Case 1 plot of log concentration versus pH

Case 1: A monobasic weak acid at a concentration = 0.002 M and pK = 6.2 . Step 1 is to calculate the logaritm (base 10) of the concentration. For Case 1 this is log(.002) = -2.699. We draw a horizontal construction line at 2.699 shown in orange on the figure. We draw a vertical construction line at the pK of 6.2, also in orange. From the intersection are drawn two sloping lines, one of slope = -45° and the other of +45°.

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 "HA". 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 HA. The transition right at the pK is not exactly linear and can be curved by eye. This is shown in the next sketch: The blue lines that overlay the construction lines are for the ionized species A-.

Case 2 plot of logarithm of concentration versus pH