Acids react with insoluble bases

Department of biology, chemistry, pharmacy

There are strong and weak acids

Hydrochloric acid and sulfuric acid are so-called. Strength Acids that immediately completely release the entire proton inventory in water:

HX → H+ + X-

This is unfavorable for removing limescale: the initially high acid strength can be dangerous for the user as well as unintentionally corroding other materials involved. On the other hand, if the acid is used up in the course of the reaction, the reaction becomes weaker and weaker and is ultimately unacceptably slow.

It is therefore better to use a weak acid to remove lime. This does not immediately release all protons but only a part. If the protons are used up, the acid replenishes them through a further reaction. So the whole thing is an equilibrium reaction. During your studies you will learn how to achieve such a balance through the Law of mass action can describe quantitatively. In the special case of the weak acid, the law of mass action results in Henderson-Hasselbalch equationthat will help you understand how buffer solutions work.

The dissolution of the lime can be formulated as follows:

CaCO3 + 2 H.+ → Approx2+ + H2CO3

H2CO3 → H2O + CO2

The driving force is that the volatile carbon dioxide is expelled from the solution from the carbonic acid formed during the decomposition. Acetic acid and, in particular, citric acid also complex the calcium ion, which is "hidden" in a certain way and can no longer form insoluble carbonates. Both effects, i.e. the weak acid strength and the masking of the cation, mean that decalcification is successful even under mild conditions.

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