BIOL%2B121%2BLab%2BBuffer%2BZone.docx

BIOL%2B121%2BLab%2BBuffer%2BZone.docx - BIOL 121 Lab...

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BIOL 121 Lab : Buffers and pH Changes Objectives: Demonstrate how buffering agents protect against pH changes Understand that buffering capacity is limited Interpret the buffer zone of a weak acid by graphing titration data Equipment: graph paper or access to a computer and knowledge of Excel pH strips The Buffer Zone A buffer is a substance that works in solution to resist pH changes. Buffers are very useful and essential in humans, because they help us maintain appropriate pH levels in our body fluids and tissues. Without biological buffers, our body systems would quickly lose their battle against the deadly conditions of acidosis and alkalosis. Because the pH scale is logarithmic, every change of 1 pH unit reflects a 10-fold difference in [H + ]. Thus, even a small change in pH reflects a large change in [H + ]. Biological systems are sensitive to small changes in pH, because H + and OH - affect macromolecular structures and functions. The carbonic acid system is one of our biological buffers, and is one of the examples we will examine in these lab exercises. A buffer system is actually a combination of two molecules -- a weak acid and its conjugate base -- that can work together to protect against pH changes by providing a large pool of available molecules for both H + and OH - to bind with. Here’s how they work: When you add acid or base to pure water, the H + concentration changes, creating a pH change. H 2 O H + + OH - But when you add acids and bases to a buffer system, the pH stays the same. As long as you stay within the “buffering capacity” (ie, you don’t overrun your pool of available molecules) the pH resists changes. You should remember this chemical equation, previously introduced in Lab 3, representing the carbonic acid system: CO 2 + H 2 O H 2 CO 3 H + + HCO 3 - Carbonic acid (H 2 CO 3 ) is the weak acid, and HCO 3 - is the conjugate base. When you add acid (like HCl) to the system, the H+ contributed by HCL bonds to the conjugate base of the buffer like a sponge, reforming intact H 2 CO 3 . The H + will 1
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