1. Equilibria of Weak Acids & Bases Part VI: Polyprotic Acids
Equilibria of Weak Acids & Bases Part VI: Polyprotic Acids & Titration Curves
Chap. 17 Sec 6
Dr. C. Yau
Fall 2013 1

2. Ionization of Polyprotic Acids
Carbonic acid is an example of a polyprotic acid (more specifically, a diprotic acid).
What is the formula of carbonic acid?
Write the ionization equations of carbonic acid.
H2CO3 + H2O H3O+ + HCO3
HCO3 + H2O H3O+ + CO32
Note that pH is based on H3O+ and there are 2 sources!
However, Kal > Ka2
so [H3O+] from 1st ioniz >> [H3O+] from 2nd ioniz
If Kal >> Ka2 then this becomes negligible.
Ka1 = 4.5x10-7
Ka2 = 4.7 x 10-11
Compare Ka1 & Ka2 2

4. Note HCO3 is produced in one step and consumed in the 2nd step.
H2CO3 + H2O H3O+ + HCO3
HCO3 + H2O H3O+ + CO32
Ka1=4.5×10-7; Ka2=4.7×10-11
Example 17.9 p. 807
Give the concentration of all species in M H2CO3 as well as the pH.
Do Pract Exer 40, 41 p. 808

5. Salts Of Polyprotic Acids…
are amphoteric, and most are basic in water.
To predict the behavior in water, compare Ka to its Kb.
Ex. Is an aqueous solution of NaHCO3 likely to be basic or acidic?
NaHCO3 is a salt of neutral Na+ and amphoteric HCO3-.
Ka=4.7×10-11; Kb=1.0x10-14/4.5×10-11=2.22 ×10-4.
Since Kb> Ka, the salt is basic in water.

6. Titration of Nitric Acid by NaOH
strong acid by strong base 1 3 5 7 9 11 10 20 30
Volume NaOH (mL) pH
Equivalence Point
What species are present at the mL range? Why is the pH increasing? What species are present at 25 mL? at 30 mL?

7. Titration of Acetic Acid by NaOH3 4 5 6 7 8 9 10 11 12 20 30
Volume NaOH (mL) pH
Equivalence Pt
pH = 4.74
Significance of this pH?
13 mL
26 mL
How is this graph different from the previous one?
Why is it starting at pH 3? Why is pH increasing faster at mL
Why is the equivalent point not at pH = 7?
What is the significance of pH at 13 mL?

8. Titration of weak vs. strong acid by strong base
Strong acid by strong base: equivalence pt is at pH = 7
Weak acid by strong base: equivalence pt is at pH > 7 (end with basic solution)
Weak acid produces strong conjugate base, which increases the pH.

9. How is this graph different from the others? Why? 2 4 6 8 10 12 5 15 20 25 30
Volume NaOH (mL) pH
(e)
(d)
(c)
(b)
(a)
How is this graph different from the others? Why?
If the acid is phosphoric acid, what species are present at (a), (b), (c), (d), (e)?

10. How is this graph different?
Why is the equivalence point at pH lower than 7?

11. Reactions Of Acids And Bases
Produce conjugate acid and bases
May not create neutral products:
strong acids + strong bases → neutral salts
weak acids + strong bases → basic salts
strong acids + weak bases → acid salts
weak acids + weak bases → amphoteric salts check Ka vs. Kb for dominance (we will not do titration calculations based on this type of reaction)

12. Color Indicators
Indicators change color within specific pH ranges because each indicator is a weak organic acid
H In + H2O H3O In-
As the concentrations of conjugate base vary, the color varies
We usually choose an indicator whose pKa=pH at the endpoint of the reaction
Select an indicator whose color change is light to dark at this pH to optimize visual detection
ChemFAQ: How do I select an indicator for a particular acid-base titration? 12

13. Acid-Base Indicators
pH 3.2 to 4.4 methyl orange
pH 4.8 to 6.4 methyl purple
pH 6.0 to 7.6 bromothymol blue
pH 8.2 to 10.0 phenolphthalein