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Indicators for Acid-Base Titrations (Sec. 9-6)

Indicators for Acid-Base Titrations (Sec. 9-6). transition range needs to match the endpoint pH as closely as possible in order to minimize titration error. Acid-Base indicators are themselves weak acids…. e.g. phenolthalein H 2 In = HIn - = In 2-. phenolthalein 8.0-9.6.

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Indicators for Acid-Base Titrations (Sec. 9-6)

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  1. Indicators for Acid-Base Titrations (Sec. 9-6)

  2. transition range needs to match the endpoint pH as closely as possible in order to minimize titration error

  3. Acid-Base indicators are themselves weak acids….. e.g. phenolthalein H2In = HIn- = In2-

  4. phenolthalein 8.0-9.6 Ch 10: Acid-Base Titrations Automated titrators determine the endpoint electronically by numerically calculating the 2nd derivative

  5. analyte = strong acid titrant = strong base pH  2 4 3 mL base  analyte = strong base titrant = strong acid pH  mL acid  Acid-Base Titrations Curves - pH (or pOH) as a function of mL of titrant added 1

  6. Initial pH 1 I. Strong Acid-Strong Base Titration Curves (Sec. 10-1) 50 mL of 0.100 M HCl is titrated with 0.100 M NaOH. Calculate the titration curve for the analysis. equivalence pt. volume:

  7. pH before the equivalence pt. pH at the equivalence pt. 2 3

  8. pH after the equivalence pt. 4

  9. Strong Acid - Strong Base Titration (both monoprotic) (analyte) (titrant) [H+] = MaVa - MbVb Vtotal [OH-] = Mb(Vb beyond eq.pt.) Vtotal pH  Eq. Pt. pH = 7 [H+] = CHA so pH = -log CHA mL base 

  10. phenolthalein 8.0-9.6 methyl red 4.2-6.2

  11. phenolthalein 8.0-9.6 Titration Error 0.02 mL/50 mL =0.04% error!

  12. HA = H+ + A- II. Weak Acid-Strong Base Titration Curve (Sec. 10-2) 50 mL of a 0.100 M soln of the weak acid HA, Ka = 1.0 x 10-5, is titrated with 0.100 M NaOH. Calculate the titration curve for the analysis.

  13. Initial pH 1 equivalence pt. volume:

  14. pH before the equivalence pt. 2

  15. pH after the equivalence pt. = same as SA-SB titration pH at the equivalence pt. 4 3

  16. Weak Acid - Strong Base Titration (both monoprotic) (analyte) (titrant) [OH-] = Mb(Vb beyond eq.pt.) Vtotal Buffer region Eq. Pt. Hydrolysis of the conjugate base pH  1/2 eq. pt. pH = pKa mL base 

  17. R = (CH3)2CHCH2 - low pH high pH Ch 11: Titrations in Diprotic Systems Biological Applications - Amino Acids (Sec. 11-1)

  18. Finding the pH in Diprotic Systems (Sec. 11-2) 1. The acidic form H2L+ The strength of H2L+ as an acid is much, much greater than HL - Ka1 = 10-2.328 = 4.7 x 10-3 Ka2 = 10-9.744 = 1.8 x 10-10 So assume the pH depends only on H2L+ and ignore the contribution of H+ from HL.

  19. Calculate the pH of 0.050M H2L+

  20. Ka1 = 10-2.328 = 4.7 x 10-3 Ka2 = 10-9.744 = 1.8 x 10-10 2. The basic form L- Strengths of conjugate bases: for L- Kb1 = Kw/Ka2 = 1.01 x 10-14/1.8 x 10-10 = 5.61 x 10-5 for HL Kb2 = Kw/Ka1 = 1.01 x 10-14/4.7 x 10-3 = 2.1 x 10-12 Since the second conj. base HL is so weak, we'll assume all the OH- comes from the L- form.

  21. Example: Calculate the pH of a 0.050M solution of sodium leucinate

  22. The Intermediate FormThe pH of a Zwitterion Solution - Leucine (HL form)

  23. assume: KwKa1 << Ka1Ka2CHL Ka1 << CHL pH of a solution of a diprotic zwitterion [H+]2 = Ka1 Ka2 -log [H+]2 = - log Ka1 - log Ka2 2 pH = pKa1 + pKa2

  24. Example:pH of the Intermediate Form of a Diprotic Acid Potassium hydrogen phthalate, KHP, is a salt of the intermediate form of phthalic acid. Calculate the pH pf 0.10M KHP and 0.010M KHP.

  25. Titration Curve for the Amino Acid Leucine

  26. equivalence pt. volumes (Ve1 & Ve2) = pt A: init. pH (H2L+ treat as monoprotic weak acid) = pts B and D: 1st and 2nd half eq. pt's =

  27. pt C: 1st eq. pt (HL) = pt E: 2nd eq. pt (L-) =

  28. Example p. 233:Titration of Sodium Carbonate (soda ash) Calculate the titration curve for the titration of 50.0 mL of 0.020 M Na2CO3 with 0.100 M HCl. equivalence pt. volumes (Ve1 & Ve2) =

  29. pt A: init. pH (CO32- treat as monoprotic weak base) = pts B and D: 1st and 2nd half eq. pt's = pt C: 1st eq. pt (HCO3-) = pt E: 2nd eq. pt (H2CO3 treat as monoprotic weak acid) =

  30. pt E: 2nd eq. pt (H2CO3 treat as monoprotic weak acid) =

  31. H3PO4 HPO42- H2PO4- PO43- Buffers of Polyprotic Acids and Bases

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