11.2 Flashcards
11.2 pH - titration curves
• To measure pH changes during acid–base titrations and to plot pH titration curves.
You will follow the pH during acid–base reactions. This will allow you to plot pH titration curves for different combinations of strong and weak, acids and bases.
You will measure the pH using a pH meter, pH probes, a data logger or narrow range pH paper, depending on availability. You can plot the pH titration curves by different methods, by hand, using software, or automatically as you record your results.
Aqueous hydrochloric acid, HCl(aq), 0.100 mol dm–3
Currently no hazard classification at this concentration
Aqueous sodium hydroxide, NaOH(aq), 0.100 mol dm–3
Currently no hazard classification at this concentration
Aqueous ethanoic acid, CH3COOH(aq), 0.100 mol dm–3
Currently no hazard classification at this concentration
Aqueous ammonia, NH3(aq), 0.100 mol dm–3
Currently no hazard classification at this concentration
note
You will also have access to
• pH probe, pH meter or narrow range pH paper
• the usual equipment for carrying out a titration.
You can record your results by hand or by using a datalogger, computer or mobile device such as a tablet or mobile phone.
Procedure
- Carry out a trial acid–base titration using 0.1 mol dm–3 hydrochloric acid and 0.1 mol dm–3 sodium hydroxide.
• Monitor how the pH changes by taking readings after adding different volumes of base during the trial titration. Make sure that the solution is mixed thoroughly before each measurement.
• You should use these trial readings to decide which measurements you are going to take during your accurate titration.
• You will need fewer readings when the pH changes little but the readings will need to be closer together when the pH changes rapidly. - Carry out an accurate titration, recording volumes of base added and the pH after each addition of base.
- Carry out further titrations using different combinations of strong and weak acids and bases.
- If you have recorded your results electronically, ensure that you save these so that you have a permanent record.
equipement
The following equipment should be provided, from which each learner or group should select appropriate apparatus. Some may suggest alternatives.
• Safety spectacles
• Burette
• White tile
• Pipette (25.0 cm3) and filler
• Clamp stand, with boss and clamp (for supporting the burette)
• Dropping pipette
• Wash bottle containing distilled or de-ionised water (about 300 cm3 will be required)
• Two conical flasks (250 cm3)
• Glass beakers (250 cm3 and 100 cm3)
Each learner or group will also need access to the following:
• A method for recording pH.
Ideally a pH meter, or pH probe with data logger would be used. Small, ‘stick’ pH meters may be suitable, but should be tested beforehand to see if usable results can be obtained. These are available from most suppliers and are relatively inexpensive.
Alternatively narrow-range pH paper can be used but several ranges will be required
Analysis of your results
- From each pH titration curve, measure the equivalence point (the centre of the steep section of the curve). What is the significance of the equivalence point?
- pKa values for several pH indicators are shown below.
thymol blue 1.7
methyl orange 3.4
methyl red 5.0
bromothymol blue 7.1
metacresol purple 8.3
phenolphthalein 9.4
Select the indicator(s) suitable for each combination of acid and base that you have titrated. - How would the shapes of your pH titration curves be different for the following changes?
a. Using 0.200 mol dm–3 acid against 0.100 mol dm–3 base
b. Using 0.100 mol dm–3 acid against 0.200 mol dm–3 base
Carry out a titration and plot a pH titration curve using 0.0500 mol dm–3 sulfuric acid, H2SO4(aq)
Explain the shape of the graph
0.0500 mol dm–3 sulfuric acid titrated with 0.1 mol dm–3 sodium hydroxide should produce a curve that looks (almost) the same as the curve for the titration of hydrochloric acid. Sulfuric acid is diprotic, so the solutions at these concentrations will react in equal volumes. Note that the Ka of the second dissociation of sulfuric acid is large enough that the first equivalence point is obscured by the second.
From each pH titration curve, measure the equivalence point (the centre of the steep section of the curve). What is the significance of the equivalence point
- The equivalence point is the point at which the volume of one solution has exactly reacted with the volume of the other solution.
a. Using 0.200 mol dm–3 acid against 0.100 mol dm–3 base
- The basic shape would be the same in all cases, but the equivalence point – and thus the steep section of the curve – would be at a different position.
a. The equivalence point would come after the addition of twice as much base.
How would the shapes of your pH titration curves be different for the following changes?b. Using 0.100 mol dm–3 acid against 0.200 mol dm–3 base
- The basic shape would be the same in all cases, but the equivalence point – and thus the steep section of the curve – would be at a different position.a. The equivalence point would come after the addition of half as much base.
