Weak Acid Base ChemistryDr. Mullins1Water Ionizes• Water partially ionizes to H+ and OH-• Water is an amphiprotic substance– Amphiprotic substance can act either as an acid or a base• The ionizable species of weak acids/bases are in equilibriumBase Acid3Weak Acids and Bases• Weak acids only partially dissociate• HA → H++ A-[ H +] [ A -] Keq= -----------------[HA]• Kais the ionization constant for a weak acid, HA, in water-log Ka= pKa• The smaller the pKa, the more acidic (stronger) the weak acid• The larger the Ka, the more acidic (stronger) the weak acid– The proton has a stronger tendency to dissociate– In other words the dissociation is pushed to the right or to H+= Ka4StrongWeakAcidWeakWeakAcidStrongWeakBaseWeakWeakBase5Relationship between pH and pKa• Weak acids will ionize (dissociate) in water• HA → H++ A-• The resulting change in pH is determined by two factors– the pKa of the conjugate acid/base pair– the concentration of the conjugate acid/base pair[deprotonated]pH = pKa+ log -------------------[protonated]• This equation is known as the Henderson-Hasselbalchequation6Henderson – Hasselbalch Relationships– When pH = pKa[A-] = [HA]– when pH < pKa, “H+on” or substance protonated– when pH > pKa“H+off” or substance deprotonatedIf the pH -pKaThe ratio of [A-]/[HA] form-3 1/1000-2 1/100-1 1/1001/1+1 10/1+2 100/1+3 1000/1[deprotonated] pH = pKa + log -------------------[protonated]From this equation, we see that[A-]pH – pKa = log -----[HA]10 7 - 10= 10 -3= [A-] / [HA][A-]------[HA]10 pH - pKa= [A-] / [HA]10 7 - 4= 10 +3= [A-] / [HA]pH < pKa < pHSolution is acidicHigh [H+]so protonated formpredominates“the soln does not needthe protons”H+H+H+H+H+H+H+H+H+H+H+H+H+H+H+Solution is less acidicLow [H+]so deprotonated formpredominates“the soln is low on protons so it wants the protons more”8Titration CurvesXTitration curves give an experimental way to determine pKa• A titration is an experiment in which measured amounts of a strong base are added to solution of acid– The strong base is added in small amounts until the acid is neutralized• A titration curve is a plot of the pH vs. the amount of strong base added• The titration curve is analyzed to give the– Inflection point -the point in an acid-base titration at which enough base has been added to neutralize 50 % of the acid[Acid] = [Base]pH = pKa9Comparison of several titration curves10Mono, Di and Triprotic acids• Compounds are classified according to the number of protons that can dissociate– One dissociable proton: monoprotic acid • 2 possible ionic forms – Two dissociable protons: Diprotic acid• 3 possible ionic forms– Three dissociable protons: triprotic• 4 possible ionic forms• Structure (ionic form) varies as a function of pH!H3A is not the same as HA2-HA ↔ A-H2A ↔ HA- ↔ A2-H3A ↔ H2A-↔ HA2-↔ A3-11H3A ↔ H2A-↔ HA2-↔ A3-12Biological Systems are Buffered• Buffers are solutions that resist changes in pH as acid and base are added– Most buffers consist of a weak acid and its conjugate base• The titration curve is flat near the pKa of the buffer– i.e. structure is resistant to change in pH – Best area to buffer ( ±1 pH unit around pKa)• Buffer capacity is related to the concentrations of the weak acid and its conjugate base– the greater the concentration of the weak acid and its conjugate base, the greater the buffer capacity13Biological Buffers• H2PO4-/HPO42-is the principal buffer in cells• H2CO3/HCO3-is an important (but not the only) buffer in bloodCO2(g) CO2(aq)CO2(aq)+H2O( l) H2CO3(aq)H2CO3(aq) H+(aq)+HCO3-(aq)CO2(g)+H2O( l) H+(aq) + HCO3-(aq)14Changes in Blood Chemistry• Breathing induced – hyperventilation can result in increased blood pH• Remove CO2 so H+ combines with HCO3-to produce more CO2-[H+] ↓ or pH↑– hypoventilation can result in decreased blood pH• Increase CO2 so it combines with H2O to produce more [H+] or pH ↓• Metabolically induced– decreases in blood pH will result in increased breathing rate• pH ↓ or [H+] ↑ so H+ combines with HCO3-to produce more CO2– Increases in blood pH ( very rare) will results in decreased breathing rate• pH↑ or [H+] ↓ so to compensate CO2 combines with H2O CO2 + H2O ↔ H++