Buffer Preparation and Capacity

In lab this week you are going to prepare an assigned buffer solution and test the buffering capacity of the solution using strong acid (HCl) and strong base (NaOH). A buffered solution resists changes in pH when acids or bases are added or when dilution occurs. The buffer is a mixture of a weak acid and its conjugate base. The buffering capacity of a solution is determined using the Henderson-Hasselbalch Equation, and the closer the ratio of acid to conjugate base is to one the higher buffering capacity. A buffer also can consist of a weak base and its conjugate acid as well.

pH = pKa + log ([A-] / [HA])

where [HA] = concentration of weak acid

and [A-] = concentration of the conjugate base

Buffer Systems Available in Lab:

1. Tris (2-Amino-2-hydroxymethyl-propane-1,3-diol) pKa = 8.082 OH

NH2

HO OH Base only

2. Phosphate Buffer (H3PO4, NaH2PO4·H2O, Na2HPO4, Na3PO4·12H2O) pKa1 = 2.15, pKa2

= 7.20, pKa3 = 12.36 OH

HO P OH

O Base and Acid

3. Acetate Buffer (HC2H3O2, NaC2H3O2) pKa = 4.76 HO

O Base and Acid 4. HEPES (2-[4-(2-hydroxyethyl)piperazin-1-yl]ethanesulfonic acid) pKa = 7.5

OH

N

N

SO O

OH

Other available solutions include:

0.1M HCl, 0.1M NaOH, 6 M HCl, and 6 M NaOH Example calculation for preparing 100.0 mL of 0.1 M phosphate buffer solution at pH = 6.80: (acid and base both available)

Now that you have what the concentrations of NaH2PO4·H2O and Na2HPO4 have to be to obtain the desired buffered solution. The grams of both compounds must be calculated in order to make 100.0 mL of buffer solution at the calculated concentrations. Once the mass of acid and conjugate base have been calculated, use the balance to obtain the calculated amount of acid and conjugate base. Both acid and conjugate base solids must be dissolved in ≈ 80 mL of water in a beaker. Once the solids are completely dissolved the pH of the solution is measured and adjusted to desired pH using 6 M HCl or 6 M NaOH. The solution at the desired pH is transferred into a 100.0 mL volumetric flask and diluted to the mark. Example calculation for preparing 100.0 mL of 0.1 M Tris buffer solution at pH = 7.50: (only base available)

Once all calculations are complete, prepare a plan of procedure to prepare your buffer solution, see above directions for preparing a buffer solution. The plan must be detailed and include all steps necessary to prepare the assigned pH buffer. Once the plan is approved by your instructor prepare the solution according to the plan of procedure.

Test your buffer to see how well it resists changes in pH by developing a plan to test the buffered solution. Once the testing plan is approved by your instructor, test the solutions and record test results in your notebook in an easily understood format such as a table. Practice Problems 1. Select the solution with the better buffering ability.

Solution A: 2.5 mol nitrous acid combined with 50 mL of water and 2.5 mol hydrochloric acid. Solution B: 2.5 mol nitrous acid combined with 50 mL of water and 2.5 mol potassium nitrite.

2. Select the solution with the better buffering ability.

Solution A: 3.0 mol sodium carbonate combined with 50 mL of water and 2.8 mol nitric acid. Solution B: 3.0 mol sodium carbonate combined with 50 mL of water and 1.5 mol nitric acid.

3. Select the solution with the better buffering ability.

Solution A: 15 mL 1.5 M ammonia plus 8.0 mL 1.5 M hydrochloric acid Solution B: 15 mL 1.5 M ammonia plus 15 mL 1.5 M hydrochloric acid

4. Select the solution with the better buffering ability.

Solution A: 35 mL of 0.4 M sodium cyanide combined with 35 mL of 0.4 M hydrochloric acid Solution B: 35 mL of 0.4 M sodium cyanide combined with 15 mL of 0.4 M hydrochloric acid

5. Select the solution with the better buffering ability.

Solution A: 20 mL of 0.35 M dimethylamine combined with 20 mL of 0.35 M dimethylammonium chloride. Solution B: 20 mL of 0.0035 M dimethylamine combined with 20 mL of 0.0035 M dimethylammonium chloride.

6. Select the solution with the better buffering ability.

Solution A: 2.0 mol ammonium chloride combined with 20 mL water and 1.0 mol hydrochloric acid Solution B: 2.0 mol ammonium chloride combined with 20 mL water and 1.0 mol potassium hydroxide

7. Select the solution with the better buffering ability.

Solution A: 1.5 M nitrous acid and 0.015 M sodium nitrite Solution B: 1.5 M nitrous acid and 1.5 M sodium nitrite