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Buffer Solutions

Written by tutor Erin M.

Buffers are aqueous solutions (i.e., dissolved in water) that are used to prevent changes in the pH of solutions. In general, buffers are made by either mixing a weak base with its conjugate acid or a weak acid with its conjugate base. The solution can then react with either an acid or a base. The weak acid (or base) and common ion serve to regulate against significant changes in the solution’s pH (a measure of the concentration of hydronium ions, H3O+, in solution). Because there are many situations in which a narrow range of pH is considered acceptable, buffers play an important role in many chemical applications. An example is blood in the human body, which maintains a typical pH of 7.3 to 7.5 through a natural buffer system. Large changes in pH would lead to major disruptions in body functions, so maintaining this range is essential.

Examples of how buffers work:

Let’s say you have a solution that contains a weak acid, such as acetic acid (CH3COOH), and a salt of the weak acid such as sodium acetate (NaCH3COO). The solution could react with a base or an acid. If a small amount of an acid or a base were to be added to the solution, the pH would remain fairly constant because of the weak acid (CH3COOH) and its common ion (CH3COO-) acting as a buffer against pH changes. If small amounts of acid (H3O+) are added to this buffer solution, the acetate ions (CH3COO-) will react with most of the added hydronium ions to form additional acetic acid (CH3COOH). The H3O+ concentration will remain about the same and therefore so will the pH.

If small amount of base (hydroxide, OH-) are added to this buffer solution, the hydronium ions in the solution will react with the added hydroxide ions to form water molecules (H2O). The acetic acid in the solutions (CH3COOH) will then ionize and replenish the hydronium that had reacted with the hydroxide. Because the hydronium concentration remains relatively constant, the pH value will as well.

If your solution contains a weak base, such as ammonia (NH3), and a salt of the weak base such as ammonium chloride (NH4Cl), the mechanism works in the same manner as the acid solution previously described. If a base (OH-) is added to the solution, the ammonium ions (NH4+) will donate a proton to the added OH- to form water. If a small amount of acid is added to the solution, hydroxide will react with the additional H3O+ to form water. Additional ammonia in the solution will then ionize to replace the OH- that had reacted with the added H3O+.

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