Lecture_2_pHpKaBuffers.pdf

pH and buffers Lecture 2 BCMB 401 Spring 24 Reading: Chapter 1.3 Acid-Base Reactions Brønsted-Lowry Definition: An acid is a proton donor. A base is a proton acceptor. Strong acids completely dissociate in water. Examples: HCl, HNO3, HClO4 Weak acids partially dissociate in water Henderson-Hasselbal...

pH and buffers Lecture 2 BCMB 401 Spring 24 Reading: Chapter 1.3 Acid-Base Reactions Brønsted-Lowry Definition: An acid is a proton donor. A base is a proton acceptor. Strong acids completely dissociate in water. Examples: HCl, HNO3, HClO4 Weak acids partially dissociate in water Henderson-Hasselbalch Equation Derived by taking the log of both sides of the acid dissociation equilibrium expression 𝒑𝑯 = 𝒑𝑲𝒂 + 𝒍𝒐𝒈 𝑩𝒂𝒔𝒆 𝑨𝒄𝒊𝒅 The ionization equilibrium of a weak acid is given by: The equilibrium constant expressed as the acid dissociation constant, Ka, is: 𝑩𝒂𝒔𝒆 [𝑯+] 𝑲𝒂 = 𝑨𝒄𝒊𝒅 [H+] is typically expressed as pH: For example: [H+] = 10-7 M → pH = 7 The pKa of an acid is defined as: Henderson-Hasselbalch Equation 𝒑𝑯 = 𝒑𝑲𝒂 + 𝒍𝒐𝒈 𝑩𝒂𝒔𝒆 𝑨𝒄𝒊𝒅 pKa = pH where [Base] = [Acid] pH of a solution can be calculated from the equation if the molar proportion of base (A-) to acid (HA) and the pKa of HA are known. Relative amounts of Base and Acid form of the molecule can be calculated from pH if pKa is known Which species predominates when pH is greater than pKa? … when pH is less than pKa? … when pH is equal to pKa? pKa = 4.75 Weak Acids as Buffers An acid-base conjugate pair resists changes in the pH of a solution. In other words, it acts as a buffer. A buffer is most effective at a pH near its pKa. Henderson-Hasselbalch equation describes how much conjugate acid-base pair is present at a certain pH (H+ as HCl) Each weak acid has characteristic pKa values. At midpoint of the titration, pH = pKa Maximum buffering action is present around the pKa value because that is where majority of protonation/deprotonation occurs Effective as buffers at pH ± 1 of pKa. (Over 80% of protonation/deprotonation) (H+ as HCl) Some acids are polyprotic (can donate multiple protons/ have multiple ionization states) Phosphoric acid is triprotic Given the pKa values shown, inorganic phosphate exists as a nearly equal mixture of H2PO4− and HPO42− in physiological systems Physiological pH is typically 7.4. Phosphoric acid/phosphate is an important buffer in biological systems pKa2 helps maintain cytoplasmic pH near 7.4 in most cells