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Titrating Polyfunctional Acids and Bases

Titrating Polyfunctional Acids and Bases. 1. Treating Complex Acid-Base Systems. Complex systems are defined as solutions made up of: (1) An acid or base that has two or more acidic protons or basic functional groups H 3 PO 4 Ca(OH) 2

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Titrating Polyfunctional Acids and Bases

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  1. Titrating Polyfunctional Acids and Bases http:\\asadipour.kmu.ac.ir 40 slides

  2. 1. Treating Complex Acid-Base Systems • Complex systems are defined as solutions made up of: (1) An acid or base that has two or more acidic protons or basic functional groups H3PO4 Ca(OH)2 (2) Two acids or bases of different strengths HCl + CH3COOH NaOH + CH3COO- http:\\asadipour.kmu.ac.ir 40 slides

  3. )3) An amphiprotic substance that is capable of acting as both acid and base HCO3- + H2O  CO32- + H3O+ HCO3- + H2O  H2CO3 + OH- NH3+CH2COO- + H2O NH2CH2COO- + H3O+ NH3+CH2COO- + H2O NH3+CH2COOH + OH- http:\\asadipour.kmu.ac.ir 40 slides

  4. Kb2 Kb3 Kb1 Ka1×Kb3=Kw Ka2×Kb2=Kw Ka3×Kb1=Kw Ka1=1×10-2 >Ka2=1×10-7> Ka3=1×10-12 Ktotal=Ka1×Ka2×Ka3=1×10-21 http:\\asadipour.kmu.ac.ir 40 slides

  5. pH of H3PO4 • Calculate the pH of 0.100M H3PO4 solution. H+ is not negligible http:\\asadipour.kmu.ac.ir 40 slides

  6. pH of HA- pH of HA- solution HA- A2- + H+ HA- H2A + OH- Ka2 Kb2 Ka1 http:\\asadipour.kmu.ac.ir 40 slides

  7. pH of HA- Calculate the pH of 0.100M NaHCO3 solution. Ka2×CHA- =1×10-10 ×1.00>>Kw…….Kw is negligible Ka1=1×10-6 >Ka2=1×10-10 http:\\asadipour.kmu.ac.ir 40 slides

  8. pH of HA- Calculate the pH of 0.0100M NaH2PO4 solution. Ka1=1×10-2 >Ka2=1×10-7> Ka3=1×10-12 Ka2×CHA- =1×10-7 ×0.01>>Kw…….Kw is negligible http:\\asadipour.kmu.ac.ir 40 slides

  9. pH of HA- Calculate the pH of 1.00×10-3M Na2HPO4 solution. Ka2×CHA- =1×10-10 ×0.001=1×10-13Kw isnot negligible Ka1=1×10-2>Ka2=1×10-7> Ka3=1×10-12 http:\\asadipour.kmu.ac.ir 40 slides

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  25. Mixture of weak and strong acids Sulfuric acid is unusual in that one of its protons behaves as a strong acid in water and the other as a weak acid (Ka2 = 1.02 X 10-2). Let us consider how the hydronium ion concentration of sulfuric acid solutions is computed using a 0.0400M solution as an example. H2SO4→H+ +HSO4- SO42- + H+ We will first assume that the dissociation of HSO4 is negligible because of the large excess of H30+ resulting from the complete dissociation of H2SO4. Therefore, [H+] ≈ [HSO4 ] ≈ 0.0400 M This result shows that [SO4- ] is not small relative to [HSO4 ], and a more rigorous so­lution is required. From stoichiometric considerations, it is necessary that [SO4] = [H+] - 0.0400 CH2SO4, = 0.0400 = [HS04- ] + [SO42-] [H+] = 0.0400 + [SO42-] [HSO4-] = 0.0800 - [H3O+] http:\\asadipour.kmu.ac.ir 40 slides

  26. Mixture of weak and strong acids Sulfuric acid is unusual in that one of its protons behaves as a strong acid in water and the other as a weak acid (Ka2 = 1.02 X 10-2). Let us consider how the hydronium ion concentration of sulfuric acid solutions is computed using a 0.0400M solution as an example. H2SO4→H+ +HSO4- SO42- + H+ We will first assume that the dissociation of HSO4 is negligible because of the large excess of H30+ resulting from the complete dissociation of H2SO4. Therefore, [H+] = 0.0400 + [SO42-] [HSO4-] = 0.0400 - [SO42-] http:\\asadipour.kmu.ac.ir 40 slides

  27. Curves for the titration of strong acid / weak acid mixture with 0.1000 M NaOH. Each titration is on 25.00 ml of a solution that is 0.1200 M in HCl and 0.0800 M in HA. http:\\asadipour.kmu.ac.ir 40 slides

  28. Curves for the titration of 25.00 ml of polyprotic acid with 0.1000M NaOH solution . A)0.1000 M H3PO4, B) 0.1000M oxalic acid, C) 0.1000 M H2SO4 Ka1=1×10-2 >Ka2=1×10-7> Ka3=1×10-12 Ka1 =5.6× 10-2 and Ka2 = 5.4 x 10-5 Ka2 = 1.02 × 10-2 http:\\asadipour.kmu.ac.ir 40 slides

  29. Titration curves for polyfunctional acids Titration of 20.00 ml of 0.1000 M H2A with 0.1000 M NaOH. For H2A, Ka1= 1.00 × 10–3 and Ka2 = 1.00 × 10–7. http:\\asadipour.kmu.ac.ir 40 slides

  30. Titration of 25.00 ml of 0.1000M maleic acid with 0.1000M NaOH. HOOC-C=C-COOH pKa1=1.89 ,pKa2=6.23 http:\\asadipour.kmu.ac.ir 40 slides

  31. E-HOOC-C=C-COOH Z-HOOC-C=C-COOH Fractional composition diagram for fumaric acid (trans-butenedioic acid). Fractional composition diagram for maleic acid (Cis-butenedioic acid). pKa1=3.05 ,pKa2=4.49 pKa1=1.89 ,pKa2=6.23 http:\\asadipour.kmu.ac.ir 40 slides

  32. amino acids alanine The amine group behaves as a base, while the carboxyl group acts as an acid. Aspartic acid http:\\asadipour.kmu.ac.ir 40 slides

  33. 1-Determining the pK values for amino acids Amino acids contain both an acidic and a basic group. NH2-CH2-COOH  +NH3-CH2-COO- Zwitterion formation +NH3-CH2-COO- + H2O  NH2-CH2-COO- + H3O+ +NH3-CH2-COO- + H2O  +NH3-CH2-COOH + OH- Ka×Kb= ??!!!! http:\\asadipour.kmu.ac.ir 40 slides

  34. 2-Determining the pK values for amino acids Amino acids contain both an acidic and a basic group. NH2-CH2-COOH  +NH3-CH2-COO- Zwitterion formation Kb Ka +NH3-CH2-COOH+NH3-CH2-COO- NH2-CH2-COO- Ka2=2×10-10 Ka1=5×10-3 http:\\asadipour.kmu.ac.ir 40 slides

  35. A B Curves for the titration of 20.00ml of 0.1000M alanine with A) 0.1000 M NaOH B) 0.1000M HCl. http:\\asadipour.kmu.ac.ir 40 slides

  36. Iso electric point: • The pH at which the average charge of the polyprotic acid iszero • The zwitterion of an amino acid, containing as it does a positive and a negative charge, has no tendency to migrate in an electric field, • whereas the singly charged anionic and cationic species are attracted to electrodes of opposite charge. • NH2-CH2-COO- +NH3-CH2-COOH • No net migration of the amino acid occurs in an electric field when the pH of the solvent is such that [anionic] = [cationic], which is pH dependent. • The pH at which no net migration occurs is called the isoelectric point; this point is an important physical constant for characterizing amino acids. The isoelectric point is readily related to the ionization constants for the species. Thus, for glycine, +NH3-CH2-COO- http:\\asadipour.kmu.ac.ir 40 slides

  37. 1-Determining iso electric point for amino acids +NH3-CH2-COO- Zwitterion formation http:\\asadipour.kmu.ac.ir 40 slides

  38. 2-Determining iso electric point for amino acids Ka2=9.87 pKa1=2.35 +NH3-CH2-COOH+NH3-CH2-COO- NH2-CH2-COO- http:\\asadipour.kmu.ac.ir 40 slides

  39. Method1=method2 Ka=Ka2,,,,,,,,,,,,Kb=Kb2

  40. Formol titration For simple amino acids, Ka and Kbare generally so small that their quantitative determination by neutralization titrations is impos­sible. Amino acids that contain more than one carboxyl or amine group can sometimes be determined. If the Kavalues are different enough (104 or more), stepwise end points can be obtained just like other polyfunctional acids or bases as long as the Kavalues +NH3-CH2-COO- + OH-  Product +NH3-CH2-COO- + HCOH  CH2=NCH2COOH http:\\asadipour.kmu.ac.ir 40 slides

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