Question

Acetic Acid has a pKa of 4.76. Explain how you would make up 100mL of an acetic acid buffer with a pH of 5.2 from a 50uM stock solution of acetic acid and sodium acetate.

Answer 1

CH3COOH ----> CH3COO-

Acetic acid buffer system

pH = pKa + log [CH3COO-] / [CH3COOH]

let

CH3COO- = x

CH3COOH = y

5.2 = 4.76 + log x/y

x/y = 2.75

x = 2.75 y ---> 1

CH3COO- + CH3COOH = Molartiy*V in L = 50uM*100 / 1000 = 5*10^-6 moles

x+y = 5*10^-6 --->2

From 1 and 2 equations

y = 1.33*10^-6 moles

x = 3.66*10^-6 moles

Volume of x = Moles /molarity = 3.66*10^-6 / 50uM = 73.2 ml

Volume of y = 1.33*10^-6 / 50 uM = 26.6 ml

Volume of CH3COOH = 26.6 ml

Volume of CH3COO- = 73.2 ml