Ch.9 - Polyprotic Acid-Base EquilibriaWorksheetSee all chapters
All Chapters
Ch.1 - Chemical Measurements
Ch.2 - Tools of the Trade
Ch.3 - Experimental Error
Ch.4 + 5 - Statistics, Quality Assurance and Calibration Methods
Ch.6 - Chemical Equilibrium
Ch.7 - Activity and the Systematic Treatment of Equilibrium
Ch.8 - Monoprotic Acid-Base Equilibria
Ch.9 - Polyprotic Acid-Base Equilibria
Ch.10 - Acid-Base Titrations
Ch.11 - EDTA Titrations
Ch.12 - Advanced Topics in Equilibrium
Ch.13 - Fundamentals of Electrochemistry
Ch.14 - Electrodes and Potentiometry
Ch.15 - Redox Titrations
Ch.16 - Electroanalytical Techniques
Ch.17 - Fundamentals of Spectrophotometry
BONUS: Chemical Kinetics
Diprotic Acids and Bases
Polyprotic Acids and Bases
Diprotic Buffers
Polyprotic Buffers
Principal Species
Isoelectric and Isoionic pH
Isoelectric and Isoionic Points 

Concept #1: Isoelectric or isoionic points represent the pH where a polyprotic acid doesn’t migrate to an electric field because it’s neutral.

Concept #2: Isoelectric and Isoionic points allow us to determine the pH of diprotic and polyprotic systems. 

Example #1: Calculate the isoelectric and isoionic pH of 0.025 M glutamine. pKa1 = 2.19, pKa2 = 9.00. 

Example #2: Draw the structures and charge of aspartic acid at pH = 9.82. 

Practice: Calculate the pI value for histidine. pKa1 (carboxyl group) = 1.60, pKa2 (ammonium group) = 9.28, pKa3 (R-group) = 5.97. 

Practice: Calculate the pI value for gluatamic acid. pKa1 (carboxyl group) = 2.16, pKa2 (ammonium group) = 9.96, pKa3 (R-group) = 4.30.