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Equilibria ofWeak Acids & BasesPart VI: Polyprotic Acids

& Titration Curves

Dr. C. Yau

Spring 2014

Chap. 17 Sec 6

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Ionization of Polyprotic AcidsCarbonic 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.2

Ka1 = 4.5x10-7

Ka2 = 4.7 x 10-11

Compare Ka1 & Ka2

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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 0.040 M H2CO3 as well as the pH.

Do Pract Exer 40, 41 p. 808

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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.

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Titration of Nitric Acid by NaOHstrong acid by strong base

What species are present at the 10-20 mL range? Why is the pH increasing?What species are present at 25 mL? at 30 mL?

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0 10 20 30Volume NaOH (mL)

pH Equivalence Point

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Titration of Acetic Acid by NaOH

How is this graph different from the previous one?Why is it starting at pH 3? Why is pH increasing faster at 10-20mL

Why is the equivalent point not at pH = 7?What is the significance of pH at 13 mL?

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101112

0 10 20 30Volume NaOH (mL)

pHEquivalence Pt

26 mL13 mL

pH = 4.74

Significance of this pH?

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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.

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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)?

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0 5 10 15 20 25 30

Volume NaOH (mL)

pH

(a) (b)

(c)(d)

(e)

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

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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)

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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

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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