Question

A 500 mL buffer is usually prepared with 46 mM carbonic acid and 58 mM sodium bicarbonate. Unfortunately you have run out of these reagents; on hand you have the following: solid citric acid, disodium maleate (salt of maleic acid), benzylamine and hydrochloric acid. Which reagent(s), and how much of each, would you need to use to produce a 500 mL buffer with the same formal concentration and pH as the carbonic acid buffer

Chemical	concentration or MW	Mass or volume needed
Citric acid	192.124g/mol
Disodium Maleate	2.6M
Benzylamine	1.4M
HCl	3M
water

I calculated the original pH of the solution to be 6.452 and the original amount of moles to be 0.052.

What I'm stuck on is how to properly make a buffer without a strong conjugate for the colution

Answer 1

The carbonic acid sodium bicarbonate buffer is described by the equation

H₂CO₃ + H₂O H₃O⁺ + HCO₃^-

pH = Pka + log salt/acid

pH = 6.34 + log 58 x 10^-3/46 x 10^-3

pH = 6.34 + 0.100

pH = 6.44

We need to use reagents that have pKa in the region of pH we are interested in

citric acid has a pKa of 3.13, benzylamine has a pKa of 9.35

disodium maleate or pKa2 of maleic acid is 6.04 so we will be using this

concentration of sodium maleacte needs to be in the same concentration of the earlier buffer so we will keep the amount of sodium maleate to be around 58 mM

pH = pKa + log salt/acid

6.44 = 6.04 + log salt/acid

log salt/acid = 0.4

salt/acid = 10^0.4

salt/acid = 2.51

58 x 10^-3/acid = 2.51

acid = 0.0231

So we need to add 23.1 x 10^-3 mM of HCl to the sodium maleate to get the buffer with 6.44 pH

Now to get 58 mM sodium maleate from 2.6 M sodium maleate to make 500 mL

we will need

58 mM x 500 mL = 2.6 M x V2

V2 = 58 x 10^-3 x 500 mL/2.6

V2 = 11.15 mL of 2.6 M sodium maleate has to be taken

23.1 x 10^-3 M x 500 mL = 3 M x V2

V2 = 23.1 x 10^-3 M x 500 mL/ 3M

V2 = 3.85 mL of 3 M HCl has to be mixed to get the right pH

Chemical	concentration or MW	Mass or volume needed
Citric acid	192.124g/mol
Disodium Maleate	2.6M	11.15 mL
Benzylamine	1.4M
HCl	3M	3.85 mL
water

Dissolve to make 500 mL buffer with pH 6.44