pH and Buffers Biochem 4511 Figures Essentials of Biochemistry 3rd Ed OSU Custom Edition Lehninger Principles of Biochemistry 5th Ed Nelson Cox Fundamentals of Biochemistry 2nd Ed Voet Voet Pratt Acids and Bases HA B HB A to the conjugate base A Br nsted acid HA can donate a proton Br nsted base B can accept a proton to form conjugate acid HB Water Serves as Both an Acid and Base H2O H H does not exist as a separate species in water H3O 2 H2O H3O Hydronium ion Hydroxide ion HA B HB A Hydronium ions H2O H2O OH H3O H does not exist as a separate species in water H3O Delocalization Proton Jumping Rapid H mobility Water as an Acid and Base Keq H2O H2O OH H3O HA H2O H3O A Equilibrium constant for acid base reactions is defined as Keq products reactants H3O A HA H2O Keq may be experimentally determined for H2O H2O H2O OH H3O Keq 1 8 x 10 16 25 C Water as an Acid and Base Kw H2O OH H With water we consider a special equilibrium constant Kw the ion product constant for water Kw is defined as the general equilibrium constant multiplied by H2O which is a constant of 55 M Kw Keq H2O H OH 10 14 Kw 1 0 x 10 14 25 C Water as an Acid and Base Kw Kw H OH 1 0 x 10 14 Kw may be used to calculate H and OH for a given water sample In neutral water H OH and therefore Kw H OH 10 14 H 1 0 x 10 7 M OH pH scale H 1 0 x 10 7 M OH These numbers are very small compared to bulk water 55 5 M However the variations are significant A new scale makes H easier to report and makes calculations simpler pH log H log 1 H For neutral water described above pH log 1 0 x 10 7 M 7 0 pH log H Log 001 3 Log 01 2 Log 1 1 Log 1 0 Log 10 1 Log 100 2 Log 1000 3 Log 10000 4 If you remember how the log scale works you should not need a calculator pH log H Less H3O more basic More OH more basic pH is higher More H3O more acidic Less OH more acidic pH is lower pH and pOH pH log H pOH log OH As pH and pOH are in equilibrium as defined by Kw Examples pH pOH 14 If H 0 1 M then pH log 1 x 10 1 M pH 1 If H 1 M then pH log 1 x 10 6 M pH 6 If OH 1 x 10 1 M then pOH 1 and pH 14 1 13 or H Kw OH 1x10 14 1x10 1 1x10 13 Temperature and pressure Affects pH Extra H2O H OH Reaction is endothermic absorbs heat By Le Chatelier s Principle if you increase the temperature the forward reaction is favored and H is increased 25 C pH of pure H2O 7 0 0 C pH 7 47 40 C pH 6 77 The pH of a solution changes in an incubator or the cold room Acids and Bases Equilibrium constant for acid base reactions HA H2O A H3O products H3O A Keq reactants HA H2O Acids and Bases Ka For acids and bases in an aqueous environment it is convenient to consider a simplified equilibrium constant termed the acid dissociation constant Ka Ka Keq H2O H3O A HA H2O H2O H3O A HA When water is the solvent H2O 55 5 M and this value essentially does not change Definition Strong Acid or Base Ka H3O A HA For a Strong Acid For a Weak Acid Ka 1 Almost completely dissociated in water Hydrochloric acid Sulfuric acid Nitric acid Ka 1 Only partially ionized in water Acetic acid Phosphoric acid Carbonic acid Can find Ka in tables if needed in 4511 we will provide Leveling Effect Ionization of Water HA H2O A H3O In aqueous solutions acids donate protons to water The strongest acid that can stably exist in water is H3O Strong acids completely dissociate B H2O HB OH In aqueous solutions bases take protons from water The strongest base in water is OH Strong bases completely dissociate Determining pH Strong Acids What is the pH of a 1 L solution of water after the addition of 10 L 1M HCl 1 Is the acid weak or strong HCl is a strong acid 2 A strong acid dissociates completely in water therefore the H is equal to the HCl in the final sample 3 pH log H See also sample calculation 2 1 Determining pH Strong Acids What is the pH of a 1 L solution of water after the addition of 10 L 1M HCl Determining pH Strong Acids What is the pH of a 1 L solution of water after the addition of 10 L 1M HCl Since HCl is a strong acid it dissociates fully moles of HCl moles of H 10 L 1 mol 1L L 1x106 L 00001 mol HCl 1x10 5 mol HCl 1x10 5 mol H 1L water 1x10 5 M H pH log H log 1x10 5 pH 5 Pure water is pH 7 0 Determining pH Strong Acids What is the pH of a 1 L solution of water after the addition of 10 L 1M HCl Since HCl is a strong acid it dissociates fully moles of HCl moles of H pH 5 0 10 L of 1M HCl shifted the pH of pure water by 2 pH units Concept Normal Molarity M mol L Concentration of a molecule Normality N mol H L Concentration of available protons 1 M HCl in water is a 1 N solution of HCl Very often used for strong acids and bases Molarity normality for polyprotic acids and bases You will see this if you spend time in a lab so keep it in mind Weak Acids Ka Calculations CH3COOH Acetic Acid weak acid H CH3COO Acetate Anion conjugate base HA H A Ka Ka What are the concentrations HA and A for an aqueous solution of 0 1 M acetic acid 1 Find the Ka Acetic acid Ka 1 76 x 10 5 M in Ka tables or provided 2 Solve for A and HA W hat are the concentrations HA and A for an aqueous solution of 0 1 M acetic acid 1 Find the Ka for Acetic acid Ka 1 76 x 10 5 M 2 Solve for A and HA Ka H A HA Ka HA H A Therefore H A H3O A x x x2 Ka 1 74 x 10 5 M HA c0 x 0 1M x This is unwieldy concentrations are very low Notation pKa Problem H and Ka cover a very wide range Example Acetic acid 1 74 x 10 5 M Solution New notation pKa logKa The Henderson Hasselbalch Equation Ka HA H A and Ka H A HA H Ka HA A log H logKa log HA A pH pKa log A HA The Henderson …