1. Acids and Bases
Titration Curves

2. Strong Acid-Base Titration Plotting Points
50.0 ml of [0.200] HNO3 is titrated with 0.100M NaOH. Calculate the pH points
a. NaOH has NOT been added. Eq: HNO3 + NaOH  NaCl + NaNO3
Since HNO3 is a strong acid pH = 0.70
b ml of M NaOH has been added. (or n=CV and moles)
Acid moles in flask = C V = [0.200] x L = mol to START
H+ left over = mol Acid – mol OH- added = mol Acid Left
C Acid left = mol H+/ ml = 0.15 M Acid left so pH=0.82
c Now another 10 ml base added for total of 20.0 mL of M NaOH
H+left over = mol – mol of OH- added = mol Acid Left
C Acid left = mol H+/ ml total volume = M Acid left so pH=0.942
50.0 mL of M NaOH has been added (half way).
H+ = mol – mol of OH- added = mol Acid left
C Acid left = mol H+/ ml = M Acid left pH=1.13

3. Strong Acid (titrant) - Strong Base Titration (analyte)
After mL of M NaOH has been added, Excess Base
pH = 12.40
Center vertical region, Graph Midpoint = equivalence point, or look at products

4. Important points:
pH increases slowly far from the equivalence point
pH changes quickly near the equivalence point
The equivalence point of a strong acid—strong base titration = 7.00
The titration of a strong base with a strong acid is almost identical, just reversed

5. COLOUR CHANGES OF SOME COMMON INDICATORS
Acid-base indicators
Must change immediately in the required pH range
over the addition of ‘half’ a drop of reagent.
COLOUR CHANGES OF SOME COMMON INDICATORS pH 1 2 3 4 5 6 7 8 9 10 11 12 13 14
METHYL ORANGE
CHANGE
LITMUS
CHANGE
PHENOLPHTHALEIN
CHANGE

6. Titration Curves
 A titration curve is a graph of the pH changes that occur during an acid-base titration versus the volume of acid or base added.
You need to be able to recognize each and then choose a suitable indicator for that titration, by looking at the PRODUCTS or at the midpoint of the titration curves steepest section
Also at that very same point is the equivalence point, and the end of a titration where the stoichiometry exactly satisfied, or moles H+ = moles OH-. It is also called the transition point or end point referring to where the indicator changes color and [HInd] = [Ind-].

7. Choosing an Indicator
The pH of the equivalence point depends on type of salt in PRODUCT.
Rule of thumb:
Not exact pH’s but good approximations
If the salt in PRODUCT is neutral the equivalence point = 7
HCl K OH → K Cl HOH
If the salt in PRODUCT is basic the equivalence point = 9
CH3COOH + K OH -  H2O + K CH3COO -
If the salt in PRODUCT is acidic the equivalence point = 5
HCl NH3 → NH4+ + Cl-

8. Titration Curve: Strong Acid and Strong Base HCl + KOH → KCl + HOH
HCl KOH → KCl HOH
50 mL mL of
0.10 M M HCl
Indicator pH = 7 Bromothymol Blue, why?
LOOK AT THE PRODUCTS, neutral water and neutral salt, so pH=7, refer to your chart of indicators pH
Volume .10 M KOH added 14 7
0.10 M KOH
0.10 M HCl
Neutral Salt KCl 25 50

9. Titration Curve: Strong Acid (analyte)and Strong Base (titrant)
HCl KOH → KCl HOH
Indicator pH = 7 Bromothymol Blue
50 mL of 0.10 M KOH is added to 50 mL of 0.10 M HCl pH
Volume .10 M KOH added 14 7 25 50

10. NEXT Strong Base into a Weak Acid

11. Strong Base (titrant) into a Weak Acid (analyte)
What is the end point and why? CH3COOH + OH -  H2O + CH3COO - Indicator pH = 9 Phenolphthalein, why? Weak base CH3COO - CH3COOH + OH -  H2O + CH3COO - Why is the pH higher at the start? It is due to the weak acid. Why? CH3COOH + OH -  H2O + CH3COO M of this weak acid is pH = 2.88

12. Strong Base (titrant) into a Weak Acid(analyte)
Can you find the buffer area? CH3COOH + OH -  H2O + CH3COO -
The titration curve levels off due to buffering effects.
Buffering happens when [HA] become equal to [A-].
At exactly the halfway point to the equivalence point
pH levels off near pKa due to HA/A- buffering effect
pH = pKa + log([A-]/[HA]) = pKa + log(1) = pKa (when [A-] = [HA])

13. NEXT Weak Acid into a Strong Base

14. Titration Curve: Weak Acid into a Strong Base HCN + OH- → CN- + HOH
20 mL of 1.0 M HCN is added to 20 mL of 1.0 M KOH
What is the end point and indicator and why?
Indicator pH = 9 Phenolphthalein, why?
The weak base CN- pH
Volume 1.0 M HCN added 14 7
1.0 M KOH
Weak base CN- 10 20

15. Titration Curve: Weak Acid into a Strong Base HCN + KOH → CN - + HOH
Find the buffer region, why is it there?
Buffer Zone
excess HCN
and CN - pH
Volume 1.0 M HCN added 14 7 10 20

16. NEXT Weak BASE into a Strong Acid

17. Titration Curve: Weak Base into a Strong Acid HCl + NH3 → NH4+ + Cl-
LAST
Titration Curve: Weak Base into a Strong Acid
  HCl NH3 → NH4+ + Cl-
60 mL of 1.0 M NH3 is added to 60 mL of 1.0 M HCl
Indicator
pH = 5
Methyl Red, why?
Weak acid in products NH4+ pH
Volume 1.0 M NH3 added 14 7
1.0 M HCl 30 60

18. Titration Curve: Weak Base to a Strong Acid HCl + NH3 → NH4+ + Cl-
HCl NH3 → NH4+ + Cl-
Indicator pH = 5 Methyl Red pH
Volume 1.0 M NH3 added 14 7 30 60
1.0 M NH3
1.0 M HCl
Weak acid has little effect on Strong Base
Acid Salt

19. Titration Curve: Strong Acid and Weak Base HCl + NH3 → NH4+ + Cl-
HCl NH3 → NH Cl-
Indicator pH = 5 Methyl Red
60 mL of 1.0 M NH3 is added to 60 mL of 1.0 M HCl pH
Volume 1.0 M NH3 added 14 7
Buffer Zone
NH3 and NH4+ 30 60

20. Next Weak with Weak Non-Stoichiometric

21. weak acid (CH3COOH) v. weak base (NH3)
Curve levels off at pH 10 due to excess 0.1M NH3
(a weak alkali)
NO SHARP
CHANGE IN pH
Steady pH change
pH 4 due to 0.1M CH3COOH (weak monoprotic acid)
Types

22. weak acid (CH3COOH) v. weak base (NH3)
PHENOLPHTHALEIN
LITMUS
METHYL ORANGE
NOTHING SUITABLE
There is no suitable indicator- none change in the ‘vertical’ portion.
The end point can be detected by plotting a curve using a pH meter.

23. POLYPROTIC ACID AND BASE TITRATION CURVES

24. Titration of a Weak Base with a Strong Acid
Similar problem to the titration of a weak acid with a strong base
Example: Titrate 100 ml of 0.10 M NH3 (Kb = 1.8 x 10-5) with 0.1 M HCl.
Notice the weak acid get hit HARD and falls away quickly, but recovers due to buffer
At ½ Equivalence point [HA] = [A-] thus pH = pKa

25. pKa2 = 10.26 pKa1 = 6.36 Titrations of Polyprotic Acids and Bases
Multiple Inflection Points = Multiple Equivalence Points will be seen
CO H HCO3- ½ way buffer point [HA]\[A-] = 1, Kb1 = KW/Ka2 = 1.8 x 10-4 (pKb1=3.74)
HCO H H2CO3 ½ way buffer point [HA]\[A-] = 1, Kb2 = KW/Ka1 = 2.3 x 10-8 (pKb2 =7.64)
½ Eq. pt Eq. pt ½ Eq. pt Eq. pt 2
pKa2 = 10.26
(pKb1 = 3.74)
pKa1 = 6.36
pKb2 = 7.64

26. Other pH curves - polyprotic acids (H3PO4)
Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps...
Step 1 H3PO NaOH ——> NaH2PO H2O
Step 2 NaH2PO4 + NaOH ——> Na2HPO H2O
Step 3 Na2HPO4 + NaOH ——> Na3PO H2O

27. Other pH curves - polyprotic acids (H3PO4)
Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps...
Step 1 H3PO NaOH ——> NaH2PO H2O
Step 2 NaH2PO4 + NaOH ——> Na2HPO H2O
Step 3 Na2HPO4 + NaOH ——> Na3PO H2O
There are three sharp pH changes
Each successive addition of NaOH is the same as equal number of moles are involved.

28. Other pH curves - polyprotic acids (H3PO4)
Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps...
Step 1 H3PO NaOH ——> NaH2PO H2O
Step 2 NaH2PO4 + NaOH ——> Na2HPO H2O
Step 3 Na2HPO4 + NaOH ——> Na3PO H2O
pH of H3PO = 1.5

29. Other pH curves - polyprotic acids (H3PO4)
Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps...
Step 1 H3PO NaOH ——> NaH2PO H2O
Step 2 NaH2PO4 + NaOH ——> Na2HPO H2O
Step 3 Na2HPO4 + NaOH ——> Na3PO H2O
pH of NaH2PO4 = 4.4
pH of H3PO = 1.5

30. Other pH curves - polyprotic acids (H3PO4)
Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps...
Step 1 H3PO NaOH ——> NaH2PO H2O
Step 2 NaH2PO4 + NaOH ——> Na2HPO H2O
Step 3 Na2HPO4 + NaOH ——> Na3PO H2O
pH of Na2HPO4 = 9.6
pH of NaH2PO4 = 4.4
pH of H3PO = 1.5

31. Other pH curves - polyprotic acids (H3PO4)
Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps...
Step 1 H3PO NaOH ——> NaH2PO H2O
Step 2 NaH2PO4 + NaOH ——> Na2HPO H2O
Step 3 Na2HPO4 + NaOH ——> Na3PO H2O
pH of Na3PO = 12
pH of Na2HPO4 = 9.6
pH of NaH2PO4 = 4.4
pH of H3PO = 1.5