pH curves
During an acid-base titration, the pH changes in a characteristic way. A pH curve is
found if the pH of the solution being titrated is plotted against the volume of solution
added.
PH Curves for titration of 50.0mL of sample of 0.10M solution of various acids with
0.10M NaOH
Similarly for titration bases with strong acids will produce the following:
Weak Acid-Base Titration
1. pH changes sharply near the equivalence
point.
2. The pKa of the weak acid = pH at the half the
volume required to neutralize the acid.
Acid-Base Indicators (In)
A visual change is required to determine when the equivalence point. Indicators serve this purpose
and only a few drops are required and so does not affect the pH of the solution. Most indicators
are made of organic dyes that are usually weak acids. The colour of the indicator depends on the
Ka value and pH.
Example: Litmus
HIn + H2O In-1 + H3O+ KIn =
Red Blue
According to Le Chatelier's principle: adding acid or H+ to the reaction above will shift the
equilibrium to the ____________ and the colour of the solution is _______________.
According to Le Chatelier's principle: adding base or OH-1 to the reaction above will shift the
equilibrium to the ___________ and the colour of the solution is ________________.
How to select the right indicator for a titration depends on knowing what the pH of the solution is
at equivalence. This is critical.
At the equivalence point pH is where half of all the indicator molecules have changed colour.
Therefore, [HIn]=[In-], this simplifies the equilibrium expression such that the pKIn = pH.
Another example is of phenolphthalein with a Ka = 1.0 x 10-8
HIn + H2O In-1 + H3O+
colourless pink
Determine the colour of indicator that would predominate when a few drops of phenolphthalein are
added to an acidic solution with a pH=1.0
By definition, usually the colour change of indicator occurs when 1/10 the initial form must be
converted to show the new colour.
Buffer Capacity and pH
Henderson-Hasselbalch equation
For weak acid and conjugate salt buffers: pH = pKa + log ([conjugate base]/[weak acid])
For weak base and conjugate salt buffers: pOH = pKb + log ([conjugate acid]/[weak base])
Review Questions
1. What is the pH of a buffer that is 0.12M lactic acid (HC3H5O3) and 0.10M sodium lactate.
Ka=1.4 x 10-4. (Ans: 3.77)
2. How many moles and mass of NH4Cl must be added to 2.0L of 0.10M NH3 to form a buffer
whose pH is 9.00. (assume the addition of ammonium chloride does not change the
volume). Kb = 1.8 x 10-5 (Ans: n =0.36mol)