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

CHAPTER 11 POLYPROTIC ACIDS AND BASES

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

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) H + ↔ 10Ca H 2 PO H 2 O

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 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

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

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

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

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

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

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- )

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

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

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

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

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