Case 2 Log Concentration Diagrams

Case 2 plot of log concentration versus pH

A weak acid with two replaceable hydrogen ions gives a graph of log concentration versus pH with more species. When a line with a slope of 1 or -1 comes directly under another pK locator point, its downward slope changes to 2 or - 2 because another equilibrium comes into play.

At low pH, it is very difficult for hydrogen ions to leave a weak acid. This makes it easy to remember that the construction line at low pH gets labeled "Unionized acid" or "H₂B". As the pH increases and approaches the pK, it is easier for the weak acid to ionize, and its concentration begins to drop. The heavier orange lines that overlay the construction lines are labeled H₂B. The transition right at the pK is not exactly linear and can be curved by eye. This is shown in the next sketch: The green lines that overlay the construction lines are for the ionized species HB¯ .