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ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 9

ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 9. DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university. CHAPTER 9 BUFFERS. HYDROLYSIS OF SALTS. - Reaction of salt with water to produce hydronium ion or

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ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 9

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  1. ANALYTICAL CHEMISTRY CHEM 3811CHAPTER 9 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university

  2. CHAPTER 9 BUFFERS

  3. HYDROLYSIS OF SALTS - Reaction of salt with water to produce hydronium ion or hydroxide ion or both (do not go to 100% completion) - Not all salts hydrolyze - The salt of a strong acid and a strong base does not hydrolyze - Neutral solution is the result - The salt of a strong acid and a weak base hydrolyzes - Acidic solution is the result

  4. HYDROLYSIS OF SALTS - The salt of a weak acid and a strong base hydrolyzes - Basic solution is the result - The salt of a weak acid and a weak base hydrolyzes Slightly acidic, neutral, or basic, depending on relative weaknesses of acid and base

  5. HYDROLYSIS OF SALTS Acidic Hydrolysis positive ion of salt Conjugate base + H2O + H3O+ - The hydronium ion makes the solution acidic NH4+ + H2O → NH3 + H3O+

  6. HYDROLYSIS OF SALTS Basic Hydrolysis negative ion of salt Conjugate acid + H2O + OH- - The hydroxide ion makes the solution basic F- + H2O → HF + OH-

  7. BUFFER SOLUTION - A mixture of a conjugate acid-base pair - Tends to resist changes in pH upon addition of an acid or a base - The resistive action is the result of equilibrium between the weak acid (HA) and its conjugate base (A-) HA(aq) + H2O(l) → H3O+(aq) + A-(aq) - Commonly used in biological systems - Enzyme-catalyzed reactions depend on pH

  8. BUFFER SOLUTION Examples HC2H3O2/C2H3O2- HF/F- NH3/NH4+ H2CO3/HCO3-

  9. BUFFER SOLUTION - When an acid is added, the conjugate base converts the excess H3O+ ion into its acid (conjugate base removes excess H3O+) H3O+(aq) + A-(aq) → HA(aq) + H2O(l) - When a base is added, the acid converts the excess OH- ion into its conjugate base and water (acid removes excess OH- ion) HA(aq) + OH-(aq) → A-(aq) + H2O(l) - These reactions go to completion (large equilibrium constants)

  10. BUFFER SOLUTION - In actual fact, the pH changes but very slightly - Large amounts of added H3O+ or OH- may overcome the buffer action and change pH of solutions - Buffers are most effective when the ratio of acid to conjugate base is 1:1 - Buffers are less efficient in handling acids if the acid is more than the conjugate base - Buffers are less efficient in handling bases if the acid is less than the conjugate base

  11. HENDERSON-HASSELBALCH EQUATION HA(aq) + H2O(l) ↔ H3O+(aq) + A-(aq) • If [HA] = [A-], then Ka = [H3O+] • Taking the negative logarithm on both sides gives • - logKa = - log [H3O+] • pKa = pH

  12. HENDERSON-HASSELBALCH EQUATION - If [HA] ≠ [A-] or - pH changes by 1 if the ratio changes by a factor of 10

  13. HENDERSON-HASSELBALCH EQUATION In general or - Ca and Cb are the analytical concentrations of the acid and the conjugate base, respectively - na and nb are the number of moles of the acid and the conjugate base, respectively Ca = na/V and Cb = nb/V (concentration ratio equals mole ratio)

  14. HENDERSON-HASSELBALCH EQUATION Addition of a Strong Acid - Some buffer base is converted to the conjugate acid Addition of a Strong Base - Some buffer acid is converted to the conjugate base

  15. PREPARING BUFFERS - Measure the amount of weak acid (HA) to be used [or weak base (B)] - Calculate the amount of strong base (OH-) to be added [or strong acid (H+)] - This makes a mixture of HA and A- which is a buffer [or B and BH+] Or - Add the correct proportions of the acid and its conjugate base, then check the pH

  16. PREPARING BUFFERS Prepare 1.00 L 0f 0.100 M tris buffer solution at pH 8.40 - Convert mole to gram tris hydrochloride (MM = 157.60 g/mol) - Weigh out sample and dissolve in a beaker with ~ 800 mL H2O - Add NaOH solution until pH is exactly 8.40 (continuous stirring) - Quantitatively transfer solution to a 1000 mL volumetric flask - Dilute to the mark and mix

  17. BUFFER CAPACITY - A measure of how well a solution resists changes in pH - Increases with increasing concentration of buffer - Maximum when pH = pKa - The greater the buffer capacity the less the pH changes upon addition of H+ or OH- - Choose a buffer whose pKa is closest to the desired pH - pH should be within pKa± 1

  18. INDICATORS - Acid-base indicators are highly colored weak acids or bases - The various protonated forms have different colors - Have very low concentrations in order not to interfere with analytes

  19. INDICATORS HIn + H2O ↔ H3O+ + In- - Indicators exist predominantly in the protonated form (HIn) in acidic solutions - Indicators exist predominantly in the deprotonated form (In-) in basic solutions - The result of color changes

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