Acid-base equilibria

Read our general notes on Risk Assessment

I was just wondering what the titration curve would look like if 10 cm³ of sulphuric acid were titrated with ammonia solution. 20 cm³ of ammonia is sufficient to neutralise the fully ionised sulphuric acid. Does the curve have two equivalence points? If so, where are they and why?

Igloo writes
 
We have no practical experience of this particular titration, so we would welcome comments from anyone who has tried this in practice.
 
I am assuming that the sulphuric acid and ammonia have the same concentration. If so, you are correct: 20 cm³ of ammonia will be needed to neutralize fully the sulphuric acid.
 
Sulphuric acid is a strong acid and ionises pretty well fully to H⁺ and HSO₄^– in aqueous solution.
 
The hydrogensulphate ion is a weak acid and is hardly ionised just in the presence of water.
 
So from an ionic point of view, the titration will take place in two stages:
 
[1] NH₃ + H⁺ + HSO₄^– -> NH₄⁺ + HSO₄^–
 
[2] NH₃ + HSO₄^– -> NH₄⁺ + SO₄^2–
 
At the start of the titration we are dealing with dilute sulphuric acid, so the pH would be very low (say 1 if it is a 0.1 M solution).
 
This starts as a titration of a strong acid with a weak base. This should have quite a sharp end point with a pH below 7 at the equivalence point.
 
Whilst sulphuric acid can form acid salts with alkali metals (e.g. NaHSO₄), we know of no evidence that it is possible to isolate a compound NH₄HSO₄), because we have never encountered this compound.
 
The second stage is the neutralisation of a weak acid by a weak base. This will not have a sharp end point. The end point should be detected with a titre which is twice that of the first end point.
 
However - we are not completely convinced by this answer so please try it and let us know.

Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website. For further guidance see our tutorial on Risk Assessment.

back to Acid-base equilibria

---

Rate this page or react
Share your views on this page, 2 ratings so far , rated at

---

updated: 10 December 2003