Question

PART I – Prepare a buffer and check the final pH using a pH meter Material notes:

Sodium phosphate, monobasic, NaH2PO4·H2O (Mw = 138)
Sodium phosphate, dibasic-pentahydrate, Na2HPO4·7H2O (Mw = 268) pH meter – calibrated using pH 4, 10, 7 reference buffers (done by TA’s)

Special safety notes:

Wear your personal protective equipment (PPE).

Procedure

1. Calculate the amounts of weak acid and conjugate base required to make 50 ml of 0.25 M solutions of each.

2. Prepare 0.25 M NaH2PO4 and 0.25 M Na2HPO4 solutions, 50 ml each:

Weigh amounts calculated above (using weigh boats) and transfer to 50 ml volumetric flasks using a funnel (e.g., rolled up weighing paper). Fill to volume with DI-H2O. *Gentle swirling and heating may be needed to solubilize Na2HPO4.

3. Calculate the amounts of weak acid and conjugate base required to prepare 100 ml of 0.02 M phosphate buffer pH 6.5. The apparent pKa’ of NaH2PO4 is 6.99 (taking into account ionic strength1). Use the Henderson-Hasselbalch equation:

pH = pKa + log[A-]/[HA]
4. Prepare 100 ml of 0.02 M phosphate buffer, pH 6.5.

Using a pipette, combine the appropriate volumes of your stock solutions in a 100 ml volumetric flask, rinsing the pipette with DI-H2O into the flask, and fill to volume with DI-H2O. Invert to mix.

5. Check the pH of your buffer using a pH meter: transfer the buffer to a beaker. Record the pH of your buffer.

Note: always rinse the pH electrode with DI-H2O between readings (squirt bottles) and gently dab away the water with a Kimwipe. Store the electrode in the KCl storage solution when not in use.

QUESTIONS:

What would have a lower pH – a solution of 1 M acetic acid with 0.5 M NaOH or a solution of 1 M lactic acid with 0.5 M NaOH? Show your calculation of the pH for each case.

If there had been CO2 in the sample, what effect would this have had on the volume of NaOH needed to reach the titration end point? How would this effect the reported %TTA value?

Is a 1% solution of acetic acid solution the same as a 0.1 M solution? Show your calculations.

From Table 2, which acids could be used to make an effective buffer at pH 7? Which could make an effective buffer at pH 5?

Answer 1

pH of a solution of 1 M acetic acid with 0.5 M NaOH = 4.75 + Log(0.5/0.5), i.e. 4.75

pH of a solution of 1 M lactic acid with 0.5 M NaOH = 3.86 + Log(0.5/0.5), i.e. 3.86

Here, 3.86 < 4.75

Therefore, a solution of 1 M lactic acid with 0.5 M NaOH has a lower pH.

If CO₂ is present in the sample, then the titration needs more volume of NaOH to reach the end point, because some of the NaOH reacts with CO₂ to form NaHCO₃ as shown below.

NaOH + CO₂ NaHCO₃

As a result of an increase in the volume of NaOH to reach the titration end point, the % Titratable Acid (TTA) does also increase.

1% solution of acetic acid = 1 g. of acetic acid is present in 100 mL of water.

No. of moles of acetic acid in 1% solution of acetic acid = 1/60

The concentration of acetic acid in 1% solution of acetic acid = (1/60) * 1000/100, i.e. 0.167 M

Therefore, 1% solution of acetic acid is not same as the 0.1 M solution of acetic acid.