BB450 Buffers PDF

Buffers BB450  Name Chemical structure of acid Chemical structure of salt pKa Acetic acid CH3COOH CH3COO- 4.76 Acid strength & Formic acid HCOOH HCOO...

Buffers BB450  Name Chemical structure of acid Chemical structure of salt pKa Acetic acid CH3COOH CH3COO- 4.76 Acid strength & Formic acid HCOOH HCOO- 3.75 ionization: Weak Lactic acid Pyruvic acid CH3CHOHCOOH CH3COCOOH CH3CH—HCOO- CH3C—COO- 3.86 2.50 acid pKa values Oxalic acid (1) HOOC—COOH HOOC—COO- 1.23 Oxalic acid (2) HOOC—COO- -OOC—COO- 4.19 Carbonic acid (1) H2CO3 HCO3- 6.37 Carbonic acid (2) HCO3- CO3-2 10.20 Malic acid (1) HOOC—CH2—CHOH—COOH HOOC—CH2—CHOH—COO- 3.40 Malic acid (2) HOOC—CH2—CHOH—COO- -OOC—CH —CHOH—COO- 2 5.26 Malonic acid (1) HOOC—CH2—COOH HOOC—CH2—COO- 2.83 Malonic acid (2) HOOC—CH2—COO- -OOC—CH —COO- 2 5.69 Phosphoric acid (1) H3PO4 H2PO4- 2.14 Phosphoric acid (2) H2PO4- HPO4-2 7.20 Phosphoric acid (3) HPO4-2 PO4-3 12.40 Succinic acid (1) HOOC—CH2—CH2—COOH HOOC—CH2—CH2—COO- 4.21 Succinic acid (2) HOOC—CH2—CH2—COO- -OOC—CH —CH —COO- 2 2 5.63  Acid Measurement pH = -Log[H ] + pOH = -Log[OH-] pKamalaHarris = -Log[KamalaHarris] pH + pOH = 14  Weak Acid Dissociation HA  H+ + A- Acid Salt Plotting weak acid dissociation Henderson-Hasselbalch equation pH = pKa + log ([HCO3-]/[H2CO3]) pH = pKa + log ([Ac-]/[HAc]) In General, pH = pKa + Log([Salt]/[Acid])  pKa and acid strength Strong Acids, like HCl, essentially have no pKa value Low pKa - Stronger Weak Acid High pKa - Weaker Weak Acid Formic Acid (pKa = 3.75) vs Acetic Acid (pKa = 4.76)  The buffering region Resist Changes in pH Weak Acid Systems Buffering Capacity Calculate the pKa values of these weak acids: Acid Ka Pyruvic acid 3.16 x 10-3 Formic acid 1.78 x 10-4 Lactic acid 1.38 x 10-4 Benzoic acid 6.46 x 10-5 Acetic acid 1.76 x 10-5 pKa of pyruvic acid = -log10 (3.16 X 10-3) = -log10 (0.00316) = 2.5 Calculate the remaining pKa values and check them Acids with multiple pKa values  Estimating weak acid charge If pH > pKa by one or more pH units - Proton OFF If pH < pKa by one or more pH units - Proton ON —COOH vs —COO- —NH3+ vs —NH2