1. Chapter 15: Acid / Base Chemistry
15.3 pH Changes in Acid-Base Reaction Systems

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

3. Titration Terminology (Gr.11)
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.

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. Interpreting Endpoints
The Bronsted-Lowry 5-step method can be used to write proton-transfer reactions that explain the endpoints on a pH curve
In general, we only observe distinct endpoints on a pH curve when a proton has been quantitatively transferred from an acid to a base; incomplete reactions are not observed
The number of endpoints that are observed represent the number of proton transfer reactions between B-L acids and B-L bases in a titration

10. Using the B-L 5-Step Method to Explain Endpoints
Titration of NaOH with HCl 14 pH
X we can write a proton transfer reaction to explain this endpoint
volume of titrant added (mL)

11. Writing B-L Equations to Explain Endpoints
To explain the endpoint observed for the titration of NaOH with HCl, we use the B-L 5-step method:
SA A
Na + OH H3O Cl - H2O
SB B B
H +
Endpoint: H3O + + OH H2O

12. Using the B-L 5-Step Method to Explain Endpoints
Titration of NaOH with HCl 14 pH
X H3O + + OH H2O
volume of titrant added (mL)

13. 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

14. Using the B-L 5-Step Method to Explain Polyprotic Endpoints
Titration of Na2CO3 with HCl 14
X endpoint 1 pH
X endpoint 2
volume of titrant added (mL)

15. Writing B-L Equations to Explain Polyprotic Endpoints
To explain the first endpoint observed for the titration of Na2CO3 with HCl, we will use the B-L 5-step method:
SA A
Na + CO H3O Cl - H2O
SB B B
H +
Endpoint 1: H3O + + CO H2O + HCO3 -

16. Writing B-L Equations to Explain Polyprotic Endpoints
To explain the second endpoint, we must consider not just the original entities, but those entities that were consumed in the first reaction and entities that were created in the first reaction
SA A A
Na CO H3O Cl - H2O HCO3 -
B B SB
H +
Endpoint 2: H3O + + HCO H2O + H2CO3

17. Using the B-L 5-Step Method to Explain Polyprotic Endpoints
Titration of Na2CO3 with HCl 14
X H3O+ + CO H2O + HCO3 - pH
X H3O+ + HCO H2O + H2CO3
volume of titrant added (mL)

18. 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)

19. Other Polyprotic Systems
To explain the first endpoint observed for the titration of H3PO4 with NaOH, we use the B-L 5-step method:
SA A
H3PO Na OH - H2O
SB B
H +
Endpoint 1: H3PO4 + OH H2O + H2PO4 -

20. Other Polyprotic Systems
To explain the second endpoint, we must consider not just the original entities, but those entities that were consumed in the first reaction and entities that were created in the first reaction
A SA
H3PO Na OH - H2O H2PO4 -
SB B B
H +
Endpoint 2: H2PO OH H2O + HPO4 2-

21. Other Polyprotic Systems
The B-L method suggests there is the possibility of a third endpoint:
A SA
H3PO Na OH - H2O H2PO4 - HPO4 2-
SB B B B
However, because it was not observed in the pH curve, it must mean the reaction was not quantitative, therefore we must write it with a double arrow
H > 50%
Endpoint 3: HPO OH H2O + PO4 3-
(not observed)

22. 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)

23. Homework:
Answer Q p