Lecture 16 10/07/05

Properties of buffers 1.Effect of dilution 2.Effect of added acid/base

What is the pH of a 1-L solution with 0.05 M OCl - and 0.1 M HOCl? What is the pH after you add 0.02 moles of HCl (assume no change in volume)? What is the pH after you add 0.02 moles of NaOH (assume no change in volume)? pK a = 7.53

How to choose a buffer Want pK a close to desired pH max buffering capacity

Buffers in real life Add buffer, then adjust pH with strong acid or base Why not just calculate the amount of buffer needed? Activity coefficients could be wrong/ignored Temperature may be different Approximations could be wrong Reported pK a probably different in lab Chance of arithmetic error Ionic strength and temperature will affect pH!

Find the pH of a solution prepared by dissolving g of tris ( g/mol) plus 4.67 g of tris hydrochloride ( g/mol) in 1 L of water. pK a = pH after adding mL of 1.00 M HCl?

pH of diprotic acids H 2 L +  HL + H + pK a1 = HL  L + H + pK a2 = 9.747

The gist – Diprotic acids When adding H 2 L + or L - Assume that first dissociation dominates Looking for pH - solve like monoprotic acid Looking for conc. of all species [L - ] = K a2 (when you added H 2 L - ) [H 2 L + ] = K b2 (when you added L - )

Adding HL Use systematic equilibrium Substitute to get equation in terms of H + and HL Charge balance K b expression K a expression K w Mass Balance 0.05 M = [H 2 L + ] + [HL] + [L - ] Assume [HL] >> [H 2 L + ] and [HL] >> [L - ] So [HL]  0.05 M Solve for H +

Alternate approximation Assume K 1 K w << K 1 K 2 [HL] Assume K 1 << [HL]

Diprotic Buffers

Principal species

CasePrincipal species pH < pK a1 H2AH2A pK a1 < pH < pK a2 HA - pH > pK a2 A-A-

Pay attention to the desired pH vs. pK How many mL of 0.8 M KOH should be added to moles oxalic acid (FW g/mol) to give a pH of 4.40 when diluted to 500 mL. pK a1 = 1.252H 2 Ox  HOx - + H + pK a2 = 4.266HOx -  Ox -2 + H +

Triprotic Acids