Acid-Base Titration Curve - Neutralization Curve

What is a titration curve?

A titration curve is the plot of the pH of the analyte solution versus the volume of the titrant added as the titration progresses.

Let’s attempt to draw some titration curves now.

1) Titration of a strong acid with a strong base

Suppose our analyte is hydrochloric acid HCl (strong acid) and the titrant is sodium hydroxide NaOH (strong base). If we start plotting the pH of the analyte against the volume of NaOH that we are adding from the burette, we will get a titration curve as shown below.

Point 1: No NaOH added yet, so the pH of the analyte is low.

As NaOH is added dropwise,H3O+ slowly starts getting consumed by OH- produced by dissociation of NaOH. Analyte is still acidic due to predominance of H3O+ ions.

Point 2: This is the pH recorded at a time point just before complete neutralization takes place.

Point 3: This is the equivalence point (halfway up the steep curve). At this point, moles of NaOH added = moles of HCl in the analyte. At this point,H3O+ ions are completely neutralized by OH- ions.The solution only has salt (NaCl) and water and therefore the pH is neutral i.e. pH = 7.

Point 4: Addition of NaOH continues, pH starts becoming basic because HCl has been completely neutralized and now excess of OH- ions are present in the solution (from dissociation of NaOH).

2) Titration of a weak acid with a strong base

Let’s assume our analyte is acetic acid CH3COOH (weak acid) and the titrant is sodium hydroxide NaOH (strong base). If we start plotting the pH of the analyte against the volume of NaOH that we are adding from the burette, we will get a titration curve as shown below.

Point 1: No NaOH added yet, so the pH of the analyte is low (it predominantly contains H3O+ from dissociation of CH3COOH).

But acetic acid is a weak acid, so the starting pH is higher than what we noticed in case 1 where we had a strong acid (HCl).

As NaOH is added dropwise,H3O+ slowly starts getting consumed by OH- (produced by dissociation of NaOH). But analyte is still acidic due to predominance of H3O+ ions.

Point 2: This is the pH recorded at a time point just before complete neutralization takes place.

Point 3: This is the equivalence point (halfway up the steep curve). At this point, moles of NaOH added = moles of CH3COOH in the analyte. The H3O+ ions are completely neutralized by OH- ions.The solution contains only CH3COONA salt and H20. 

Let me pause here for a second - can you spot a difference here as compared to case 1 (strong acid versus strong base titration)??? In the case of a weak acid versus a strong base, the pH is not neutral at the equivalence point. The solution is basic (pH ~ 9) at the equivalence point. Let’s reason this out.

As you can see from the above equation, at the equivalence point the solution contains CH3COONA salt.This dissociates into acetate ions CH3COO- and sodium ions NA+.As you will recall from the discussion of strong/ weak acids in the beginning of this tutorial,CH3COO- is the conjugate base of the weak acid CH3COOH.So CH3COO- is is relatively a strong base (weak acid)

CH3COOH has a strong conjugate base), and will thus react with H2O  to produce hydroxide ions (OH-)  thus increasing the pH to ~ 9 at the equivalence point. 

Point 4: Beyond the equivalence point (when sodium hydroxide is in excess) the curve is identical to HCl-NaOH titration curve (1) as shown below.

3) Titration of a strong acid with a weak base

Suppose our analyte is hydrochloric acid HCl (strong acid) and the titrant is ammoni  that we are adding from the burette,  If we start plotting the pH of the analyte against the volume of NH3 we will get a titration curve as shown below a NH3 (weak base).