Acid Base Titrations

1. Acid Base Titrations

2. Key Terms – significance? • Titration • Titrant • Indicator • Equivalence point • Endpoint • Midpoint

3. Endpoint Equivalence point Midpoint

4. 5 key regions: Initial pH Buffering region Midpoint Vertical rise Equivalence point

5. Strong Acid/Strong Base • example”: HCl and NaOH • pH at equivalence point = 7 When a strong acid is combined with a strong base it produces a salt that has no tendency to affect the pH of a solution • Initial pH H+ from HCl • Up to EqPt OH- + H+ H2O xs H+ • At EqPt mol H+ = mol OH- pH from H2O • After EqPt xs OH- in total volume

6. Strong Acid/Strong Base

7. Strong Base/Strong Acid

8. Weak Acid/Strong Base • HC2H3O2 and NaOH • pH at equivalence point > 7 For a weak acid, the salt contains the conjugate base, which is able to recombine with a proton. Thus the solution is slightly basic • Initial pH H+ from HC2H3O2 use Ka • Up to EqPt OH- + HC2H3O2  C2H3O2- + H2O • At EqPt C2H3O2-+ H2O OH- + HC2H3O2 • After EqPt xs OH- in total volume

9. Weak Acid/Strong Base

11. Weak Acid-Strong Base Midpoint after one-half of the acid has been stoichiometrically titrated … [HA] = [A-]. At this point the pH = pKa. The titration profile is relatively flat around the pH = pKa point. This means that within this region the pH is not changing much upon the addition of small amounts of base. This is the definition of a "buffered" solution, and explains why the most effective buffering is at a pH value equal to the pKa.

12. Strong vs Weak

13. Weak Base/Strong Acid • NH3 and HCl • pH at equivalence point < 7 The solution contains the conjugate base of the strong acid and conjugate acid which can donate a proton, thus, at the equivalence point the pH is lower than neutral (pH 7.0) • Initial pH OH- from NH3 use Kb • Up to EqPt H+ + NH3  NH4+ + OH- • At EqPt NH4++ H2O NH3 + H3O+ • After EqPt xs H+ in total volume

14. Weak Base/Strong Acid

17. Choosing indicators for titrations Remember that the equivalence point of a titration is where you have mixed the two substances in exactly equation proportions. You obviously need to choose an indicator which changes color as close as possible to that equivalence point. That varies from titration to titration.

18. Strong acid v strong base

19. Strong acid v weak base

20. Weak acid v strong base

21. Weak acid v weak base

22. Neither indicator is any use. Phenolphthalein will have finished changing well before the equivalence point, and methyl orange falls off the graph altogether. On the whole, you would never titrate a weak acid and a weak base in the presence of an indicator.

23. Titrations of Polyprotic Acids • Polyprotic acids can potentially donate more than one proton • Each proton will have an associated Ka value • The titration curve will reflect the separate Ka values • There will be two unique equivalence points, associated with the separate Ka values

24. Titration curve for diprotic acid

25. For a diprotic acid, there are two buffering regions and two equivalence points. • This proves the earlier assertion that polyprotic acids lose their protons in a stepwise manner.