1. Changes in Acid-Base Reaction Systems
Acid / Base Titrations
Changes in Acid-Base Reaction Systems

2. Titration Terminology
Titration – the progressive transfer of a solution from a buret (called the titrant) into a measured volume of another solution (called the sample).
Equivalence point – the volume of titrant required to neutralize the sample (# mol acid = # mol base).
Endpoint – the pH at the equivalence point of a titration.
Indicator – a chemical which is added to the sample that changes colour at the equivalence point of a titration.
Buffering region – a horizontal region of the pH curve where pH is not changing significantly.

3. pH curves
A pH curve is a graph showing the continuous change in pH during a titration (an acid-base reaction)
Titration of NaOH and Na2CO3 with HCl 14 pH 7
vol. of titrant (HCl) added (mL)

4. Interpreting pH curves
Titration of NaOH with HCl 14
buffer region titrant
pH endpoint equivalence point X
equivalence point volume
buffer region
volume of titrant added (mL)

5. Which is the titrant? pH pH Take 2 titrations involving NaOH and HCl
vol. of titrant vol. of titrant
HCl is titrant NaOH is titrant
(pH high to low) (pH low to high)

6. Choosing an Indicator
pH curves are used to determine which indicator(s) are suitable for a particular titration
The goal is to get an indicator that changes colour in the same region as the endpoint of your titration
Titration of NaOH with HCl
unsuitable: alizarin yellow (too early)
pH good indicators: bromothymol blue,
litmus and phenol red
unsuitable: orange IV (too late)
vol. of titrant (HCl) added (mL)

7. Multiple Endpoints Titration of Na2CO3 with HCl 14 endpoint 1 8 X
pH endpoint 2
equivalence point volume 1
X equivalence point volume 2
volume of titrant added (mL)

8. Choosing Indicators for Multiple Endpoints
Titration of Na2CO3 with HCl 14
endpoint 1 - metacresol purple ( ) X
pH endpoint 2 – methyl orange ( ) X
volume of titrant added (mL)

9. Polyprotic Acids and Bases
Polyprotic acids are those which can donate more than one proton; in most cases, it is equal to the number of hydrogens in the chemical formula
E.g. H2SO4 : can donate 2 protons
E.g. H3PO4 : can donate 3 protons
Polyprotic bases are those which can accept more than one proton; in most cases it is equal to the magnitude of the charge on the anion
E.g. CO3 2- : can accept 2 protons
E.g. PO4 3- : can accept 3 protons

10. Explaining Polyprotic Endpoints
Titration of Na2CO3 with HCl 14
X endpoint 1 pH
X endpoint 2
volume of titrant added (mL)

11. Explaining Polyprotic Endpoints
Titration of Na2CO3 with HCl 14
X H3O+ + CO H2O + HCO3 - pH
X H3O+ + HCO H2O + H2CO3
volume of titrant added (mL)

12. Other Polyprotic Systems
Consider the titration of phosphoric acid, H3PO4, with sodium hydroxide, NaOH: 11
X endpoint 2 pH
X endpoint 1
Vol. of NaOH (mL)

13. Other Polyprotic Systems
Consider the titration of phosphoric acid, H3PO4, with sodium hydroxide, NaOH:
HPO42- + OH H2O + PO43- (not observed)
H2PO4- + OH- H2O + HPO X pH
X H3PO4 + OH H2O + H2PO4 -
Vol. of NaOH (mL)