Titrating Polyfunctional Acids and Bases 920207 http:\\asadipour.kmu.ac.ir 40 slides

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- 920207 http:\\asadipour.kmu.ac.ir 40 slides

http:\\asadipour.kmu.ac.ir 40 slides )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- 920207 http:\\asadipour.kmu.ac.ir 40 slides

http:\\asadipour.kmu.ac.ir 40 slides 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 920207 http:\\asadipour.kmu.ac.ir 40 slides

http:\\asadipour.kmu.ac.ir 40 slides pH of H3PO4 Calculate the pH of 0.100M H3PO4 solution. H+ is not negligible 920207 http:\\asadipour.kmu.ac.ir 40 slides

http:\\asadipour.kmu.ac.ir 40 slides pH of HA- pH of HA- solution HA- A2- + H+ HA- H2A + OH- Ka2 Kb2 Ka1 920207 http:\\asadipour.kmu.ac.ir 40 slides

http:\\asadipour.kmu.ac.ir 40 slides 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 920207 http:\\asadipour.kmu.ac.ir 40 slides

http:\\asadipour.kmu.ac.ir 40 slides 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 920207 http:\\asadipour.kmu.ac.ir 40 slides

http:\\asadipour.kmu.ac.ir 40 slides pH of HA- Calculate the pH of 1.00×10-3M Na2HPO4 solution. Ka2×CHA- =1×10-10 ×0.001=1×10-13 Kw isnot negligible Ka1=1×10-2 >Ka2=1×10-7> Ka3=1×10-12 920207 http:\\asadipour.kmu.ac.ir 40 slides

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http:\\asadipour.kmu.ac.ir 40 slides 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+] 920207 http:\\asadipour.kmu.ac.ir 40 slides

http:\\asadipour.kmu.ac.ir 40 slides 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-] 920207 http:\\asadipour.kmu.ac.ir 40 slides

http:\\asadipour.kmu.ac.ir 40 slides 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. 920207 http:\\asadipour.kmu.ac.ir 40 slides

http:\\asadipour.kmu.ac.ir 40 slides 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 920207 http:\\asadipour.kmu.ac.ir 40 slides

http:\\asadipour.kmu.ac.ir 40 slides 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 . 920207 http:\\asadipour.kmu.ac.ir 40 slides

http:\\asadipour.kmu.ac.ir 40 slides Titration of 25.00 ml of 0.1000M maleic acid with 0.1000M NaOH. HOOC-C=C-COOH pKa1=1.89 ,pKa2=6.23 920207 http:\\asadipour.kmu.ac.ir 40 slides

http:\\asadipour.kmu.ac.ir 40 slides 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 920207 http:\\asadipour.kmu.ac.ir 40 slides

http:\\asadipour.kmu.ac.ir 40 slides amino acids alanine The amine group behaves as a base, while the carboxyl group acts as an acid. Aspartic acid 920207 http:\\asadipour.kmu.ac.ir 40 slides

http:\\asadipour.kmu.ac.ir 40 slides 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= ??!!!! 920207 http:\\asadipour.kmu.ac.ir 40 slides

http:\\asadipour.kmu.ac.ir 40 slides 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 920207 http:\\asadipour.kmu.ac.ir 40 slides

http:\\asadipour.kmu.ac.ir 40 slides A B Curves for the titration of 20.00ml of 0.1000M alanine with A) 0.1000 M NaOH B) 0.1000M HCl. 920207 http:\\asadipour.kmu.ac.ir 40 slides

http:\\asadipour.kmu.ac.ir 40 slides Iso electric point: The pH at which the average charge of the polyprotic acid is zero 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- 920207 http:\\asadipour.kmu.ac.ir 40 slides

http:\\asadipour.kmu.ac.ir 40 slides 1-Determining iso electric point for amino acids +NH3-CH2-COO- Zwitterion formation 920207 http:\\asadipour.kmu.ac.ir 40 slides

http:\\asadipour.kmu.ac.ir 40 slides 2-Determining iso electric point for amino acids Ka2=9.87 pKa1=2.35 +NH3-CH2-COOH +NH3-CH2-COO-  NH2-CH2-COO- 920207 http:\\asadipour.kmu.ac.ir 40 slides

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

http:\\asadipour.kmu.ac.ir 40 slides Formol titration For simple amino acids, Ka and Kb are 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 Ka values are different enough (104 or more), stepwise end points can be obtained just like other polyfunctional acids or bases as long as the Ka values +NH3-CH2-COO- + OH-  Product +NH3-CH2-COO- + HCOH  CH2=NCH2COOH 920207 http:\\asadipour.kmu.ac.ir 40 slides