How to construct plots of log concentration versus pH

Plots of minus logarithm of concentration versus pH develop straight lines with a very small curvature where they intersect. The curvature can be drawn by hand with little error or can simply be neglected for many applications. Use of such plots is illustrated by examples.

Case 1: A monobasic weak acid at a concentration = 0.002 M and pK = 6.2

Case 2: A dibasic acid at 0.005 M, pK1 = 4.5, pK2 = 8.9

Step 1 is to calculate the logarithm (base 10) of the concentration. For Case 1 this is log(.002) = -2.699

For Case 2, log(.005) = -2.301

Now the graph is minus log of C vs pH. Remember that hydrogen ions and hydroxyl ions are present. Since the definition of pH is - log of H+, the plot of log C vs pH for hydrogen ion is a straight line from 0,0 to 14,14. The plot of OH¯ is the mirror image or 14,0 to 0,14. This is shown on the next figure where orange is H+ and blue is for OH¯ . The figure is too short, and the lines do not get to the opposite sides.

  • Case 1 plot of logarithm of concentration versus pH
  • Java applet for constructing diagrams
  • Explanation of why equilibria are straight lines