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

Diprotic species involve the movement of two acidic hydrogens. 

Diprotic Acids and Bases

Concept #1: Diprotic acids can donate two acidic hydrogens and as a result possess two Ka values. 

Concept #2: Diprotic bases can accept two acidic hydrogens and as a result possess two Kb values. 

Concept #3: With the presence of different forms it becomes essential to know which Ka or Kb value to use. 

Diprotic Acids and Bases Calculations

Example #1: Sulfurous acid, H2SO3, represents a diprotic acid with a Ka1 = 1.6 x 10-2 and Ka2 = 4.6 x 10-5. Calculate the pH and concentrations of H2SO3, HSO3 and SO32– when given 0.200 M H2SO3

Example #2: Determine the pH of 0.080 M Na2S. Hydrosulfuric acid, H2S, contains Ka1 = 1.0 x 10-7 and Ka2 = 9.1 x 10-8.

Example #3: If Ka1 = 4.46 x 10-7 and Ka2 = 4.69 x 10-11 for H2CO3 what is the pH for a 0.15 M solution of NaHCO3

Practice: An unknown diprotic acid has an initial concentration of 0.025 M. What is the pH of the solution if pka1 is  3.25 and pKa2 is 6.82?