lec 3 - BL/CH 401 Lecture #3 - Acids, Bases, and Buffers 1....

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BL/CH 401 Lecture #3 - Acids, Bases, and Buffers 1. Principle of Ionization of Weak Acids: The Fundamental Concept of Buffers is: A Buffer Resists Change pH buffers resist change in pH when either acid (H+) or base (OH-) is added to it. Chemicals which are pH buffers are weak acids or bases Acids = Proton (H+) donors Bases = Proton Acceptors {*Figure 1*} Acids and their conjugate bases are in equilibrium. Since equilibria are related to the properties of the reactants and products, so for weak acids, the tendency to give up its proton determines its buffering property. This tendency to ionize can be put in terms of an equation for the equilibrium: {*Figure 2*} Where [ ] = Molar concentration; K = Ionization constant Simplest example is water (H 2 O): {*Figure 3*} But since [H 2 O] (water concentration) = Constant (55.5 M), Kw = [H+][OH-] = 10 -14 M In pure water, [H+] = [OH-] = 10 -7 M
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To make this easier to use, the pH scale was invented. pH = -log [H+]; thus when [H+] = 10 -7 M, pH = 7 This is called Neutral pH because it is in the middle of the pH scale. At pH greater than neutral, the solution is alkaline; while at pH less than neutral, the solution is acid. {*Figure 4*} 2. Titration of a Weak Acid illustrating its Ionization and Buffering Property: {*Figure 5*} All weak acids have titration curves like this one. Bases (like ammonium, NH4+) are also weak acids and have similar titration curves. The position where the Buffering zone is on the pH scale is related to the chemical nature of the weak acid : Acetic acid ionizes in the Acidic portion of the pH scale {*Figure 6*}
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Titration curves can also be put in the form of an equation: {*Figure 7*} The pK of a weak acid is the pH where [A-] = [HA] At pH below the pK, [HA] > [A-] At pH above the pK, [HA] < [A-] Therefore the pK determines the buffering zone for a weak acid. For example, Acetic Acid has a pK = 4.8 and a buffering zone from pH 3.8 to 5.8. So a weak acid will be an effective buffer at pH = pK +/- 1 pH unit. To buffer at a specific pH, use a weak acid with a pK near the pH you want to maintain. 3. Ionization of Simple Amino Acids: Amino acids are more complicated than simple weak acids since amino acids have at least 2 ionizing groups . Glycine (abbrevation is Gly), for example, has both a carboxylic acid and an amino group that can ionize: {*Figure 8*}
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If we dissolve the free base of Gly in pure water (ie neutral pH), it will ionize by protonating itself. {*Figure 9*} The equilibrium is far to the right so most of the Gly is in the charged form called the Zwitterion and Gly is still neutral because the + charge is neutralized by the - charge. Gly is always in the Zwitterion form at neutral pH. Now if we put Gly at an acid pH where it is fully protonated (ie. it has all the protons bound to it
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lec 3 - BL/CH 401 Lecture #3 - Acids, Bases, and Buffers 1....

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