Polyprotic Equations Slide

Polyprotic Acid-Base Equations

After plugging these terms into a proper charge balance equation we can produce an advanced solution to any acid-base problem.

For quick and dirty pH calculations for most points in the polyprotic titration curve, we can divide the curve into the following regions:

At 0 mL HCl added, we have a solution of pure Na₂CO₃. Try treating this as a monoprotic weak base, where K_b = K_w/K₂

Between 5 mL and 15 mL we are in one of the buffer regions. Here

At 20 mL HCl, we have the first end point, and a solution of pure NaHCO₃ in water, an amphiprotic substance.

From 25 to 35 mL we have the other buffer region. Here

At 40 mL HCl added, we have the equivalent of pure H₂CO₃. Try treating this as a monoprotic weak acid where K_a = K₁.

David L. Zellmer, Department of Chemistry, California State University, Fresno. March 17, 1997