Question

For an experiment, you need two buffers, one with pH = 3.1 and one with pH = 9.7. You have the following reagents available. Which pair would you use for each buffer and what must the buffer ratio be in each case?

1) Hydrobromic acid (HBr)

2) Sodium bromide (NaBr)

3) Hydrofluoric acid (HF)

4) Sodium fluoride (NaF)

5) Pyridine (C₅H₅N)

6) Pyridinium chloride (C₅H₅NH⁺Cl^-)

7) Triethylamine ((CH₃CH₂)₃N)

8) Triethylammonium chloride ((CH₃CH₂)₃NH⁺Cl^-)

9) Sodium hydroxide (NaOH)

Answer 1

for pH = 3.1 buffer

suitable pairs :

a) Hydrofluoric acid (HF) & Sodium fluoride (NaF)

pKa of HF = 3.20

pH = pKa + log [NaF/HF]

3.10 = 3.20 +  log [NaF/HF]

[NaF/HF] ratio = 0.794

b) Pyridinium chloride (C5H5NH+Cl-) & Triethylamine ((CH3CH2)3N)

pKa of Triethylamine ((CH3CH2)3N) = 3.25

pH = pKa + log [C5H5NH+Cl-] / [CH3CH2)3N]

3.10 = 3.25 + log [C5H5NH+Cl-] / [CH3CH2)3N]

[C5H5NH+Cl-] / [CH3CH2)3N] ratio = 0.708

c) for 9.7 buffer :

suitable pair :

Pyridine (C5H5N) & Pyridinium chloride (C5H5NH+Cl-)

pH = pKa + log [C5H5NH+Cl-]/[C5H5N]

9.7 = 8.77 + log [C5H5NH+Cl-]/[C5H5N]

[C5H5NH+Cl-]/[C5H5N] = 0.117