Chemguide: Support for CIE A level Chemistry
Learning outcome 7.2
The Bronsted-Lowry theory of acids and bases
Statements 7.2.9 and 7.2.10
These statements deal with titration curves and the use of indicators. Before you do anything else, read these statements in your syllabus.
Statement 7.2.9: Titration curves
Start by reading the page about pH (titration) curves.
This is quite a complicated page, and it is easy to get confused between the shapes of the various curves. In the first instance, stop when you get to the heading "More complicated titration curves".
What I would suggest you do is ignore these more complicated graphs until after you have read about indicators later on. I will remind you further down this page.
Don't worry about the references to buffer solutions throughout the page. Once you know about buffer solutions later, you could come back and look at this again, but it is more important to be able to draw the curves.
Concentrate on how the curves relate to each other. Make sure that you can draw the four basic curves shown in the summary section (just before you get into the complicated shapes).
And make sure that you can understand why they are different - just in terms of the likely pHs involved.
If you understand this, you should be able to draw a reasonable approximation to these curves.
Now do the same thing for the curves where you add an alkali to an acid. Look carefully at the graphs on the pH curves page and work out what happens if you start with a strong or weak acid, and add a strong or weak base. You need to know what your likely start and finish points are going to be.
Statement 7.2.10: Indicators
Read the page about acid-base indicators. Ignore any reference to Kind. CIE don't want you to know about this.
Ignore the final bit about the titration of sodium carbonate solution for now.
You don't need to know about the structure of methyl orange (or any other indicator), but you do need to know:
And a final comment on this last point:
CIE have in the past mentioned the use of the indicator bromothymol blue in this context, to demonstrate the fact that it won't give a good result in a weak base / weak acid titration.
Bromothymol blue has a pH range of 6.0 to 7.6, and so bridges the end point of a typical weak acid / weak base titration.
However, the colour change isn't sharp. It will change gradually from blue through green to yellow while you add perhaps 1 cm3 of weak acid to a weak base. You can't get an accurate titration out of this.
In an exam, if CIE ask you about any other indicators, they will always tell you their pH ranges and their colours.
Note: It is important that you know how to produce titration curves for all the combinations, that you know that an indicator has to change colour on the steep bit of the curve, and you know the pH ranges of methyl orange and phenolphthalein. If you know all that, you can work out the rest if you need it.
The more complicated titration curves
Once you are reasonably happy about this, go back to the pH (titration) curves page, and read the end section on "More complicated titration curves".
Then look at how a careful choice of indicators lets you pick up both end points in the titration of sodium carbonate solution with dilute hydrochloric acid on the page about acid-base indicators.
Don't spend too much time on this, but you should be aware that it is possible to get these more complex titration curves under some circumstances.
© Jim Clark 2020