Chapter 12

1. Chapter 12 Krissy Kellock Analytical Chemistry 221

2. Acid / Base Indicators • Indicators display a color that is dependent on the pH of the solution in which they are dissolved so the solution can be identified as acidic or basic. • The acid / base indicator is usually a weak organic acid or weak organ base whose undissociated form differs in color from its conjugate form. • An acid indicator will change color when an acidic H+ attaches to it and show its base color when the H+ comes off: • HIn + H2O ↔ In- + H3O+

3. Acid and Base Indicators • Pure acid color will be seen when [HIn] / [In-] ≥ 10 / 1 • Pure base color will be seen when [HIn] / [In-] ≤ 1/ 10 • For full acid color the hydronium concentration need o change it to that color will be: • [H3O+] = 10Ka • And for base it would be: • [H3O+] = 0.1Ka

4. Problem 12-23 • 20ml HCl ( 0.2000M HCl) = 4.0 mmol HCl • [HCl] = 4 mmol HCl / 20ml + 25ml = 0.0889M • pH = -log 0.0889 = 1.05 • 1.05 • HCl = 4 mmol HCl – (25ml)(0.132M NaOH) / 45ml = -log = 1.81 • 1.81 • [NaOH] = 25ml(0.232M NaOH) – 4 mmol HCl / 45ml = -log –14 = 12.60

5. Titration Curve • curve produced as the acid progresses through preequivalence, equivalence and postequivalence points. • preequivalence point – concentration of the acid is determined from its starting concentration and the amount of base that has been added. • Equivalence point – point at which hydronium and hydroxide ion concentrations are equal (pH 7). • Postequivalence point – analytical concentration of the base is calculated and that is assumed to be equal or a multiple of the hydroxide concentration

6. pH 2 = preequiv pH 4 = equiv pH 10 = potequiv http://jeffline.tju.edu/CWIS/DEPT/biochemistry/pH_tutorial/chap9

7. Problem 12-19 • 9.00g HCl x 1.098g soln x 1mmol NaOH 100g ml 0.04g NaOH • 2.471M = [OH-] • -log 2.471 = -0.393= • pOH = 14 – (-0.393) = 14.393

8. Buffer Solutions • A buffer is a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid that resists changes in pH of a solution • A buffer solution consisting of a conjugate acid/base pair formed when a weak acid is titrated with a strong base or a weak base with a strong acid. • Buffers are used to maintain a pH of a solution at a constant and predetermined level

9. Buffer Properties • the pH of a buffer remains basically independent of dilution • resist pH change after addition of small amounts of strong acids or bases • the buffer capacity of a buffer is the number of moles of strong acid or strong base that causes one liter of the buffer to change pH by one unit

10. Calculating pH in Weak Acid or Weak Base Titrations • At the beginning the solution contains only a weak acid or a weak base and the pH can be calculated from the concentration of that solute and its dissociation constant • After various increments of titrant have been added, the solution consists of a series of buffer. The pH of each buffer can be calculated from the analytical concentrations of the conjugate base or acid and the residual concentrations of the weak acid or base.

11. Calculating pH in Weak Acid or Weak Base Titrations • At the equivalence point, the solution contains only the conjugate of the weak acid or base being titrated and the pH is calculated from the concentration of this product. • Beyond the equivalence point, the excess of the strong acid or base titrant represses the acidic or basic character of the reaction product to such an extent that the pH is governed largely by the concentration of the excess titrant