pH and Buffers PDF - Mai 2024

pH and Buffers Year 2 – Unit I, Problem #3 buffers resist change Mai Sater, MD PhD Department of Medical Biochemistry Academic Year 2024- 2025  Concept Map...

pH and Buffers Year 2 – Unit I, Problem #3 buffers resist change Mai Sater, MD PhD Department of Medical Biochemistry Academic Year 2024- 2025  Concept Map Acids and Bases Acid Base Ionization of H2O Neutralization H+ Donor H+ Acceptor H3O+ OH- Salt Water Strong Weak Strong Weak Weak Acid Acid Acid Base Base Kw = [H3O+] [OH-] or Base -Log [H3O+] Buffer pH pH The Structure of Water Water is the medium of life; its d- properties are essential to macromolecular structure and function Strongly electronegative O d Loading… dominates shared electron pairs, which results in: H are electron-poor, have potential + for H-bonding H2O has a strong dipole; water is a very polar medium H2O has unique properties (surface tension, boiling point..etc) The 2 H are electrons shared spend covalently - more time with - neighboring all molecule oxygen bound to oxygen sharing : have hydrogen bond of electrons atom rather than hydrogen atom  Auto-ionization of Water Water is essentially a neutral molecule, but ionizes slightly H2O + H2O ⇄ H3O + OH¯ Water acts as acid (proton donor) and base (proton acceptor) H2O ⇄ H + OH¯ The equilibrium for the auto-ionization of water is given by ion product constant of water (Kw), which at 25ºC is: Kw = [H ] ✕ [OH¯] = 1.0 x 10-14 M2 where [H+] = hydrogen ion concentration, [OH¯] = hydroxide ion concentration. Because Kw is a constant, [H ] and [OH-] can vary independently: An increase in the concentration of one of the ions will cause an equilibrium shift that decreases the other ion. The [H ] ✕ [OH¯] = 1.0 x10-14 is always maintained. In pure water [H+] = [OH-] = 10-7 M, pH =7.0 Ionization of Water Loading…  pH A solution is neutral when [H ] and [OH¯] are equal in solution. [H ] and [OH¯] concentration in pure H2O at 25ºC = 1.0 ✕ 10-7M. pH scale: log scale that corresponds to [H3O+] of aqueous solutions, calculated as the negative log (base 10) of [H3O+] : pH = -log [H3O ] / log 1/[H3O ] pH (at 25ºC) of: – Neutral solution = -log (1.0 ✕ 10-7) = 7.00 – Acidic solution: < 7.00, – Basic solution: > 7.00. Most body fluids have pH values in the range 6.5 - 8.0, also referred to the physiological pH range (Note that the pH of our gastric juice is 1 – 3). pH and pOH formulas  pH in Living Systems Compartment pH Gastric acid 0.70 Lysosomes 4.50 Granules of chromaffin cells 5.50 Urine 6.00 Neutral H2O (at 37°C) 6.81 Cytosol 7.20 Cerebrospinal fluid (CSF) 7.30 Blood 7.34 – 7.45 Mitochondrial matrix 7.50 Pancreas secretions 8.10 [H+] and [OH-] in Different pH  Ionization of acids and bases [H ] [OH¯] Kw = ; (pH = -logH) [H2O] weak acids and bases lonkes partially - While Part of them will remain in lonized form strong acidslonizes completely - Weak acids and bases found in biological systems do not completely ionize at physiological pH There is a measurable equilibrium between the weak acid and its conjugate base or between the weak base and its conjugate acid HA + H2O H+ + A- The dissociation constant of an acid (Ka) is: [H ] [A¯] Ka = ; (pKa = -logKa) [HA] The smaller the pKa the stronger the acid Strong acid Weak acid HA HA Loading… H+ A- H+ A- bicarbonate = most important buffer in our body  Acids/Bases Strength Strong Acid Weak Acid  BUFFERS or weak base and it's acid & congugate A mixture of a weak acid or a weak base and its salt that resists changes in pH when small amounts of an acid or base are added. Maximum buffering occurs at pKa + 1 pH unit. Physiological buffering systems: – I. Bicarbonate/Carbonic Acid [HCO3-/H2CO3] – 2. Phosphate [HPO42-/H2PO4-] – 3 Protein [Pr-/HPr]  Ka = [H+] [A-] [HA] HA HA HA H+ A- H+ A- H+ A- Maximum buffering occurs at pKa + 1 pH unit.  Henderson-Hasselbalch Equation concentration of base by undissaciated conjugate acld divide pH = pKa + log [A¯] / [HA] [HA] = concentration of the undissociated weak acid; [A¯] = concentration of the conjugate base of [HA]. If [HA] = [A-], pKa = pH  Henderson-Hasselbalch Equation Carbonic acd The pKa of H2CO3 is 6.1 In normal blood, the [HCO3-]/[H2CO3] ratio is 20:1 (log 20 = 1.3) Therefore in normal blood: pH = 6.1 + log [20/1] pH = 6.1 + 1.3 pH = 7.40