1. Strength of Acids Strength of an acid is measured by the extent it reacts with water to form hydronium ions (H 3 O + ). Strong acids ionize ~100% so pH = -log [ acid ]. eg. HCl, HBr, HNO 3, H 2 SO 4

2. Strength of Acids Weak acids (or bases) only partially ionize; typically only 1-5%. eg. CH 3 COCOOH, pyruvic acid CH 3 CHOHCOOH, lactic acid CH 3 COOH, acetic acid

3. Strength of Acids Strength of an acid is indicated by its K a or pK a (= -log(K a )). HA + H 2 O H 3 O + + A - Larger K a (smaller pK a ) values indicate more dissociation and stronger acids. K a = [H 3 O + ][A - ] [HA]

4. Strength of Acids K a pK a CH 3 COCOOH 3.2x10 -3 2.5 CH 3 CHOHCOOH 1.4x10 -4 3.9 CH 3 COOH 1.8x10 -5 4.8 Larger K a and smaller pK a values indicate stronger acids.

5. Monitoring Acidity The Henderson-Hasselbalch (HH) equation is derived from the equilibrium expression for dissociation of a weak acid.

6. HA A - + H + Ka = [A - ][H + ] [HA] pH = pK a + log [A - ] [HA] Henderson-Hasselbalch Equation

7. The HH equation enables us to calculate the pH of a weak acid solution during a titration and to make predictions regarding buffer solutions. What is a titration? It is a process in which carefully measured volumes of a base are added to a solution of an acid while monitoring the change in pH.

9. When chemically equivalent amounts of acid and base are present during a titration, the equivalence point is reached. At the equivalence point, only the salt (eg. Na + CH 3 COO - ) is present in solution.

10. Titration Curve (CH 3 COOH with NaOH) moles OH - per mole HA pH

11. Titration Curve At the inflection point, equal moles of A - and HA are present in solution. [A - ] = [HA] pH = pK a + log (1) = pK a + 0 [A - ] [HA] pH = pK a + log

12. Sample calculation The pK a for acetic acid is 4.76. 1. Calculate the relative amounts of acetic acid (HA) and acetate ion (A - ) when 0.7 equivalents of NaOH have been added to an acetic acid solution. 2. Use the Henderson-Hasselbalch equation to calculate the pH at this point. (Click for answer.)

13. Titration Curve (HOAc with NaOH)-4 0.7 equivalents of NaOH neutralizes 0.7 eq of acid (HA) producing 0.7 eq of acetate (A - ) and leaving 0.3 eq of HA. pK a of HOAc is 4.76 pH = 4.76 + log [0.7] [0.3] 30% acid and 70% salt. pH=5.13

14. Buffer Solutions Buffer: a solution that resists change in pH when small amounts of strong acid or base are added. A buffer consists of: a weak acid and its conjugate base or a weak base and its conjugate acid

15. Buffer Solutions Maximum buffer effect occurs at the pK a for an acid. Effective buffer range is +/- 1 pH unit of the pK a for the acid or base. eg. H 2 PO 4 - /HPO 4 2-, pK a =7.20 buffer range 6.20-8.20 pH

18. Buffer Solutions Within cells the primary buffer is phosphate : H 2 PO 4 - /HPO 4 2- The primary buffer in blood is bicarbonate : HCO 3 - /H 2 CO 3. Proteins also provide buffer capacity. Some side chains can accept or donate protons.

19. Buffer Solutions Buffers work by chemically neutralizing added acid or base. Eg.: HCO 3 - + H 3 O + H 2 CO 3 + H 2 O H 2 CO 3 + OH - HCO 3 - + H 2 O As long as amount of added acid or base is small, log [A-]/[HA] will not change significantly and pH will not change.

20. Buffer Solutions Calculate the ratio of lactic acid to lactate in a buffer at pH 5.00. The pK a for lactic acid is 3.86 5.00 = 3.86 + log [lactate] [lactic acid] 5.00-3.86 = log [lactate] [lactic acid] antilog 1.14 = [lactate] [lactic acid] = 13.8