Question

Calculate the mass of glycine required to prepare 50 mM glycine (250 mL), and the anticipated volume of 2.5 N NaOH required to titrate the glycine buffer to a pH of 10. Calculate the pH of 50 mM glycine prior to addition of NaOH. The pKa values for glycine are 9.6 for amino group and 2.34 for carboxyl group.

Answer 1

First, you can calulate the mass of glycine required to prepare 250 mL of 50 mM glycine solution:

Now, you must take into account the two equilibrium dissociation reactions for glycine:

NH₃⁺‐CH₂‐COOH NH₃⁺‐CH₂‐COO^- + H⁺ NH2‐CH₂‐COO^- + H⁺

To make things easier, we will called NH₃⁺‐CH₂‐COOH as Glycine⁺, NH₃⁺‐CH₂‐COO^- as Glycine^+/- and NH2‐CH₂‐COO^- as Glycine^-.. pK_a-COOH=2.34 and pK_a-NH3+=9.6 respectively.

Now, you can determine the equilibrium constants:

;

As you know the glycine concentration, you can calculate [H⁺] for each equilibrium reaction as follows:

For the first equilibrium:

For the last equilibrium:

Then, you can calculate the pH of 50 mM glycine solution prior the addition of NaOH, as follows:

pH_{Glycine-50 mM}=3.32.

Now, you can calculate the required volume to titrate the glycine buffer to a pH of 10:

Finally:

V_{NaOH-2.5 N}=0.0478 mL