Water pH and Buffers PDF

Water pH and buffers Learning outcomes: 1. Describe the properties of water. 2. Use structural formulas to represent several organic compounds that can serve as hydrogen bond donors or acceptors. 3. Explain the orientation, in an aqueous environment, of the polar and nonpolar regions of...

Water pH and buffers Learning outcomes: 1. Describe the properties of water. 2. Use structural formulas to represent several organic compounds that can serve as hydrogen bond donors or acceptors. 3. Explain the orientation, in an aqueous environment, of the polar and nonpolar regions of macromolecules. 4. Explain the relationship of pH to acidity and alkalinity. 5. Describe what buffers do, how they do it. Introduction Approximately 60% of our body weight is water. Importance of water: 1. Water is the solvent of life. 2. Dissolves and transports compounds in the blood. 3. Provides a medium for movement of molecules into and throughout cellular compartments. 4. Separates charged molecules. 5. Dissipates heat. 6. Participates in chemical reactions. Water: Water structure: A water molecule is an irregular tetrahedron with oxygen at its center. Water structure:  A molecule with electrical charge distributed asymmetrically about its structure is referred to as a dipole, As a result, there is an electrostatic attraction between the oxygen atom of one water molecule and the hydrogen of another called a hydrogen bond. Hydrogen bond:  Hydrogen bonds are relatively weak. Hydrogen bonding influences the physical properties of water and accounts for its relatively high boiling point. Some biological important hydrogen bonds: Water as a solvent: Water is a polar solvent, It readily dissolves most biomolecules, which are generally charged or polar compounds, water forms hydrogen bonds and electrostatic interactions with these molecules. Water as a solvent: (polar compounds) Water as a solvent: (charged compounds) Classification of compounds according to their solubility Hydrophilic compounds that dissolve easily in water. Hydrophobic compounds that undissolved in water Amphipathic compounds Amphipathic compounds contain regions that are polar (or charged) and regions that are nonpolar. Most biomolecules are amphipathic. Amphipathic compounds in water: When an amphipathic compound is mixed with water, the polar, hydrophilic region interacts with the solvent and tends to dissolve, but the nonpolar, hydrophobic region tends to avoid contact with the water. These stable structures of amphipathic compounds in water, called micelles. Ionization (dissociation) of water: Water molecules have a slight tendency to undergo reversible ionization to yield a hydrogen ion (a proton) and a hydroxide ion. The dissociation constant of water The dissociation constant for water, Kd, expresses the relationship between the hydrogen ion concentration [H+], the hydroxide ion concentration [OH-], and the concentration of water [H2O] at equilibrium. pH: The concentration of hydrogen ions in a solution is usually denoted by the term pH. pH is the negative log10 of the hydrogen ion concentration expressed in moles per liter (mol/L). pH = - log [H+] Therefore, the pH of pure water is 7 pH scale: A pH of 7 is termed neutral because [H+] and [OH-] are equal. Acidic solutions have a greater hydrogen ion concentration and a lower hydroxide ion concentration than pure water (pH 7.0). pH scale Example 1: What is the pH of a solution whose hydrogen ion concentration is 3.2 × 10−4 mol/L? pH = - log[H+ ] -log (3.2X 10-4 ) =3.5 Example 2: What is the pH of a solution whose hydroxide ion concentration is 4.0 × 10−4 mol/L? pH+ pOH=14 [OH- ] = 4.0X 10-4 pOH = -log[OH ] -log (4.0X 10-4) = 3.4 pH = 14- pOH 14 - 3.4 = 10.6 How to calculate the pH of weak acid and weak base? The tendency of the week acid or the week base to dissociate and donate a hydrogen ion or hydroxide ion to solution is week. Its denoted by its Ka, the equilibrium constant for dissociation of a weak acid. The higher the Ka, the greater is the tendency to dissociate a proton. The pH of a solution of a weak acid (or base) and its salt is given by the Henderson-Hasselbalch equation. Buffer Almost every biological process is pH-dependent; a small change in pH produces a large change in the rate of the process. Cells and organisms maintain a specific and constant cytosolic pH, usually near pH 7; change of pH is controlled by biological buffers. Buffer: Are aqueous systems consist of a weak acid and its conjugate base or week base and its conjugate acid tend to resist changes in pH when small amounts of acid (H+) or base (OH-) are added. The major buffer systems in the body: 1.The bicarbonate–carbonic acid buffer system, which operates principally in extracellular fluid (ECF). 2.The hemoglobin buffer system in red blood cells. 3.The phosphate buffer system in all types of cells. 4.The protein buffer system of cells and plasma Reference: 1. Harpers Biochemistry. 2. Lehninger principle of Biochemistry. 3. Lippincotts Thank you