1. ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 11 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university

2. CHAPTER 11 POLYPROTIC ACIDS AND BASES

3. POLYPROTIC ACIDS - Have more than one acidic proton Examples phosphoric acid, carbonic acid, amino acids

4. POLYPROTIC ACIDS Tooth Decay - Bacteria on teeth metabolize sugar into lactic acid CH 3 CH(OH)CO 2 H - Tooth enamel contains hydroxyapatite (calcium hydroxyphosphate) Ca 10 (PO 4 ) 6 (OH) 2 - Tooth decay is the result of reaction between lactic acid and hydroxyapatite to produce phosphoric acid Ca 10 (PO 4 ) 6 (OH) 2 + 14H + ↔ 10Ca 2+ + 6H 2 PO 4 - + 2H 2 O

5. POLYPROTIC ACIDS Erosion of limestone or marble (calcium carbonate) - Calcite (calcium carbonate) is soluble in acidic solutions (insoluble in neutral or basic solutions) - Calcite dissolves in acid rain causing erosion on buildings CaCO 3 (s) ↔ Ca 2+ + CO 3 2- CO 3 2- + H + ↔ HCO 3 - Acid Rain - SO 2, NO x, and CO 2 combine with water vapor and sunlight to produce sulfuric acid, nitric acid, and carbonic acid

6. POLYPROTIC ACIDS Amino Acids - Building blocks of proteins - Have acidic carboxylic acid group and basic amino group - The acidic proton resides on the N of the amino group - Have positive site (amino group) and negative site (acid group) - Called zwitterion - Both groups are protonated at low pH and depotonated at high pH

7. DIPROTIC SYSTEMS - Contain two acidic protons H 2 A ↔ HA - + H + (K a1 ) HA - ↔ A 2- + H + (K a2 ) - Acid dissociation constants: K a1 > K a2 A 2- + H 2 O ↔ HA - + OH - (K b1 ) HA - + H 2 O ↔ H 2 A + OH - (K b2 ) - Base association constants: K b1 > K b2

8. DIPROTIC SYSTEMS H 2 A ↔ HA - + H + (K a1 ) + HA - + H 2 O ↔ H 2 A + OH - (K b2 ) = H 2 O ↔ H + + OH - K a1 x K b2 = K w K a2 x K b1 = K w

9. DIPROTIC SYSTEMS K a1 >>>> K a2 - A solution of a diprotic acid behaves like a solution of a monoprotic acid with K a = K a1 K b1 >>>> K b2 - The fully basic form of a diprotic acid can be considered as monobasic with K b = K b1

10. DIPROTIC SYSTEMS The Intermediate Form - Is both an acid and a base - Can donate or accept a proton - Called amphiprotic

11. TRIPROTIC SYSTEMS K a1 x K b3 = K w K a2 x K b2 = K w K a3 x K b1 = K w First Intermediate (H 2 A - )Second Intermediate (HA 2- )

12. PREDOMINANT SPECIES - From the Henderson-hasselbalch equation - pH changes by 1 if the ratio changes by a factor of 10 pH = pK a + 1 if [A - ]/[HA] = 10 pH = pK a - 1 if [A - ]/[HA] = 0.10

13. PREDOMINANT SPECIES Monoprotic Systems [A - ] = [HA] when pH = pK a A - is the predominant form when pH > pK a HA is the predominant form when pH < pK a

14. PREDOMINANT SPECIES Diprotic Systems There are two pK a values [H 2 A] = [HA - ] when pH = pK a1 [HA - ] = [A 2- ] when pH = pK a2 H 2 A is the predominant form when pH < pK a1 HA - is the predominant form when pK a1 < pH < pK a2 A 2- is the predominant form when pH > pK a2

15. Triprotic Systems There are three pK a values [H 3 A] = [H 2 A - ] when pH = pK a1 [H 2 A - ] = [HA 2- ] when pH = pK a2 [HA 2- ] = [A 3- ] when pH = pK a3 H 3 A is the predominant form when pH < pK a1 H 2 A - is the predominant form when pK a1 < pH < pK a2 HA 2- is the predominant form when pK a2 < pH < pK a3 A 3- is the predominant form when pH > pK a3 PREDOMINANT SPECIES

16. Diprotic acids (two equivalence points) TITRATION CURVES pH pK a2 pK a1 H 2 A/HA - HA - /A 2- Excess OH - volume of OH - added