CO 3 2- ≈2H + 2)Titrate the mixture with H + In presence of Phenol phetalein (C ml) Titrant ≈ CO 3 2- 3) (A-2C)=Bml ≈ H CO 3 -

20 Carbonate and Carbonate Mixtures 1392020820http:\\asadipour.kmu.ac.ir 24 slides

21 The Determination of Organic functional Groups Carboxylic acid group Most CA have between 10 –4 > K a > 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. 1392020821http:\\asadipour.kmu.ac.ir 24 slides

22 Amine group A) titrated in aqueous media. 1) Aliphatic amines generally have K b 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 (K b  10 –10 ). 4) The same is true for cyclic amines, such as pyridine and its derivatives. 1392020822http:\\asadipour.kmu.ac.ir 24 slides

23 Ester groups Esters are commonly determined by saponification with a measured quantity of standard base: R 1 COOR 2 + OH –  R 1 COO – + HOR 2 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. (CH 3 CO) 2 O + ROH  CH 3 COOR + CH 3 COOH 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 : (CH 3 CO) 2 O + H 2 O  2CH 3 COOH 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. 1392020823http:\\asadipour.kmu.ac.ir 24 slides

24 Carbonyl groups Many aldehydes and ketones can be determined with a solution of hydroxylamine hydrochloride. The reaction, which produces an oxime, is R 1 R 1 C=O + NH 2 OH  HCl  C=NOH + HCl + H 2 O R 2 R 2 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. 1392020824http:\\asadipour.kmu.ac.ir 24 slides


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