1 Chapter 2 – Water and pH - Water one of the most important molecules in life. 70% of the bodies mass is water • 2/3 of total body water is intracellular (55 - 66% body weight of men and 10% less for women) • the blood plasma. • The rest is interstitial fluid of which 25% is in - The body tightly controls both the volume and pH of water. pH • The bicarbonate system is crucial for blood maintenance changes of pH greater than 0.1 are dangerous and can lead to coma - diabetics • Properties of water • Polarity • Hydrogen bonding potential Specific heat, heat of vaporization • It is the unique combination of properties of water • Nucleophilic that make it the perfect solvent for biological • Ionization systems. We will discuss each of these properties in Water is an ideal • more detail. biological solvent Water is close to a tetrahedral shape with the unshared 3 hybridized orbital are in two corners and the electrons on the two sp - hydrogen in others o (109 Compared to a tetrahedron, CH ) or NH the bond angle is smaller 4 3 o o o ) vs.104.5 (109.5 and 107 Water has hydrogen bonding potential covalent, weak • H - bonds are non - interactions O is both a Hydrogen donor and acceptor H • 2 One H - O can form up to four H • bonds 2 What Are the Properties of Water? comparison of ice and water, in terms of H bonds and Motion A - Ice: 4 H bonds per water molecule • Water: 2.3 H bonds per water molecule • • Ice: H - bond lifetime - about 10 microsec about 10 psec • Water: H - bond lifetime - (10 psec = 0.00000000001 sec) • ent from Its Polar Nature The Solv Properties of Water Derive Water has a high dielectric constant • Ions are always hydrated in water and carry around a "hydration shell" • Water forms H bonds with polar solutes • The Solvent Properties of Water Derive from Its Polar Nature solvent ability dielectric constant (D) is What makes this molecule important? - easily disrupts ionic compounds - high (measure of the ability to keep ions apart) – Large electronegativity creates a strong ionic type bond (dipole). has a higher density than solid water – Liquid water (ice). Is this normal? Think of why this is important? solvating shells – - orderliness ability to take place in many hydrogen bonds (up to – 4 at a time)
2 What makes this molecule important? ... Strong nucleophile Specific Heat ... ... High specific heat High heat of vaporization ... Water is nucleophilic Water participates in many chemical reactions – it is electron rich – it is a weak nucleophile – it is present in high concentration pH - pouvoir hydrogene Acid & Base and pH (the power of hydrogen) Water undergoes ionization Water ionizes to form the hydronium (hydroxyl) ion and hydroxide ions Water can act as both an acid and base The equilibrium constant for the ionization of water is: The concentration of pure water § 1 liter = 1000g MW of water is 10.015 9 - the final concentration of water is 55M and H+ concentration is about 1.8 x 10 § § Very little water actually dissociates § – not easily measured or easy to use So Keq is very small Instead a different constant is used where the denominator is ignored - 14 K = 1.0 X 10 w pH is a measure of the proton concentration of a solution - + ] = [OH ] the solution is when [H + - ] > [OH when [H ] the solution is - + Note the relationship! A change in 1 pH units is a ten fold ] < [OH when [H ] the solution is change in hydrogen ion concentration
3 The extent of ionization of a weak acid is a function of its acid dissociation constant pKa and bases Bronsted and Lowry acid – acid donates protons – bases accepts protons Consider a weak acid, HA The acid dissociation constant is Strong acids dissociate nearly fully given by: - + Weak acids only partially dissociate HA + A H ⇄ Acids with Ka < 1 are considered weak acids - 5 > difficult to work with so instead use log scale: Ka for acetic acid is 1.76 x 10 - pK = - log K a a - 5 log 1.76 x 10 So the pKa of acetic acid is = - = 4.75 The pH is a measure of acidity and the pKa is a measure of acid strength Are Buffers, and What Do They Do? What 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 K lat near the p Note how the plot of pH versus base added is f a Buffers can only be used reliably within a pH unit of their p K a The Henderson - Hasselbalch Equation Know this! You'll use it constantly. the solution pH is For any acid HA, the relationship between the pK , the concentrations existing at equilibrium and a - + given by: HA ⇄ + A H - Hasselbalch equation The relationship between pH and pKa is described by the Henderson
4 What is the H H Equation Used For? - This is used to determine the concentration of acid and base at a given pH. It is Also used to determine the pH of a known solution. These concepts are used to calculate buffer strength and understand the pH of a biological solution. rs are mixtures of weak acids and their conjugate bases that resist pH by shifting the Remember that buffe equilibrium between the acid and base in response to the pH of a solution. Case 1) when the concentration of base equals the acid. Case 2) when the pH is above or below 1 pH unit of the pKa The pKa of acetic acid is 4.75. Calculate the pH of a mixture of 250 mM acetic acid and 100 mM Na acetate. Start with the HH equation What is the ratio of lactic acid to lactate in a buffer at pH of 5.00. • The pKa of lactic acid is 3.86? What is the concentration of base and acid you need to add to make a 50 mM solution of lactate buffer at pH 4.0? The MW of Lactic acid is 91 amu and sodium lactate is 102 amu. This is for you to take home try and we will calculate an answer with Dr Provost - !
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