pH (TITRATION) CURVES
This page describes how pH changes during various acid-base titrations.
The equivalence point of a titration
Sorting out some confusing terms
When you carry out a simple acid-base titration, you use an indicator to tell you when you have the acid and alkali mixed in exactly the right proportions to "neutralise" each other. When the indicator changes colour, this is often described as the end point of the titration.
In an ideal world, the colour change would happen when you mix the two solutions together in exactly equation proportions. That particular mixture is known as the equivalence point.
For example, if you were titrating sodium hydroxide solution with hydrochloric acid, both with a concentration of 1 mol dm-3, 25 cm3 of sodium hydroxide solution would need exactly the same volume of the acid - because they react 1 : 1 according to the equation.
In this particular instance, this would also be the neutral point of the titration, because sodium chloride solution has a pH of 7.
But that isn't necessarily true of all the salts you might get formed.
For example, if you titrate ammonia solution with hydrochloric acid, you would get ammonium chloride formed. The ammonium ion is slightly acidic, and so pure ammonium chloride has a slightly acidic pH.
That means that at the equivalence point (where you had mixed the solutions in the correct proportions according to the equation), the solution wouldn't actually be neutral. To use the term "neutral point" in this context would be misleading.
Similarly, if you titrate sodium hydroxide solution with ethanoic acid, at the equivalence point the pure sodium ethanoate formed has a slightly alkaline pH because the ethanoate ion is slightly basic.
To summarise:
· The term "neutral point" is best avoided.
· The term "equivalence point" means that the solutions have been mixed in exactly the right proportions according to the equation.
· The term "end point" is where the indicator changes colour. As you will see on the page about indicators, that isn't necessarily exactly the same as the equivalence point.
Simple pH curves
All the following titration curves are based on both acid and alkali having a concentration of 1 mol dm-3. In each case, you start with 25 cm3 of one of the solutions in the flask, and the other one in a burette.
Although you normally run the acid from a burette into the alkali in a flask, you may need to know about the titration curve for adding it the other way around as well. Alternative versions of the curves have been described in most cases.
Titration curves for strong acid v strong base
We'll take hydrochloric acid and sodium hydroxide as typical of a strong acid and a strong base.
Running acid into the alkali
You can see that the pH only falls a very small amount until quite near the equivalence point. Then there is a really steep plunge. If you calculate the values, the pH falls all the way from 11.3 when you have added 24.9 cm3 to 2.7 when you have added 25.1 cm3.
Running alkali into the acid
This is very similar to the previous curve except, of course, that the pH starts off low and increases as you add more sodium hydroxide solution.
Again, the pH doesn't change very much until you get close to the equivalence point. Then it surges upwards very steeply.
