1. Acid-Base Titrations Introduction 3.)Overview  Titrations are Important tools in providing quantitative and qualitative data for a sample.  To best understand titrations and the information they provide, it is necessary to understand what gives rise to the shape of a typical titration curve.  To do this, acid-base equilibria are used to predict titration curve shapes. Biochemistry, Vol. 41, No. 22, 2002 6945 pK a of His in the His-Asp catalytic dyad that catalyzes the oxidation of glucose 6-phosphate

2. Acid-Base Titrations Titration of Strong Base with Strong Acid 1.)Graph of How pH changes as Titrant is Added  Assume strong acid and base completely dissociate  Any amount of H + added will consume a stoichiometric amount of OH -  Reaction Assumed to go to completion  Three regions of the titration curve - Before the equivalence point, the pH is determined by excess OH- in the solution - At the equivalence point, H + is just sufficient to react with all OH - to make H 2 O - After the equivalence point, pH is determined by excess H + in the solution.

3. Acid-Base Titrations Titration of Strong Base with Strong Acid 1.)Graph of How pH changes as Titrant is Added  Remember, equivalence point is the ideal goal  Actually measure End Point - Marked by a sudden physical change: color, potential  Different Regions require different kinds of calculations - Illustrated examples  The “true” titration reaction is: Titrant Analyte

4. Acid-Base Titrations Titration of Strong Base with Strong Acid 2.)Volume Needed to Reach the Equivalence Point  Titration curve for 50.00 mL of 0.02000 M KOH with 0.1000 M HBr  A t equivalence point, amount of H + added will equal initial amount of OH - mmol of HBr at equivalence point mmol of OH - being titrated When 10.00 mL of HBr has been added, the titration is complete. Prior to this point, there is excess OH- present. After this point there is excess H + present.

5. Acid-Base Titrations Titration of Strong Base with Strong Acid 3.)Before the Equivalence Point  Titration curve for 50.00 mL of 0.02000 M KOH with 0.1000 M HBr - Equivalence point (V e ) when 10.00 mL of HBr has been added - When 3.00 mL of HBr has been added, reaction is 3/10 complete Fraction of OH - Remaining Initial concentration of OH - Dilution Factor Initial volume of OH - Total volume Calculate Remaining [OH - ]: Calculate [H + ] and pH:

6. Acid-Base Titrations Titration of Strong Base with Strong Acid 4.)At the Equivalence Point  Titration curve for 50.00 mL of 0.02000 M KOH with 0.1000 M HBr - Just enough H + has been added to consume OH - - pH determined by dissociation of water - pH at the equivalence point for any strong acid with strong base is 7.00 - Not true for weak acid-base titration KwKw K w = 1x10 -14 x

7. Acid-Base Titrations Titration of Strong Base with Strong Acid 5.)After the Equivalence Point  Titration curve for 50.00 mL of 0.02000 M KOH with 0.1000 M HBr - Adding excess HBr solution - When 10.50 mL of HBr is added Calculate excess [H + ]: Initial concentration of H + Dilution factor Volume of excess H + Total volume Calculate volume of excess H + : Calculate pH:

8. Acid-Base Titrations Titration of Strong Base with Strong Acid 6.)Titration Curve  Rapid Change in pH Near Equivalence Point - Equivalence point is where slope is greatest - Second derivative is 0  pH at equivalence point is 7.00, only for strong acid-base - Not True if a weak base-acid is used

9. Acid-Base Titrations Titration of Weak Acid with Strong Base 1.)Four Regions to Titration Curve  Before any added base, just weak acid (HA) in water - pH determined by K a  With addition of strong base  buffer - pH determined by Henderson Hasselbach equation  At equivalence point, all HA is converted into A - - Weak base with pH determined by K b KaKa KbKb

10. Acid-Base Titrations Titration of Weak Acid with Strong Base 1.)Four Regions to Titration Curve  Beyond equivalence point, excess strong base is added to A - solution - pH is determined by strong base - Similar to titration of strong acid with strong base 2.)Illustrated Example:  Titration of 50.00 mL of 0.02000 M MES with 0.1000 M NaOH - MES is a weak acid with pK a = 6.27 - Reaction goes to completion with addition of strong base K

11. Acid-Base Titrations Titration of Weak Acid with Strong Base 3.) Volume Needed to Reach the Equivalence Point  Titration of 50.00 mL of 0.02000 M MES with 0.1000 M NaOH - Reaction goes to completion with addition of strong base - Strong plus weak react completely mmol of base mmol of HA

12. Acid-Base Titrations Titration of Weak Acid with Strong Base 4.) Region 1: Before Base is Added  Titration of 50.00 mL of 0.02000 M MES with 0.1000 M NaOH  Simply a weak-acid problem F - x x x KaKa K a = 10 -6.27 Calculate [H + ]: Calculate pH:

13. Acid-Base Titrations Titration of Weak Acid with Strong Base 5.) Region 2: Before the Equivalence Point  Titration of 50.00 mL of 0.02000 M MES with 0.1000 M NaOH  Adding OH - creates a mixture of HA and A -  Buffer  Calculate pH from [A - ]/[HA] using Henderson-Hasselbach equation Calculate [A - ]/HA]: Relative Initial quantities (HA≡1)1 -- Relative Final quantities- Amount of added NaOH is 3 mL with equivalence point is 10 mL Calculate pH: Simply ratio of volumes Simply the difference of initial quantities

14. Acid-Base Titrations Titration of Weak Acid with Strong Base 5.) Region 2: Before the Equivalence Point  Titration of 50.00 mL of 0.02000 M MES with 0.1000 M NaOH  pH = pK a when the volume of titrant equals ½ V e Relative Initial quantities (HA≡1)1 -- Relative Final quantities-

15. Acid-Base Titrations Titration of Weak Acid with Strong Base 5.) Region 3: At the Equivalence Point  Titration of 50.00 mL of 0.02000 M MES with 0.1000 M NaOH  Exactly enough NaOH to consume HA  The solution only contains A -  weak base Relative Initial quantities (HA≡1)11 -- Relative Final quantities-- 11 KbKb F - x x x

16. Acid-Base Titrations Titration of Weak Acid with Strong Base 5.) Region 3: At the Equivalence Point  Titration of 50.00 mL of 0.02000 M MES with 0.1000 M NaOH Calculate Formal concentration of [A - ]: A - is no longer 0.02000 M, diluted by the addition of NaOH Initial concentration of HA Dilution factor Initial volume of HA Total volume

17. pH at equivalence point is not 7.00 Acid-Base Titrations Titration of Weak Acid with Strong Base 5.) Region 3: At the Equivalence Point  Titration of 50.00 mL of 0.02000 M MES with 0.1000 M NaOH Calculate [OH - ]: Calculate pH: pH will always be above 7.00 for titration of a weak acid because acid is converted into conjugate base at the equivalence point

18. Acid-Base Titrations Titration of Weak Acid with Strong Base 5.) Region 4: After the Equivalence Point  Titration of 50.00 mL of 0.02000 M MES with 0.1000 M NaOH  Adding NaOH to a solution of A - - NaOH is a much stronger base than A - - pH determined by excess of OH - Calculate excess [OH - ]: Initial concentration of OH - Dilution factor Volume of excess OH - Total volume Calculate volume of excess OH - : Calculate pH: Amount of added NaOH is 10.10 mL with equivalence point is 10 mL

19. Acid-Base Titrations Titration of Weak Acid with Strong Base 5.) Titration Curve  Titration of 50.00 mL of 0.02000 M MES with 0.1000 M NaOH  Two Important Features of the Titration Curve Equivalence point: [OH-] = [HA] Steepest part of curve Maximum slope pH=pK a V b = ½V e Minimum slope Maximum Buffer Capacity

20. Acid-Base Titrations Titration of Weak Acid with Strong Base 5.) Titration Curve  Depends on pK a or acid strength  Inflection point or maximum slope decreases with weaker acid - Equivalence point becomes more difficult to identify Strong acid  large slope change in titration curve Easy to detect equivalence point weak acid  small slope change in titration curve Difficult to detect equivalence point

21. Acid-Base Titrations Titration of Weak Acid with Strong Base 5.) Titration Curve  Depends on acid concentration  Inflection point or maximum slope decreases with lower acid concentration - Equivalence point becomes more difficult to identify - Eventually can not titrate acid at very low concentrations High concentration  large slope change in titration curve Easy to detect equivalence point Low concentration  small slope change in titration curve Difficult to detect equivalence point At low enough concentration, can not detect change

22. Titration in Polyprotic Systems 1.) Principals for Monoprotic Systems Apply to Diprotic and Triprotic Systems  Multiple equivalence points and buffer regions  Multiple Inflection Points in Titration Curve Acid-Base Titrations K b2 Two equivalence points K b1

23. Acid-Base Titrations End Point Determination 1.) Indicators: compound added in an acid-base titration to allow end point detection  Common indicators are weak acids or bases  Different protonated species have different colors pK = 1.7 pK = 8.9

24. Acid-Base Titrations End Point Determination 2.) Choosing an Indicator  Want Indicator that changes color in the vicinity of the equivalence point and corresponding pH  The closer the two match, the more accurate determining the end point will be Bromocresol green will change color Significantly past the equivalence point resulting in an error. Bromocresol purple color change brackets the equivalence point and is a good indicator choice

25. Acid-Base Titrations End Point Determination 2.) Choosing an Indicator The difference between the end point (point of detected color change) and the true equivalence point is the indicator error Amount of indicator added should be negligible Indicators cover a range of pHs

26. Acid-Base Titrations End Point Determination 3.) Example: a) What is the pH at the equivalence point when 0.100 M hydroxyacetic acid is titrated with 0.0500 M KOH? b) What indicator would be a good choice to monitor the endpoint?