Buffering Keeping the pH of a Solution Constant (Nearly)

Buffer  A combination of a weak acid and a strong base  HAc ↔ Ac - + H +  In the range of pH where the two forms are present H + added from acid will taken up to form HAc H + consumed by addition of base will be replenished from HAc  A combination of a weak base and a strong acid  H 2 CO 3 ↔ HCO H +  In the range of pH where the two forms are present H + added from acid will taken up to form H 2 CO 3 H + consumed by addition of base will be replenished from H 2 CO 3

Behavior of Vinegar pH of solution barely changes when base added in the range of

Behavior of Bicarbonate pH Changes little with addition of acid in the range of pH

There Must Be a Formula! When pH 3-11, this simplifies to Where β max is the maximum amount of acid or base that can be absorbed given some concentration, c, of buffer.

Don’t Sweat the Formula  Buffers work best ± 1 pH unit from their pKa  If you expect acids to form, starting pH should be pKa+1  If you expect base to form (acid consumed), starting pH should be pKa-1  A 10 mM buffer can “absorb” about 5 mM acid

Volatile Buffers  Sometimes we want to evaporate all the water to concentrate a product like DNA or protein  If the Buffer used is non-volatile like Tris, the salt is left behind  If the buffer components are volatile they will evaporate as well, leaving behind a “clean” product Acetic acid and formic acid are good acids Pyridine and ammonia are examples of good bases to use