PAG 11.3: pH – Acids & Buffers
- PAG 11.3 comes in three parts:
- Preparing hydrochloric acid solutions of a range of concentrations by dilution and measuring their pH values
- Preparing a range of buffer solutions and measuring their pH values
- Using both types of solutions, investigating how the pH changes with the addition of sodium hydroxide solution.
Preparing hydrochloric acid solutions with different concentrations
- This can be done by serial dilution
- Serial dilution is any dilution in which the concentration decreases by the same factor in each successive step
Serial dilution method
-
- Example 1
- Adding 1.0 cm3 of HCl (aq) to 9.0 cm3 of distilled water is a 1 : 10 dilution
- This means that the concentration will decrease by a factor of 10
- Example 2
- Adding 1.0 cm3 of HCl (aq) to 4.0 cm3 of distilled water is a 1 : 5 dilution
- This means that the concentration will decrease by a factor of 5
- Example 1
Method
- Label 7 test tubes 1 - 7
- Add 10.0 cm3 of 0.1 mol dm-3 hydrochloric acid to test tube 1
- Test tube 1 should not be diluted with distilled water
- Test tube 1 will, therefore, only contain 0.1 mol dm-3 HCl (aq)
- Transfer 1.0 cm3 of this hydrochloric acid, from test tube 1, into a 10.0 cm3 measuring cylinder
- Add 9.0 cm3 of distilled water to the measuring cylinder
- The measuring cylinder should now contain 10.0 cm3 of liquid
- Transfer the resulting diluted solution from the measuring cylinder into test tube 2
- This is a 1 : 10 dilution
- Theoretically, the concentration of test tube 2 will be = 0.01 mol dm-3
- This process now repeats
- Transfer 1.0 cm3 of this hydrochloric acid, from test tube 2, into a 10.0 cm3 measuring cylinder
- Add 9.0 cm3 of distilled water to the measuring cylinder
- Transfer the resulting diluted solution from the measuring cylinder into test tube 3
- Theoretically, the concentration of test tube 3 will be = 0.001 mol dm-3
- Transfer 1.0 cm3 of this hydrochloric acid, from test tube 3, into a 10.0 cm3 measuring cylinder
- Add 9.0 cm3 of distilled water to the measuring cylinder
- Transfer the resulting diluted solution from the measuring cylinder into test tube 4
- Continue this process until test tubes 1 - 6 are prepared
- Test tube 7 should only contain distilled water
- The pH of test tubes 1 - 7 can then be tested in the usual ways
- Use of a calibrated pH probe
- Addition of universal indicator solution and comparison to its colour chart
Specimen results
Test Tube | Concentration (mol dm-3) | pH |
1 | 0.1 | 1.00 |
2 | 0.01 | 2.00 |
3 | 0.001 | 3.00 |
4 | 0.0001 | 4.00 |
5 | 0.00001 | 5.00 |
6 | 0.000001 | 6.00 |
7 | 0.0000001 | 7.00 |
- The resulting solutions can be reacted with sodium hydroxide solution
- This will cause a change in pH
- It can be monitored by titrimetric methods - using a pH probe / meter to track the pH as the titration proceeds
- This should produce results / graphs similar to those seen in 5.3.5 Neutralisation
Preparing a buffer solution
- A buffer solution is a solution that is resistant to small changes in pH
- Eventually enough acid or base can be added to overcome that capacity
- The amount of acid or base needed to change the pH of a buffer is known as the "buffer capacity"
Method
- The volume and concentration of ethanoic acid used in this preparation is 5 cm3 and 0.30 mol dm-3 respectively
- First, calculate the concentration of ethanoic acid that will be present in 100 cm3
- Number of moles of ethanoic acid in 5 cm3 = = 0.0015 moles
- Therefore, this is the number of moles in 100 cm3
- Therefore the concentration = = 0.015 mol dm-3
- Add 50 cm3 of distilled water to a 100 cm3 beaker
- Use a pipette to add 5 cm3 of 0.30 mol dm-3 of ethanoic acid to the beaker
- Calculate the mass of sodium ethanoate required by determining the concentration of [A–] using:
- pH = pKa + log10
- The pKa of ethanoic acid is 4.76
- 5.00 = 4.76 + log10
- 100.24 =
- [A–] = 0.0261 mol dm-3
- Therefore the number of moles of A– needed in 100 cm3 is
- = 0.00261 moles
- Therefore the mass of sodium ethanoate required is
- 0.00261 x 82.04 = 0.214 g
- Measure 0.22 g of sodium ethanoate into a weighing boat
- Slightly more sodium ethanoate than required should be measured because some will be left behind when transferring in the next steps
- Calibrate a pH meter and add it to the beaker
- Slowly add the sodium ethanoate to the beaker, stirring with a glass rod to dissolve the solid
- Do not stir using the pH probe
- Continue until the pH reaches 5.00
- Measure the weighing boat that contained the sodium ethanoate
- Transfer your made buffer solution to a 100 cm3 volumetric flask
- Add distilled water up to the mark, cap and mix
Preparing a buffer solution with a higher buffer capacity
- A buffer solution with a pH of 5.0 with a higher "buffer capacity" can be prepared using 0.50 mol dm-3
- The method to make this buffer solution will be the same as above but the calculations will change:
- Number of moles of ethanoic acid in 5 cm3 = = 0.0025 moles
- Therefore this is the number of moles in 100 cm3
- Therefore the concentration = 0.025 mol dm-3
- Calculate the mass of sodium ethanoate required by determining the concentration of [A–] using:
- pH = pKa + log10
- The pKa of ethanoic acid is 4.76
- 5.00 = 4.76 + log10
- 100.24 =
- [A-] = 0.0434 mol dm-3
- Therefore the number of moles of A- needed in 100 cm3 is
- = 0.00434 moles
- Therefore the mass of sodium ethanoate required is
- 0.0434 x 82.04 = 0.356 g
Testing the buffer capacity
- Calibrate a pH probe
- Rinse the pH probe with distilled water and shaken to remove excess water
- Place the tip of the pH probe in pH 4 buffer solution ensuring bulb is fully immersed
- Allow the probe to sit until the pH stabilises
- Adjust reading to the pH of the buffer
- Repeat this process with a pH 9 buffer solution
- Ensure that the first step, washing with distilled water, is not missed
- Using a volumetric pipette, transfer 25.0 cm3 of the first buffer solution into a conical flask
- Load a 50 cm3 burette with a standardised NaOH solution
- Use the pH probe to monitor the titration of the buffer until the pH changes 1 unit
- Repeat with the second buffer solution